INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
ENVIRONMENTAL HEALTH CRITERIA 160
ULTRAVIOLET RADIATION
This report contains the collective views of an international group of
experts and does not necessarily represent the decisions or the stated
policy of the United Nations Environment Programme, the International
Labour Organisation, or the World Health Organization.
An Authoritative Scientific Review of Environmental and Health
Effects of UV, with Reference to Global Ozone Layer Depletion
Published under the joint sponsorship of
the United Nations Environment Programme,
the International Labour Organisation,
and the World Health Organization
World Health Orgnization
Geneva, 1994
The International Programme on Chemical Safety (IPCS) is a joint
venture of the United Nations Environment Programme, the International
Labour Organisation, and the World Health Organization. The main
objective of the IPCS is to carry out and disseminate evaluations of
the effects of chemicals on human health and the quality of the
environment. Supporting activities include the development of
epidemiological, experimental laboratory, and risk-assessment methods
that could produce internationally comparable results, and the
development of manpower in the field of toxicology. Other activities
carried out by the IPCS include the development of know-how for coping
with chemical accidents, coordination of laboratory testing and
epidemiological studies, and promotion of research on the mechanisms
of the biological action of chemicals.
WHO Library Cataloguing in Publication Data
Ultraviolet radiation
(Environmental health criteria: 160)
1. Ultraviolet rays 2. Radiation effects
3. Environmental exposure I.Series
ISBN 92 4 157160 8 (NLM Classification: WD 605)
ISSN 0250-863X
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CONTENTS
PREFACE
1. SUMMARY AND CONCLUSIONS
1.1. Physical characteristics
1.2. Action spectrum and minimum erythemal dose
1.3. Cellular and molecular studies
1.4. Animal studies
1.5. Health effects on humans
1.5.1. Skin
1.5.2. Immune system
1.5.3. Eye
1.6. Environment
1.7. Guidelines on exposure limits and protective measures
2. PHYSICAL CHARACTERISTICS
2.1. Electromagnetic spectrum
2.2. Radiometric quantities and units
2.3. UV production
2.3.1. Thermal emitters
2.3.2. Electrical gaseous discharges
2.3.3. Stimulated emission
2.4. Summary
3. UV SOURCES
3.1. The sun
3.1.1. Factors affecting solar UV levels
3.1.2. Ozone depletion effects
3.1.3. Trends in UV 19
3.1.4. Theoretical models
3.1.5. UV monitoring
3.1.6. Analysis of UV data
3.1.7. Conclusions
3.2. Artificial sources
3.2.1. Incandescent sources
3.2.2. Gaseous discharge sources
3.2.3. Gas welding
3.2.4. Arc welding
3.2.5. Lasers
3.2.6. Sunbeds
4. HUMAN EXPOSURE
4.1. Sunlight
4.2. Skin exposure geometry
4.3. Ocular exposure geometry
4.4. Workplace
4.4.1. Outdoor work
4.4.2. Indoor work
4.4.3. Research
4.4.4. Commerce
4.4.5. Medicine and dentistry
4.5. Elective exposure
5. DOSIMETRIC CONCEPTS
5.1. Interaction of UV with matter
5.2. Biological weighting factors and spectrally
weighted quantities
5.3. Measurement techniques
5.3.1. Detectors
5.3.2. Radiometers
5.3.3. Spectroradiometers
5.3.4. Personal dosimetry
5.4. Calibration
6. CELLULAR AND MOLECULAR STUDIES
6.1. Introduction
6.2. Interactions with biomolecules
6.2.1. Cellular chromophores
6.2.2. Cellular targets
6.3. Action spectra
6.4. Biomolecular damage
6.4.1. Nucleic acids
6.4.2. Membranes
6.4.3. Proteins
6.5. Cellular defenses
6.5.1. DNA
6.5.2. Human excision repair disorders
6.5.3. Antioxidant pathways
6.5.4. Summary
6.6. Cellular consequences of damage
6.6.1. Membrane disruption
6.6.2. Activation of genes
6.6.3. Cell death
6.6.4. Mutation, chromosomal damage and transformation
6.7. Conclusions
7. ANIMAL STUDIES
7.1. Skin carcinogenesis
7.1.1. Domestic animals
7.1.2. Experimental animals
7.1.3. Interactions between radiations
of different wavelengths
7.1.4. Dose response
7.1.5. Effect of pattern of exposure
7.1.6. Action spectrum
7.1.7. Interaction between UV and chemicals
7.1.8. Mechanisms of UV carcinogenesis
7.1.9. Conclusions
7.2. Immune responses
7.2.1. Immune function assays
7.2.2. Susceptibility to rumours
7.2.3. Susceptibility to infectious disease
7.2.4. Susceptibility to immunologically-mediated
disease
7.2.5. Conclusions
7.3. Ocular studies
7.3.1. Introduction
7.3.2. General effects
7.3.3. Caractogenesis
7.3.4. Retinal effects
8. HUMAN STUDIES: THE SKIN
8.1. Characteristics
8.1.1. Structure and optical properties
8.1.2. Skin types
8.2. Beneficial effects
8.2.1. Vitamin D3
8.2.2. Skin adaptation
8.2.3. Other benefits
8.3. Acute effects
8.3.1. Erythema and sunburn
8.3.2. Skin pigmentation and tanning
8.3.3. Photosensitization
8.4. Chronic effects on the skin other than cancer
8.5. Cancer
8.5.1. Nonmelanocytic skin cancer
8.5.2. Cutaneous melanoma
8.5.3. Cancer of the lip
8.5.4. Ocular cancers
8.5.5. Other cancers
8.5.6. Action spectrum
8.5.7. Dose response
8.5.8. Effects of pattern of exposure
8.5.9. Interactions between UV and other agents
8.5.10. Mechanisms of UV carcinogenesis
8.6. Conclusions
9. HUMAN STUDIES: IMMUNE FUNCTION
9.1. Immune function assays
9.2. Susceptibility to tumours, infectious and autoimmune
diseases
9.3. Conclusions
10. HUMAN STUDIES: THE EYE
10.1. Introduction
10.2. The eye
10.3. Study design
10.4. Diseases of the external eye
10.4.1. Photokeratitis and photoconjunctivitis
10.4.2. Climatic droplet keratopathy
10.4.3. Pinguecula
10.4.4. Pterygium
10.4.5. Hyperkeratosis, carcinoma-in-situ, and squamous
cell carcinoma of the conjunctiva
10.5. Diseases of the lens
10.5.1. Cataract
10.5.2. Exfoliation syndrome
10.5.3. Anterior lens capsule
10.6. Diseases of the choroid and retina
10.6.1. Uveal melanoma
10.6.2. Age-related macular degeneration
10.7. Conclusions
11. EFFECTS ON PLANT AND AQUATIC ECOSYSTEMS
11.1. Introduction
11.2. Effects on terrestrial plants
11.2.1. UV penetration into the leaf
11.2.2. Changes in growth
11.2.3. Effects on plant function
11.2.4. Species competition
11.2.5. Plant diseases
11.2.6. UV-protection systems
11.3. Effects on aquatic ecosystems
11.3.1. Effects on phytoplankton
11.3.2. UV increase and primary biomass production
11.4. Conclusions
12. HEALTH HAZARD ASSESSMENT
12.1. Introduction
12.2. Elective exposures
12.2.1. Medical exposures
12.2.2. Phototherapy of seasonal effective disorder
(SED)
12.2.3. Sunbeds
12.2.4. Sunbathing
12.3. Adventitious exposures
12.3.1. Outdoor exposures
12.3.2. Artificial sources
13. INTERNATIONAL GUIDELINES ON EXPOSURE STANDARDS
14. PROTECTIVE MEASURES
14.1. Introduction
14.2. Education
14.3. Protection factors
14.4. Clothing
14.5. Sunscreens
14.6. Tanning devices
14.7. Occupational protection
14.8. Protection in medicine and dentistry
14.9. Nutrition
14.10. Additional protective agents
14.11. Eye protection
15. FUTURE RESEARCH
15.1. Introduction
15.2. INTERSUN
15.3. Solar and personal UV monitoring
15.3.1. Solar monitoring
15.3.2. Personal monitoring
15.4. Terrestrial plants
15.5. Aquatic ecosystems
15.6. Human health
15.6.1. Skin
15.6.2. Immune system
15.6.3. Eye
15.7. Laboratory studies
15.8. Education
15.9. Administration
16. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES
16.1. United Nations Environment Programme
16.1.1. Ozone
16.1.2. Human health
16.2. International Agency for Research on Cancer
16.3. World Health Organization
16.4. International Commission on Non-Ionising
Radiation Protection
REFERENCES
PREFACE
The World Health Organization (WHO), in collaboration with the
United Nations Environment Programme (UNEP) and the International
Non-Ionizing Radiation Committee (INIRC) of the International
Radiation Protection Association (IRPA), published the first
Environmental Health Criteria (EHC) monograph on Ultraviolet
Radiation in 1979. At the United Nations Conference on the
Environment and Development (UNCED) in 1992 it was declared under
Agenda 21 that there should be activities on the effects of
ultraviolet radiation. Specifically:
"(i) Undertake, as a matter of urgency, research on the effects on
human health of the increasing ultraviolet radiation reaching the
earth's surface as a consequence of depletion of the stratospheric
ozone layer;
(ii) On the basis of the outcome of this research, consider taking
appropriate remedial measures to mitigate the above-mentioned
effects on human beings".
Within the United Nations mandate, and that of the 1993 WHO
Global Strategy for Health and Environment, this monograph has been
drafted to provide the essential authoritative review on which
future research programmes in UV can progress.
IRPA initiated activities in NIR by forming a Working Group on
Non-Ionizing Radiation in 1974. This Working Group later became the
INIRC at the IRPA meeting held in Paris in 1977. In May 1992 the
INIRC was chartered as an independent scientific commission called
the International Commission on Non-Ionizing Radiation Protection
(ICNIRP). ICNIRP continues the work of the IRPA/INIRC by reviewing
the scientific literature on NIR and making assessments of health
risks of human exposure to such radiation. Using the Environmental
Health Criteria monographs, developed in conjunction with WHO,
ICNIRP recommends guidelines on exposure limits, drafts codes of
practice, and works in conjunction with other international
organizations to promote safety and standardization in the
non-ionizing radiation fields.
A UNEP/WHO/ICNIRP Task Group to review the final draft of the
updated Environmental Health Criteria on Ultraviolet (UV) Radiation
met in Geneva from 8-11 December, 1993. Dr W. Kreisel, Executive
Director, World Health Organization opened the meeting on behalf of
WHO. Dr H. Gopalan and Mr R. Matthes welcomed the participants on
behalf of UNEP and ICNIRP respectively.
The first revision of this publication was compiled by members
of the IRPA/INIRC and more recently by members of ICNIRP. Chapters
were prepared by Drs B. Armstrong, J-P. Cesarini, L. Court, P.
Dolan, G. Johnson, A. Kricker, A. McKinlay, M. Repacholi, M.
Selgrade, D. Sliney and R. Tyrrell. These chapters together with the
report of an informal consultation held in Geneva, in August 1993 on
"The Effects of Solar UV Radiation on the Eye", the International
Agency for Research on Cancer (IARC) report of the meeting on
"Health, Solar UV Radiation and Environmental Change " held in Lyon
in October 1992, and the meeting on Immunotoxicology held in
Bilthoven in July 1993, were used by Drs M. Repacholi and D. Sliney
to compile the review draft in September 1993. Comments were
received from a broad cross-section of specialists in the UV fields
and their reviews were gratefully received. These comments were then
incorporated for Task Group review. Final scientific editing of the
text was completed by Drs J-P. Cesarini, A. McKinlay, M. Repacholi
and D. Sliney. Sincere thanks to Christine Cornish and Nancy Smith
for their assistance in the preparation of this text. An editorial
group consisting of Drs M. Repacholi, H. Gopalan and T. Kjellström
coordinated the preparation of this monograph.
This monograph comprises a review of the data on the effects of
exposure on biological systems pertinent to the evaluation of human
health risks. Its purpose is to give an overview of the known
biological effects of UV, identify gaps in knowledge and provide
direction for further research. This monograph will assist health
authorities, regulatory and similar agencies to provide guidance on
health risks from exposure to UV and limits for occupational and
general public exposure.
Earlier reports were not necessarily included, as they were
reviewed in the 1979 monograph. Every effort has been made to
distinguish clearly between established biological effects and those
that have been reported as preliminary or isolated results, or as
hypotheses proposed to explain observed results. The conclusions are
based on established knowledge of interactions of UV with biological
systems.
Subjects reviewed include: the physical characteristics of UV;
measurement techniques; applications and sources of exposure;
mechanisms of interaction; biological effects; guidance on
protective measures; and recommendations on exposure limits.
This monograph will also serve as a scientific database for the
planned WHO/UNEP/IARC/ICNIRP International Research Programme on
Health, Solar UV Radiation and Environmental Change (INTERSUN). The
general objectives of INTERSUN are to:
(i) evaluate the quantitative relationship between solar UV at the
surface of the earth and human health effects, develop reliable
predictions of the health consequences of changes in UV, provide
baseline estimates of the incidence of health effects of UV in
representative populations around the world, and develop practical
ways of monitoring change in these effects over time in relation to
environmental and behavioural change;
(ii) provide essential input into the development of environmental
and public health policies and actions in relation to depletion of
stratospheric ozone; and
(iii) provide a framework for monitoring the effects of solar UV and
the impact of prevention programmes.
Health agencies and regulatory authorities are encouraged to
set up and develop programmes that ensure effective protection
against the health effects of UV. It is hoped that this criteria
monograph will provide useful information for such endeavours.
TASK GROUP MEETING ON THE REVISION OF ENVIRONMENTAL HEALTH CRITERIA
ON UV RADIATION
Members
Dr J.P. Cesarini LRTPH, Fondation Rothschild, Paris, France
Dr L. Court Centre de Recherches du Service de Santé
des Armées, La Tronche Cédex, France
Dr A. McKinlay National Radiological Protection Board,
Didcot, United Kingdom
Dr D. Sliney US Army Environmental Hygiene Agency,
Aberdeen Proving Ground, MD, USA
Dr M.J. Selgrade Immunotoxicology Branch, USEPA, Research
Triangle Park, NC, USA
Dr G. Johnson Institute of Ophthalmology, University of
London, London, United Kingdom
Dr B. Armstrong International Agency for Research on
Cancer, Lyon, France
Dr H. Van Loveren National Institute of Public Health &
Environmental Protection, Bilthoven,
Holland
Dr A. Koppikar Office of Health & Environmental
Assessment, USEPA, Washington DC, USA
Dr R.M. Tyrrell Swiss Institute for Cancer Research,
Epalinges/Lausanne, Switzerland
Dr J.C. van der Leun Academisch Ziekenhuis Utrecht, CX Utrecht,
Holland
Dr I.A. Badr King Khaled Eye Specialist Hospital, King
Saud University, Riyadh, Saudi Arabia
Dr G. Kulandaivelu School of Biological Sciences, Madurai
Kamaraj University, Madurai, India
Dr E. Turowski Institute for Water, Soil and Air Hygiene,
Berlin, Germany
Observers
Dr B. Weatherhead Surface Radiation Research Branch, Boulder,
Colorado, USA
Dr H. Gopalan United Nations Environment Programme
(UNEP), Nairobi, Kenya
Dr A. Kulmula World Meteorological Organization (WMO),
Geneva, Switzerland
Dr A. Kricker International Agency for Research on
Cancer, Lyon, France
Dr U. Feister Deutscher Wetterdienst, Potsdam, Germany
Dr W.H. Weihe President, International Society of
Biometeorology (ISB), Brannenburg, Germany
Dr R. Matthes Bundesamt fur Strahlenschutz, Ingolsädter,
Oberschleissheim, Germany
Dr C. Remé Professor of Ophthalmology,
Universitätsspit, Zurich, Switzerland
WHO Secretariat
Dr W. Kreisel Executive Director, Health and Environment
Mr G. Ozolins Manager, Prevention of
Environmental Pollution
Dr T. Kjellström Prevention of Environmental Pollution
Dr M. Repacholi Prevention of Environmental Pollution
(Secretary)
Dr B. Thylefors Manager, Programme for the Prevention of
Blindness
Dr P.-H. Lambert Chief, Microbiology and Immunology Support
Services
Dr A.-D. Négrel Programme for the Prevention of Blindness
Dr V. Koroltchouk Cancer and Palliative
Care
NOTE TO READERS OF THE CRITERIA MONOGRAPH
Every effort has been made to present information in the
criteria monographs as accurately as possible without unduly
delaying their publication. In the interest of all users of the
Environmental Health Criteria monographs, readers are kindly
requested to communicate any errors that may have occurred to the
Director of the International Programme on Chemical Safety, World
Health Organization, Geneva, Switzerland, in order that they may be
included in corrigenda, which will appear in subsequent volumes.
1. SUMMARY AND CONCLUSIONS
This monograph is concerned with the effects of ultraviolet
(UV) radiation exposure on human health and the environment. Such a
review of the scientific literature is considered timely in view of
the consequences of increased levels of UV at the surface of the
earth resulting from depletion of stratospheric ozone.
1.1 Physical Characteristics
Exposure to UV occurs from both natural and artificial sources.
The sun is the principal source of exposure for most people. Solar
UV undergoes significant absorption by the atmosphere. With
depletion of the stratospheric ozone people and the environment will
be exposed to higher intensities of UV. The consequences of this
added UV exposure are considered so serious that it was a major
topic for discussion at the World Environment Conference, held in
Rio de Janeiro in 1992. In Agenda 21, adopted by the Conference, it
was specifically recommended to "undertake, as a matter of urgency,
research on the effects on human health of the increasing
ultraviolet radiation reaching the earth's surface as the
consequence of depletion of the stratospheric ozone layer." It is
this issue that underscores the current need to better understand
the potential health and environmental risks of UV exposure.
UV is one of the non-ionizing radiations in the electromagnetic
spectrum and lies within the range of wavelengths 100 nm (which
corresponds to a photon energy of approximately 12 eV) to 400 nm.
The short wavelength limit of the UV region is often taken as the
boundary between the ionizing radiation spectrum (wavelengths < 100
nm) and the non-ionizing radiation spectrum. UV can be classified
into UVA (315 - 400 nm), UVB (280 - 315 nm) and UVC (100 - 280 nm)
regions, although other conventions for UVA, UVB and UVC wavelengths
bands are in use.
Most artificial sources of UV, except for lasers, emit a
spectral continuum of UV containing characteristic peaks, troughs
and lines. These sources include various lamps used in medicine,
industry, commerce, research and the home.
UV-induced biological effects depend on the wavelengths of the
radiation. It is necessary for a proper determination of hazard to
have spectral emission data. These consist of spectral irradiance (W
m-2 nm-1) measurements or calculations of emissions from the
source. The total irradiance (W m-2) is obtained by summing over
all wavelengths emitted. The biological or hazard weighted
irradiance (W m-2 effective) is determined by multiplying the
spectral irradiance at each wavelength by the biological or hazard
weighting factor (which quantifies the relative efficacy at each
wavelength for causing the effect) and summing over all wavelengths.
Such factors are obtained from action spectra.
1.2 Action Spectrum and Minimum Erythemal Dose
An action spectrum is a graph of the reciprocal of the radiant
exposure required to produce the given effect at each wavelength.
All the data in such curves are normalized to the datum at the most
efficacious wavelength(s). By summing the biologically effective
irradiance over the exposure period, the biologically effective
radiant exposure (J m-2 effective) can be calculated. For UV
induced erythema, the action spectrum adopted by the International
Commission on Non-Ionizing Radiation Protection (ICNIRP),
International Commission on Illumination (CIE), the International
Electrotechnical Commission (IEC) and various national bodies, is a
composite curve obtained by statistical analysis of many research
results on the minimum radiant exposure of UV at different
wavelengths necessary to just cause erythema.
The most commonly used quantity for describing the erythemal
potential of an exposure to UV is the number of minimum erythemal
doses (MEDs) represented by the exposure. An MED is the radiant
exposure of UV that produces a just noticeable erythema on a
previously unexposed skin. It corresponds to a radiant exposure of
monochromatic radiation at the maximum spectral efficacy for
erythema (around 300 nm) of approximately 150 to 2000 J m-2
effective, depending on skin type. In this text 200 - 300 J m-2
effective is used as the value of 1 MED for comparative safety
purposes for white skin.
1.3 Cellular and Molecular Studies
To produce any change, UV must be absorbed by the biomolecule.
This involves absorption of a single photon by the molecule and the
production of an excited state in which one electron of the
absorbing molecule is raised to a higher energy level. The primary
products caused by UV exposure are generally reactive species or
free radicals which form extremely quickly but which can produce
effects that can last for hours, days or even years. DNA is the most
critical target for damage by UVB and UVC radiations. While a
considerable amount of knowledge is available concerning the
interaction of UV with nucleic acids, controversy exists as to which
lesion constitutes the most important type of pre-mutagenic damage.
Cell death, chromosome changes, mutation and morphological
transformations are observed after UV exposure of procaryotic and
eucaryotic cells. Many different genes and several viruses
(including HIV) are activated by UV exposure. The genes activated by
UVB and UVC are different from those activated by UVA. Studies of
DNA repair defective disorders have clearly established a link
between UV induced DNA damage in skin and various types of cancer.
1.4 Animal Studies
Solar UV has been shown to produce cancers in domestic and food
animals. In experimental animals UV causes predominantly squamous
cell carcinomas. UVB is most effective at producing SCCs, although
they are produced by UVA but at much higher intensities, similar to
the levels needed for erythema and tanning. The effectiveness of UVC
is unknown except at one wavelength (254 nm). At this wavelength the
effectiveness is less than UVB.
Melanomas are much less common and only two animal models have
been found for induction of melanoma by UV alone. An initial action
spectrum determined for a type of hybrid fish indicates a peak in
the UVB range but also shows a high level of effectiveness in the
UVA. Basal cell carcinomas are rare in animals.
Exposure to suberythemal doses of UV have been shown to
exacerbate a variety of infections in rodent models. UV affects
infections both at the site of exposure and at distant sites. Recent
work indicates that systemic infections without skin involvement may
be affected. Enhanced susceptibility appears to result from T-helper
cell activity. The mechanisms associated with this suppression
appear to be the same as those identified with suppression to
contact and delayed type hypersensitivity responses. Suppression of
these immune responses appears to be mediated by release of soluble
mediators from UVB exposed skin which alters the antigen
presentation by Langerhans and other cells so that they fail to
activate TH 1 cells. The resulting immune suppression is antigen
specific, can occur regardless of whether or not antigen is applied
at the site of exposure, and is relatively long lasting. UV exposure
also prevents the development of protection immunity to a variety of
infections in mice and rats.
Many studies in experimental animals have demonstrated that UV
exposure can cause both acute and delayed effects such as cataract,
photokeratitis, damage to the corneal epithelium and various retinal
effects. Studies of photochemical retinal injury in aphakic monkeys
have shown that the retina is six times more vulnerable to
photochemical damage from UV than the visible wavelengths.
1.5 Health Effects on Humans
1.5.1 Skin
Acute effects on the skin consist of solar erythema, "sunburn",
which, if severe enough, may result in blistering and destruction of
the surface of the skin with secondary infection and systemic
effects, similar to those resulting from a first or second degree
heat burn. Although UVC is very efficiently absorbed by nucleic
acids, the overlying dead layers of skin absorb the radiation to
such a degree that there is only mild erythema and, usually, no late
sequelae, even after repeated exposures. Much less is known about
the biological effects of UVA. However, doses of UVA, which alone
may not show any biological effect, can, in the presence of certain
environmental, consumer and medicinal chemical agents, result in
injury to tissues (phototoxicity, photoallergy, enhancement of
photocarcinogenesis).
Chronic skin changes due to UV consist of skin cancer (both
melanoma and non-melanocytic), benign abnormalities of melanocytes
(freckles, melanocytic naevi and solar or senile lentigines), and a
range of other chronic injuries resulting from UV exposure to
keratinocytes, blood vessels and fibrous tissue, often described as
"photoaging" (solar elastosis). The much increased rates of skin
cancer in patients with xeroderma pigmentosum, who have a deficiency
in the capacity to repair UV-induced DNA damage, suggest that direct
UV damage of the DNA may be a step in the cause of these cancers.
This suggestion has also been supported by the observation of UV
specific mutations of the p53 tumour suppressor gene in a proportion
of patients with non-melanocytic skin cancer. Oxidative and immune
suppressant effects may also contribute to the capacity of UV to
cause skin cancers.
Cancer of the lip is much more common in fair than dark skin
populations and is associated with outdoor work. However possible
confounding with tobacco and alcohol use has not been adequately
controlled in any study.
The worldwide incidence of malignant melanoma has continued to
increase. Strong epidemiological evidence exists that sun exposure
causes cutaneous melanoma and non-melanocytic skin cancer. Their
incidence is less in darker than light skin groups living in the
same geographical area. Risk of skin cancer decreases with
increasing pigmentation. The anatomical site most seen for squamous
cell carcinoma (SCC) is the head and neck, areas most exposed to the
sun. Incidence of both melanoma and non-melanocytic skin cancer are
increased in areas of high ambient solar UV radiation. Melanoma is
strongly related to frequency of recreational exposure to the sun
and to history of sunburns.
There is suggestive evidence that exposure to sunlamps may
increase the risk of melanoma, but the studies conducted so far have
not consistently controlled confounding factors.
1.5.2 Immune system
A number of studies suggest that UV exposures at environmental
levels suppress immune responses in both rodents and man. In rodents
this immune suppression results in enhanced susceptibility to
certain infectious diseases with skin involvement and some systemic
infections. Mechanisms associated with UV-induced immunosuppression
and host defence mechanisms which provide for protection against
infectious agents, are similar in rodents and man. It is therefore
reasonable to assume that exposure to UV may enhance the risk of
infection and decrease the effectiveness of vaccines in humans.
However additional research is necessary to substantiate this.
1.5.3 Eye
The acute effects of UV on the eyes consist of the development
of photokeratitis and photoconjunctivitis, which are unpleasant but
usually reversible and easily prevented by appropriate eyewear.
Chronic effects on the eye consist of the development of pterygium
and squamous cell cancer of the conjunctiva and cataracts. A review
of the studies suggests that there is sufficient evidence to link
acute ocular exposure to photokeratitis but our knowledge of the
effects of chronic exposure is less certain. While there is
sufficient evidence that cortical and posterior subcapsular
cataracts (PSC) can be caused by UVB in laboratory animals, there is
limited evidence to link cortical and PSC cataracts in humans to
chronic ocular exposure to UVB. Insufficient information is
available to separate out the other factors contributing to cataract
formation, or to state the proportion of cataracts which can be
attributed to UVB exposure. There is also limited evidence to link
the development of climatic droplet keratopathy and pterygium, but
insufficient evidence to link uveal melanoma with UV exposure.
1.6 Environment
Increased levels of UV due to ozone layer depletion may have
serious consequences for living organisms. A 10% reduction in ozone
could lead to as much as a 15-20% increase in UV exposure depending
on the biological process being considered. While the impact on
human health, crop production, fisheries etc. is largely unknown,
adverse effects of increased exposure to UVB have been reported on
plant growth, photosynthesis and disease resistance. Further, the
impact of increased UV levels on aquatic ecosystems (the major
contributor to the earth's biomass) may be substantial.
Phytoplankton, at the base of the aquatic food chain, serves as food
for larvae of fish and shrimp. These in turn are consumed by fish,
which subsequently provide an essential food source for many human
beings and other animals. A significant reduction in phytoplankton
from increased UVB exposure will directly affect the human and
animal marine food source.
1.7 Guidelines on Exposure Limits and Protective Measures
Guidance on exposure limits for UV are described in chapter 13.
International guidelines define exposure limits (ELs) below which it
is expected that nearly all people may be repeatedly exposed without
adverse effects. The ELs are intended to be used to evaluate
potentially hazardous exposures from, for example, solar radiation,
arcs, gas and vapour discharges, fluorescent lamps and incandescent
sources. The ELs are generally below levels which are often used for
the UV exposure of patients required as part of medical treatment
and below levels associated with sunbed exposure. ELs are not
intended to apply to exposure of pathologically photosensitive
individuals, to people concomitantly exposed to photosensitising
agents or to neonates.
Finally this monograph describes protection and control
measures such as the containment of UV sources, and methods for
personal protection including the use of sunscreen preparations,
clothing, eye and skin protection, and behavioural modifications.
While topical application of sunscreen is a preferred method of
absorbing UVB, some preparations do not absorb the longer wavelength
UVA effectively. Moreover, some have been found to contain
ingredients that are mutagenic in sunlight. There is still much
research necessary before the impact on health of increased levels
of UVA will be known. In the meantime people using sunscreens should
use only those with the highest sun protection factor (SPF) and be
aware that they are for their protection from the sun and not for
tanning purposes. Use of wide brimmed hats, protective clothing and
UV absorbing eye glasses is still the best personal protection
against the adverse effects of UV exposure.
With increasing levels of solar UV resulting from depletion of
the ozone layer, and the continuing rise in the level of melanoma
worldwide, people should become more aware of their UV exposure and
take appropriate precautions. These precautions include staying out
of the sun during the period around noon (the period when the UV
levels are highest), or wearing UV protective clothing, hats and sun
glasses. Broad spectrum (UVB and UVA protective) sunscreens should
be used when other means of protection are not feasible. These
sunscreens should be used to reduce exposure rather than lengthen
the period of exposure to the sun. Protection of young children is
particularly important for the prevention of long-term consequences
of UV exposure. In general behavioural patterns must change to
protect against increasing solar UV levels.
2. PHYSICAL CHARACTERISTICS
2.1 Electromagnetic Spectrum
Oscillations of electromagnetic fields can cause energy to be
transported in the form of electromagnetic radiation. Examples of
this type of radiative transport of energy include radiofrequency
waves, light and x rays. Ultraviolet (UV) radiation is one form of
electromagnetic energy in the optical region of the electromagnetic
spectrum. All electromagnetic radiation is characterized by
frequency f and wavelength lambda. These two quantities are linked
through the relationship:
f = c/lamda
where c is the speed of light (3 108 m s-1). The energy of a
single photon is determined by the wavelength of the photon as
described by the relationship
(photon energy) = hf = hc/lamda where h = 1.24 eV nm
Non-ionizing radiation (NIR) is the term generally applied to
all forms of electromagnetic radiation whose primary mode of
interaction with matter is other than by producing ionization. NIR
refers to electromagnetic fields and radiation with wavelengths
exceeding 100 nm, which is equivalent to quantum (photon) energies
below 12.4 eV, the minimum energy needed to break the weakest
macromolecular bonds. The non-ionizing spectrum encompasses all
fields of radiation from UV to DC fields.
For purposes of health protection, the optical portion of NIR
can be subdivided into several wavelength ranges, as shown in table
2.1. The nomenclature was standardized by the International
Commission on Illumination (CIE) however, some scientists use a
modification of this system by shifting the 315 nm break point
between UVA and UVB to 320 nm; the reader should always check the
definition given in each publication. For the purposes of this
document, the CIE convention is followed.
Table 2.1 Optical Radiation Spectral Bands
Ultraviolet radiation 100 - 400 nm
UVA 315 - 400 nm
UVB 280 - 315 nm
UVC 100 - 280 nm
Visible radiation (light) 400 - 760 nm
Infrared radiation (IR) 760 - 106 nm = 1 mm
UV of wavelengths less than 180 nm has no direct biological
effect on humans since it is effectively absorbed in a few
centimetres of air. For this reason, the spectral region below 180
nm is frequently referred to as the vacuum ultraviolet region.
2.2 Radiometric Quantities and Units
Radiometric quantities are absolute physical quantities used to
describe the characteristics of a source or radiation field. For UV,
ten generic radiometric terms are summarized in table 2.2. Each of
these quantities can be defined for a certain wavelength or
frequency range, or can be integrated over the whole spectrum of a
given source. Since UV is normally absorbed over a surface, with
very limited penetration depth, the most common quantities used to
describe exposure dose and dose rate to UV are: radiant exposure
(incident energy divided by the receptor surface area) and
irradiance (incident power divided by the receptor surface area)
respectively. Radiant exitance is the power per area of the emitted
radiation at the emitting surface. The receptor surface is most
often considered as a flat plane. However, for some biological and
chemical purposes, the radiation incident on a cylindrical or
spherical surface may also be considered.
For the purpose of radiation protection, physical quantities
are needed to describe sources and fields of radiation as well as
the interaction of this optical radiation with matter. These are
described in IRPA/INIRC(1985).
2.3 UV Production
Sources of electromagnetic radiation can be categorized in
several different ways; for example they can be grouped according to
the type of material or the type of equipment that produces the
radiation. At the submicroscopic level, the manner in which the
radiation originates can be described in terms of nuclear,
electronic or molecular transitions between energy states or by the
acceleration of charged particles. Common sources of UV emission
involve energy transitions between electronic states of molecules in
materials.
2.3.1 Thermal emitters
When the temperature of a material increases, electrons in the
molecules are raised to higher energy states, a variety of energy
transitions take place and photons are emitted. The higher the
temperature the greater is the fraction of these photons at higher
energies. Matter at temperatures above 2500 K emits a significant
number of photons in the UV spectral range. The emission spectra of
such incandescent sources are characteristically smooth - so-called
"continuum", possibly with superimposed spectral emission lines.
Table 2.2 Radiometric terminology for UV:
Useful radiometric quantities and units
Quantity Symbol Defining equation Unit and abbreviation
Radiant energy Qe Qe = Integral Operator Phie dt joule (J) = 1 watt second
dQe
Radiant energy We We = joule per cubic metre
density dV (J m-3)
dQe
Radiant flux Phie , P Phie = watt (W)
(radiant power) dt
dPhie
Radiant exitance Me Me = watt per square metre
dA (W m-2)
= Integral Operator Le cosTheta dOmega
Irradiance or dPhie
Radiant Flux Ee Ee = watt per square metre
Density dA (W m-2)
dPhie
Radiant intensity Ie Ie = watt per steradian (W sr-1)
dOmega
d 2Phie
Radiance1 Le Le = watt per steradian per
dOmega dA cosTheta square metre (W sr-1 m-2)
Table 2.2 (contd).
Quantity Symbol Defining equation Unit and abbreviation
Radiant exposure He = dQe joule per square metre
(dose in He (J m-2)
photobiology) dA
P
Radiant efficiency2 ne ne = unitless
(of a source) Pi
Optical density3 De De = - log10(gammae) unitless
NOTE: All terms in this table are radiometric terms and should not be confused with photometric terms.
The symbol A represents surface area, Omega is solid angle, Theta is the incident (zenith) angle,
V represents volume, t is time. The units may also refer to narrow spectral bands in which the term is
preceded by the word spectral and the unit is then per wavelength interval and the symbol has a
subscript lamda. For example, spectral irradiance Egamma has units of W m-2 nm-1 or W cm-2 nm-1. While
the metre is the preferred unit of length, the centimetre is still commonly used for many of the above
terms and the nm or µm are most commonly used to express wavelength.
1. At the source L = dI and at a receptor L = dE
dA cosTheta dOmega cosTheta
2. Pi is electrical input power in watts
3. This formula applies only to situations where the radiation is not scattered, but only absorbed.
In this case gamma represents the fraction of transmitted energy.
The spectral emission of a heated material is governed by basic
physical laws. The Stefan-Boltzmann Law expresses the total
radiation emitted by a heated material as a function of its
temperature. The total radiant power emitted by a theoretically
perfect radiator, a so-called black body, is proportional to the
fourth power of its temperature (in kelvin, K). Any practical
thermal emitter in equilibrium emits less radiant power than its
equivalent black body at the same temperature and unlike a black
body the amount depends on the nature of its surface. The spectral
distribution of black body radiation is described by the Planck
Radiation Law. Wien's Displacement Law is illustrated in figure 2.1,
and describes mathematically the spectral distribution of emission
from a black body and the wavelength of the maximum emission shift
to shorter wavelengths as temperature increases. When a material is
heated so that incandescence occurs, it first appears red and, with
increasing temperature, progresses to white or blue.
2.3.2 Electrical gaseous discharges
Radiation may be produced when an electric current is passed
through a gas or vapour. Atoms may be ionized if sufficient energy
is transferred from a moving electron. Alternatively the moving
electron may not impart sufficient energy for ionization, but
instead may impart energy to raise the electrons of the gas to an
excited (higher) energy level. When they return to a lower energy
level, or their ground state, radiation of one or more
characteristic wavelengths is emitted. The wavelengths of emission
are determined by the type of gas or vapour present in the discharge
and appear as spectral emission lines. The width of the lines and
the amount of radiation in the interval between them depends on the
pressure of the discharge. At low pressure, fine lines with little
or no continuum are produced. As the pressure of the discharge is
raised the lines broaden and their relative magnitudes alter. The
magnitude of the continuum increases. The electrical gaseous
discharge is the basis of operation of many UV emitting lamps.
2.3.3 Stimulated emission
Radiation can be produced by the specific electronic transition
process of stimulated emission. This depends on the ability of the
radiating medium to undergo "population inversion", i.e., achieving
a condition where there are more atoms or molecules in a higher
energy excited state than in a lower one. Once population inversion
occurs an avalanche of photons can be generated by stimulated
emission. Initially, spontaneously emitted photons stimulate other
excited atoms to emit photons of the same energy in phase with one
another. This is the basis of operation of the laser.
2.4 Summary
UV is a part of the non-ionizing region of the electromagnetic
spectrum. Its position in the spectrum can be characterised by
wavelength and it is quantified using radiometric quantities and
units. Production of UV is possible by different mechanisms that are
all based on atomic or molecular excitation. Excitation in a medium
can be achieved by thermal, electrical or optical energy absorption.
Emission of radiation is caused either by de-excitation or by
stimulated synchronised processes.
3. UV SOURCES
3.1 The Sun
The sun is the main source of UV. The broad spectrum and
intensity of UV from the sun are due to the high temperature at its
surface and its size. The intensity of solar UV reaching the earth's
atmosphere would probably be lethal to most living organisms on the
earth's surface without the shielding afforded by the atmosphere.
Solar UV undergoes absorption and scattering as it passes through
the earth's atmosphere with absorption by molecular oxygen and ozone
being the most important processes. The ozone layer prevents almost
all UV of wavelengths lamda < 290 nm and a substantial fraction (in
excess of 90% of the total energy) from 290 - 315 nm from reaching
the earth's surface. Thus the terrestrial environment is exposed to
UV between 290 nm and 400 nm. The spectrum, both before passage
through the atmosphere and at sea level, is shown in figure 3.1
The solar radiation reaching the top of the earth's atmosphere
is affected by the solar output and the earth-sun distance.
Variations in solar output are much smaller than the variations
caused by atmospheric attenuation factors. The irradiance incident
on the top of the earth's atmosphere is summarised in table 3.1, but
will vary with the exact distance of the earth from the sun at a
particular time. The extreme values associated with this variation
are approximately ± 3.3% above and below the annual mean and occur
in January and July respectively.
Table 3.1 Spectral distribution of solar radiation prior to
attenuation by the earth's atmosphere (Frederick et al., 1989).
Wavelength band Irradiance (W m-2) Percent of total
UVC 6.4 0.5
UVB 21.1 1.5
UVA 85.7 6.3
Total UV 113.2 8.3
Visible and IR 1254 91.7
The atmosphere has a profound effect on the irradiance which
reaches the surface of the earth. In January (in the northern
hemisphere) or July (in the southern hemisphere) when the solar
elevation is low, direct UV travels a longer path through the
atmosphere and a large amount of scattering occurs. In addition,
much of the resultant scattered UV propagates downwards to the
earth's surface at angles to the horizontal that are larger than the
solar elevation, hence travelling a shorter and less absorptive
path. This results in large ratios of scattered to direct UV. During
the summer the ratio of diffuse to direct UV is smaller.
3.1.1 Factors affecting solar UV levels
The total solar UV reaching the earth's surface, termed global
UV can be divided into two components: direct and diffuse. Global UV
reaching a horizontal surface is the quantity most often measured.
For biological entities such as people and trees, UV hitting
cylindrical or spherical surfaces may be more important.
The amount and spectral distribution of solar UV irradiance
reaching the earth's surface depends on a number of factors,
including
(a) wavelength of the UV
(b) solar zenith angle, which depends on latitude, date of the year
and time of day
(c) solar source spectrum incident at the top of the atmosphere (d)
ozone column thickness and vertical distribution
(e) molecular absorption and scattering (including localized
gaseous pollutants)
(f) aerosol absorption and scattering (including anthropogenic
(g) absorption, scattering and reflection by clouds
(h) reflectance characteristics (albedo) of the ground
(i) shadowing by surrounding objects
(j) altitude above sea level
The presence of cloud cover, air pollution, haze, or even
scattered clouds, plays a significant role in attenuating UV. UVB
and UVA irradiances are reduced due to scattering by water droplets
and/or ice crystals in the clouds. Clouds can block a significant
portion of the UV which would have otherwise reached the surface.
Cloud cover and type are highly variable. The transmission of
UV radiation through clouds depends on cloud height, type and
optical density. The resultant effect on UV transmission is
difficult to assess particularly in the case of partial cloudiness.
The effect of cloudiness on the solar irradiation of a horizontal
plane can be approximated by
F = 1 - 0.056 C
where C is the total cloud index in tenths of sky covered from 0 to
10, 10 being complete sky cover. Thus for complete cloud cover, the
transmitted UV irradiance would decrease by 72% and for half cloud
cover by 44%. In extreme cases cloud cover can decrease UV
irradiance by over 90%. Estimates of the average reduction of UVB
due to clouds (relative to cloudless skies) based on satellite
measurements of backscattered solar UV are 30% at 60 degrees
latitude, 10% at 20 degrees latitude and 20% at the equator.
3.1.2 Ozone depletion effects
Over 90% of the total atmospheric ozone resides in the
stratosphere (the upper atmosphere). The total ozone column is
important for filtering solar UV. Only UVB is affected by changes in
the ozone column. UVC is almost completely absorbed by ozone and
oxygen in the atmosphere; even with severe ozone reduction UVC would
still be effectively absorbed by the remaining oxygen.
As stratospheric ozone levels decrease, the resulting higher
levels of solar UVB could increase the production of reactive OH
radical molecules, potentially increasing the chemical reactivity of
the troposphere. In polluted areas with sufficient concentrations of
oxides of nitrogen (NOx) (above 0.5 ppb by volume) and hydrocarbon
compounds this enhanced reactivity is calculated to result in
greater levels of tropospheric ozone and other potentially harmful
oxidized products, such as hydrogen peroxide and acids.
The amount of ozone in the free troposphere is increasing for
example over central europe and other parts of the globe. The effect
of these trends on UV radiation needs to be studied further (WMO,
1993). For high sun angles, tropospheric ozone is a more effective
absorber of UV radiation than stratospheric ozone because of the
increased path length of scattered radiation in the lower atmosphere
(Brühl & Crutzen, 1989).
The total ozone column is not uniform but varies with latitude
and time of year. At the same latitude, away from the equator and
tropics, the total ozone column tends to be greater in spring than
in autumn. Thus, even though the sun angles are the same on March 21
and September 21, the differences in total ozone column result in
more UVB in early autumn than in early spring. If cloud cover and
atmospheric pollution are not taken into account, in the tropics,
the relatively constant ozone column and the similar solar angles
throughout the year, result in little variation in solar UVB with
season.
Changes in total column ozone were observed by both ground
based and satellite based instruments from the WMO Global Ozone
Observing System. The measurements show pronounced ozone depletion
over Antarctica in months 9-12 (local early spring) (UNEP-WMO,
1989). Increases in biologically effective UVB were observed during
this same time period (NSF, 1993). Over representation of UVA in the
common broad band meter action spectrum, relative to most biological
action spectra, produces less pronounced relative and absolute
changes in assessed UVB. Therefore under cloud free conditions
common broad band meters are less sensitive in assessing the ground
level UVB effects of ozone depletion than detectors with a steeper
action spectrum response such as for DNA damage or for erythema.
3.1.3 Trends in UV
There have been no significant changes in ozone at the equator.
Total column ozone over the Northern mid-latitudes has decreased by
several percent over the past two decades. Efforts to detect changes
in UV over long time periods have failed due to changes in column
ozone.
Besides the problems indicated in the interpretation and
comparison of measurement data from different sites and sources,
there are also insufficient direct solar terrestrial UVB
measurements for constructing a global climatology or trend
assessment due to some of the following (Driscoll 1992):
(a) problems in establishing appropriate instrumentation: either
highly sophisticated and reliable spectral instruments or broad
band instruments which fairly represent the sensitivities of
different biological and chemical targets (with different
wavelength dependencies and sensitivity to different
orientations),
(b) difficulty of maintaining accurate field instrument
calibrations over many years, and
(c) practical limitations in establishing a global monitoring
network especially with the potential for disturbance from
locally polluted areas.
With insufficient spectral measurement data collected over long
periods, data from the more extensively used broad-band measurement
systems have been scrutinised. UV data from eight stations in the US
showed decreases in UVB between 0.5% and 1.1% per year during the
time period of 1974-1985 (Scotto et al., 1988). However, this
result does not agree with theoretical predictions and may be due to
problems in the long-term calibration of meters or local pollution
in mainly urban or semi-urban sites (Munkata, 1993; Smith & Ryan
1993). In Russia, a 12% decrease of UVB was observed in Moscow
between 1968 and 1983 with a concurrent 15% increase in turbidity
and a 13% increase in cloudiness. At the Jungfraujoch observing
station in the Swiss alps (3.6 km above sea level), increases of
0.7±0.2% per year in UVB were observed under clear sky conditions
between 1981 and 1989. A comparison of summer spectral data weighted
with the CIE erythemal action spectrum using the same
instrumentation at Lauder, New Zealand (45°S) and Neuherberg,
Germany (48°N) showed weighted UV irradiances were 1.6 times larger
in New Zealand due to decreased ozone column thickness (24% lower
than in Germany). Spectral filter measurements at 39o N between 1976
though 1990 showed large increases in monthly maximum values which
were not statistically significant (Correll et al., 1992).
3.1.4 Theoretical models
To overcome some of the deficiencies in interpreting the
results from solar UV measurements, theoretical models have been
used to predict UV levels. However, the theoretical determination of
the spectral distribution of global UV including the effects of
cloud cover and ground reflection is extremely complex. There are
three categories of models to predict UV transmission. Empirical and
semi-empirical models are useful for assessing daily or annual
erythemal doses as a function of latitude, solar zenith angle, cloud
cover and ground albedo. These models have been used for a long time
at a variety of locations. Two of the most common examples are the
models of Green et al. (1974) and Diffey (1977)
Two stream models simulate the radiative transfer through the
atmosphere. They have been well verified with field data from
Antarctica as well as mid-latitudes (e.g. Frederick et al., 1989).
Multistream models allow for the angular distribution of UV
transmission. This is useful for more detailed dosimetry (Stamnes
et al., 1988).
The maximum erythemally effective UVB irradiances, calculated
by the Diffey model, are shown in table 3.2 for the Northern
Hemisphere, as a function of latitude and time of year at sea level
(Driscoll 1992). There are strong seasonal and latitudinal
variations in UVB. Under cloudless skies, the UVB is more intense in
summer and at all times of year is greater at lower latitudes. The
solar elevation angle determines the length of the path of the sun's
rays as they penetrate the atmosphere. When the sun is low in the
sky, the path through the atmosphere is longer and the filtering
action of the air is therefore increased. When the sun is directly
overhead, the sun's rays have the shortest path through the
atmosphere. Approximately 50% of the daily UV is received during the
middle four hours around noon when the sun is high in the sky
(Sliney, 1987).
The number of minimum erythemal doses (MEDs) in a 3 hour
exposure period around noon for fair skin is shown as a function of
latitude and time of year in table 3.3 for the Northern Hemisphere.
For example, at 55°N (Newcastle), a calculated value of 6 MED would
be received for a sensitive skin type in a 3 hour exposure around
noon on a clear day in July (1 MED per half hour exposure).
Comparing the values in table 3.2 with the results of
calculations using the model developed by Frederick et al. (1989)
for clear sky conditions at noon, the latter are typically 50%
higher. This may be due to differences in the biological action
spectra used or in the spectral irradiance data calculated. The R-B
meter response characteristic used by Frederick et al. (1989) does
not follow the CIE reference erythema action spectrum used in the
model developed by Diffey (1977) at wavelengths greater than 300 nm.
3.1.5 UV monitoring
Recent public and scientific concern about ozone depletion and
increased UV have lead to the establishment of many UV monitoring
centres in the last few years. Five years ago less than fifty UV
monitoring stations were operating around the world. Today more than
250 monitoring centres are underway for a variety of reasons.
Governmental agencies, scientific institutions, universities and
private groups have begun to monitor UV. The World Meteorological
Organization (WMO) has established a global network called Global
Atmosphere Watch (GAB). It presently has eight observatory stations
that make continuous spectral and broad band UV measurements. The
Global Environment Facility is supporting the creation of 10-15
additional stations in developing countries. Various national and
multi-national agencies are also operating and establishing UV
monitoring networks.
Table 3.2 Calculated clear sky noontime erythemally effective UVB
irradiances (mW m-2 CIE erythemally weighted) on a horizontal
surface as a function of latitude (°) and time of year for the sea
level in the Northern Hemisphere using typical ozone values
(Driscoll, 1992).
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 229 251 260 229 210 207 214 233 278 248 226 221
5 212 232 242 235 214 213 220 241 258 227 207 206
10 203 220 229 248 226 225 232 255 244 215 198 175
15 155 199 201 227 221 226 234 240 223 198 155 135
20 127 183 191 215 234 239 247 227 212 184 127 111
25 95 134 165 185 203 227 219 200 194 143 101 84
30 79 110 157 176 192 215 208 190 186 117 85 56
35 42 80 114 146 160 188 182 166 148 93 50 31
40 28 61 93 139 152 178 172 158 121 73 33 21
45 16 33 66 105 126 158 153 128 93 42 21 13
50 12 22 54 85 119 150 145 104 77 28 15 8
55 6 14 33 65 92 130 114 81 48 18 8 4
60 4 10 21 53 75 105 93 67 31 12 5 2
65 0 6 14 35 59 85 76 42 21 7 2 0
70 0 3 10 23 48 70 62 29 15 4 0 0
75 0 0 6 15 30 53 39 20 9 0 0 0
80 0 0 3 11 20 33 26 14 5 0 0 0
85 0 0 2 7 14 23 18 9 2 0 0 0
90 0 0 0 4 10 16 13 5 0 0 0 0
Table 3.3 Number of MEDs in a 3 h exposure period for a sensitive
skin type (1 MED = 200 J m-2 effective) for the erythemally
effective UVB irradiances given in table 3.2 as a function of
latitude (°) and time of year for the Northern Hemisphere (Driscoll
1992).
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0 12 14 14 12 11 11 12 13 15 13 12 12
5 11 13 13 13 12 12 12 13 14 12 11 11
10 11 12 12 13 12 12 13 14 13 12 11 9
15 8 11 11 12 12 12 13 13 12 11 8 7
20 7 10 10 12 13 13 13 12 11 10 7 6
25 5 7 9 10 11 12 12 11 10 8 5 5
30 4 6 8 10 10 12 11 10 10 6 5 3
35 2 4 6 8 9 10 10 9 8 5 3 2
40 2 3 5 8 8 10 9 9 7 4 2 1
45 1 2 4 6 7 9 8 7 5 2 1 1
50 1 1 3 5 6 8 8 6 4 2 1 0
55 0 1 2 4 5 7 6 4 3 1 0 0
60 0 1 1 3 4 6 5 4 2 1 0 0
65 0 0 1 2 3 5 4 2 1 0 0 0
70 0 0 1 1 3 4 3 2 1 0 0 0
75 0 0 0 1 2 3 2 1 0 0 0 0
80 0 0 0 1 1 2 1 1 0 0 0 0
85 0 0 0 0 1 1 1 0 0 0 0 0
90 0 0 0 0 1 1 1 0 0 0 0 0
The reasons for monitoring UV are generally divided into the
following four areas:
(i) to provide information to the public on UV levels and
variations,
(ii) establishing a basic UV climatology,
(iii) studying cause and effects of UV transmission, and
(iv) detecting long term variability.
While these reasons are not mutually exclusive they can often
dictate the type of care taken with instrument maintenance. The
quality and care needed for trend detection requires a great deal
more effort than for UV measurements to provide public information.
There are many difficulties in making accurate field
measurements of ambient UV. Care must be taken that no part of the
instrument is shaded from either direct or indirect sun; nearby
surfaces must not change during the time period of monitoring; and
routine maintenance such as snow and debris removal must be
conducted systematically to avoid trend bias. The value of the
measurements for trend detection rely on diligent maintenance of the
site including co-located measurements. The data must have spectral,
temporal and angular resolution appropriate for validation of model
calculations and for evaluation of biological effects (spectral
resolution better than <1 nm). Broad band instruments are useful
for establishing climatology and operations where spectral
instruments are not practical.
Frequent and reliable calibration of the instrumentation is
necessary. UV monitoring for the detection of trends is difficult
and failure to detect a trend may be due to lack of awareness of
changes in instrument behaviour and correction of it.
Intercomparison of different types of UV monitoring instruments have
shown that individual instrument characteristics can cause
substantial differences in measurements. Further intercomparisons
are necessary to understand and reduce the uncertainties of
measurements.
3.1.6 Analysis of UV data
Atmospheric UV data are difficult to analyze. To date all UV
data have been analyzed separately by each investigator and in a
different manner. This variation in analysis makes the
interpretation and comparison of results difficult. Unfortunately,
the data are normally not easily obtained and re-analyzed for
comparative studies.
UV data pose difficult problems because they are not generally
independent and normally distributed, therefore robust time series
techniques should be used to analyze the data correctly. Standard
techniques, depending on how they are applied, can greatly over or
under estimate the confidence in the observed trends, as well as
supply spurious trends when used on UV data. Bishop (1992) has
suggested that using proper statistical techniques, at least ten
years of data will be necessary to correctly detect a trend of 5%
per decade. This estimate was made based on analysis of a single
station's data independently. However, analysing groups of data may
allow trend detection in less time.
3.1.7 Conclusions
The earth's atmosphere has a profound effect on the UV
irradiance reaching the earth's surface. It absorbs a large fraction
of the incident UV and changes part to diffuse radiation. Variations
in UV intensity depend on solar zenith angle, atmospheric ozone,
cloudiness, aerosol load and other factors.
One of the most important tools to determine climatological
values and long-term changes in solar UV radiation is the monitoring
of solar UV. Atmospheric parameters which modulate UV need to be
observed in parallel to help explain the changes observed in the
measurements. The existing data base is much too scarce to derive
climatological values and trends on global or regional scales.
Efforts are being made to establish national and international
networks for measurements of solar UV radiation.
Models that simulate typical values and predictions of changes
in UV radiation for different atmospheric conditions are also used.
The results from these models need to be compared with measurements.
3.2 Artificial Sources
Artificial sources of UV are commonplace. There are few
artificial sources that result in human exposure to UV greater than
that from the sun. However, exceptions are those used for medical
therapy and diagnosis, cosmetic tanning. Industrial sources are
generally effectively enclosed, but accidental exposure may occur.
There are very few non-laser sources of optical radiation that emit
UV solely.
Any unfiltered optical source, whose emissions are due to the
heating of a material e.g. a filament lamp, that emits significant
quantities of UV will also emit visible and infrared radiations. In
the case of high temperature tungsten halogen lamps biologically
significant amounts of shorter wavelength UVB are also emitted.
Essentially the same holds true for high intensity (gaseous)
discharge (HID) lamps. Some incandescent and HID lamps have
sufficient intrinsic filtration in the glass envelope of the lamp.
However, additional filtration, afforded by incorporation of the
lamp in a suitably filtered luminaire, may be necessary.
Most man-made sources of UV can be grouped together in the
categories shown below. The spectrum of the UV emitted varies from
one source to another
Incandescent sources
tungsten lamps
Gas discharges
mercury lamps (low-, medium- and high-pressure)
mercury lamps with metal halides
xenon lamps
hydrogen and deuterium lamps
flash tubes
Electric discharges
welding arcs
carbon arcs
Fluorescent lamps
fluorescent lighting tubes
fluorescent sunlamps (UVB emitters)
fluorescent UVA tubes
Lasers
excimer laser
dye laser
gas laser
3.2.1 Incandescent sources
When a material is heated a large number of energy transitions
occur within its molecules and optical photons are emitted. An
ideally efficient emitter (radiator) is termed a black body
radiator. The total radiant power and its spectral distribution
depend only on the temperature of the black body. The spectral
emissions in terms of spectral radiant exitance of a black body
radiator for different temperatures are illustrated in figure 3.2
and tabulated in table 3.4 (McKinlay et al., 1988). The wavelength
corresponding to the peak of the spectral emission of a black body
radiator varies inversely as its temperature and as the temperature
increases an increasing amount of UV is emitted. Incandescent
sources whose temperatures are greater than about 2900 K emit
significant amounts of UV with respect to possible effects on human
health. The optical emission of the sun corresponds approximately to
that of a black body radiator at a temperature of around 6000 K.
By integrating over all wavelengths the area under an absolute
plot of spectral radiant exitance against wavelength, the total
radiant exitance M can be determined by the formula:
M = sigmaT4
where sigma = Stefan-Boltzmann constant (5.67 x 10-3 W m-2 K-4)
and T = absolute temperature (K).
Table 3.4 Approximate radiant exitances of black body sources at
different temperatures
Temperature UVB/C UVA UVA
K 200-315 nm 315-340 nm 340-400 nm
[W m-2] [W m-2] [W m-2]
1500 8.6 x 10-5 5.6 x 10-4 2.7 x 10-2
2000 2.4 x 10-1 7.9 x 10-1 14.2
2500 29 62 634
3000 760 1147 8095
3500 8050 9220 5 x 104
4000 4.8 x 104 4.4 x 104 2 x 105
5000 6.3 x 105 4 x 105 1.4 x 106
6000 3.6 x 106 1.7 x 106 5 x 106
10000 1.4 x 108 3.2 x 107 6.9 x 107
In practice no material emits radiation with a black body
spectrum. However, tungsten at high temperatures (such as used for
the filaments of ordinary and tungsten halogen incandescent lamps)
and molten metals approximate to theoretical black bodies. Such
sources are termed grey bodies. The ratio of the actual spectral
radiant exitance to the theoretical black body spectral radiant
exitance is a measure of how closely the grey body approximates in
its spectral emission at a particular wavelength to a black body and
is termed the spectral emissivity (epsilonlamda). Similarly, by
summation over all wavelengths, the total radiant exitance M of a
material is given by
M = epsilon sigma T4
where epsilon is the emissivity.
The emissivity depends on the material and on its surface
structure, and is a function of wavelength. For example, for
tungsten at 3000 K (typical of a tungsten halogen lamp filament),
the emissivity varies from an average of about 0.45 in the UVA
region to about 0.2 in the IRB ( 1.4 - 3.0 µm) region.
The incandescent lamp is the oldest type of electric lamp still
in common use. The optical radiation from incandescent lamps results
from the heating of tungsten filaments which, with the exception of
lamps for some photographic and graphic arts applications, are not
heated to temperatures in excess of about 3000 K. Typical gas-filled
incandescent filament lamps operate between 2700 and 3000 K with
electrical input powers up to 500 W and the peaks of their spectral
emissions are in the infra-red (IRA) region. The emissions of UV by
tungsten filament lamps are generally negligible with respect to
human health.
In applications where greater power (up to 5 kW) is required,
tungsten-halogen (often called quartz-halogen) lamps are often used.
These are quartz envelope tungsten filament lamps to which a halogen
vapour (usually iodine) is added. The presence of the vapour enables
operation at an increased gas pressure compared with conventional
incandescent lamps, and evaporation from the filament is minimized.
These features result in improved luminous efficacy and longer life.
In order to operate efficiently, the temperature of the wall of the
bulb must be maintained at not less than 260 °C. The majority of the
bulbs of tungsten-halogen lamps are made from silica (quartz) whose
thermal properties are most suitable. The combination of filament
temperatures which are likely to be in the range 2900 to 3450 K and
quartz bulbs results in a significantly higher level of emission of
potentially harmful UV compared with ordinary tungsten filament
lamps. The incorporation of suitable secondary filtration to reduce
UV emissions to an acceptable level is an important feature of the
design of any illumination system using tungsten halogen lamps
(McKinlay et al., 1989).
3.2.2 Gaseous discharge sources
The electrical excitation of a gas or vapour is a typical
mechanism for generating optical radiation. An electric current is
passed through a gas or a mixture of gases ionized to produce
electrons and positive ions. Emissions are the result of electronic
transitions in the atoms of a material from low to high energy
states (absorption and excitation) followed by transitions from the
high to low energy states (de-excitation and emission). This process
is often combined with the process of luminescence, whereby the
characteristic (line) photon emissions of the gas are absorbed by a
luminescent material (luminophor or phosphor) which in turn emits
optical radiation, typically as a continuum over a range of longer
wavelengths. The 253.7 nm UV emission from a low pressure mercury
vapour discharge is used as a source of excitation in low pressure
fluorescent lamps. By raising the pressure of the discharge to a few
atmospheres the emission lines increasingly broaden effectively
forming a continuum. In some cases the 253.7 nm line emission will
be self-absorbed by the vapour of the discharge.
Low pressure discharge lamps
Germicidal lamps
The low pressure mercury-discharge lamp is often used for the
purpose of germicide and disinfection. Such lamps are very efficient
emitters of UV. Approximately 50% of the electrical power is
converted to UV of which up to 95% is emitted at a wavelength of
253.7 nm (see figure 3.3). Germicidal type lamps are available in a
range of sizes, shapes and powers. Small, low-wattage (5-10 W)
germicidal lamps are often used as fluorescence-inducing lamps for
the purposes of, for example, chromatographic analysis and the
fluorescence identification and authentication of documents. The
quartz envelopes of some lamps in this category transmit 185 nm
wavelength radiation characteristic of mercury vapour.
Fluorescent lamps
The most common application of the low-pressure discharge is
fluorescent lamps. These operate by means of a discharge between two
electrodes through a mixture of mercury vapour and a rare gas,
usually argon. Light is produced by conversion of 253.7 nm mercury
emission to longer wavelength radiations by means of a phosphor
coating on the inside of the glass envelope of the lamp. Lamps are
available with many different phosphors and envelopes to produce a
wide range of spectral emissions covering the visible (light), UVA
and UVB regions. While the continuum emissions of fluorescent lamps
are characteristic of the phosphors the narrow peak, spectral
emissions are dominated by the characteristic line emission spectrum
of the low-pressure mercury vapour discharge.
General lighting fluorescent lamps
These lamps are available in a range of physical sizes, powers
and phosphors. The range of phosphors includes a large selection of
"near white" and "special colour" lamps. In relation to other light
sources the fluorescent lamp is particularly efficient, with about
20% of the input energy resulting in useful light. Detailed spectral
analysis of the UV emissions of different general lighting
fluorescent lamps have shown that in general UVB and UVC emissions
are extremely low due to the marked attenuation of wavelengths <
320 nm afforded by the glass envelope.
Frequently in office and industrial environments where
fluorescent lamps are used, the luminaire assembly incorporates a
diffuser or controller. Three materials are commonly used in the
construction of diffusers; opal acrylic, opal styrene and opal
polycarbonate. Controllers are luminaire covers that are configured
with small prisms or lenses. Two commonly used materials are clear
acrylic and clear styrene. Some luminaires incorporate opal
(diffusing) sides and a clear figured [controller] base. The use of
diffusers and controllers results in the absorption and reflection
of the radiation emitted by the associated lamp. The UV-attenuating
properties of different diffusers is demonstrated by the measurement
data in table 3.5.
During the past few years the further development and improved
design of general lighting fluorescent lamps have been evident in
the production of compact fluorescent lamps. These lamps are
essentially low power small diameter fluorescent tubes folded in a
compact form. They are most readily available commercially with cool
white phosphors but other phosphors are also available. For a given
illuminance their spectral emissions of UV are essentially no
different from those of full size tubular fluorescent lamps as shown
in table 3.6.
Table 3.5 Measurements of UV irradiance from various diffusers/controllers
with a white fluorescent lamp as the source. Irradiance totals in each
waveband in mW m-2 (percentage in parentheses), (McKinlay et al., 1988).
Diffuser Type UVA UVB UVACGIH**
mW m-2 mW m-2 [mW m-2]effective
Bare lamp 22.32 (100d) 3.45 (100) 59 x 10-3 (100)
Clear acrylic+ 16.35 (73) 2.91 (84) 48 x 10-3 (81)
Clear styrene+ 2.87 (13) 0 (0) 0 (0)
Opal styrene* 0.92 (4) 3 x 10-3 (< 0.1) 0.02 x 10-3 (<0.1)
Opal polycarbonate* 0.20 (< 1) 12 x 10-3 (< 1) 0.09 x 10-3 (<1)
+ Surface figured with small prisms
* Reeded surface
** ACGIH occupational hazard weighted irradiance: 1 maximum permissible
exposure for an 8 h working day is equivalent to 10-3 [W m-2]effective ..
Table 3.6 Measurements of UV from compact fluorescent lamps,
normalised to an illuminance of 500 lux (Whillock et al., 1990)
Lamp type UVA UVB
mW m-2 µW m-2
Luma (7 W) LC7 47 x 103 0
Luma (7 W) LC7 with diffuser 197 0
Osram (11 W) Dulux EL 38 x 103 0.1
Philips (9 W) SL9 37 x 103 0
Sylvania (13 W) Lynx CFD 43 x 103 30.81
Thorn (16 W) 2D 54 x 103 2.48
Tungsram (16 W) Globulux 663 0
"Special" applications fluorescent lamps
Apart from a number of colour-rendering fluorescent lamps,
which are essentially variations of general lighting fluorescent
lamps, a number of special applications fluorescent lamps have been
developed.
A common example of a UVB emitting fluorescent lamp is the FS
type lamp with spectral emission shown in figure 3.4. Such lamps
were previously used for cosmetic tanning and are now often used as
a source of UVB in biological experiments. The blacklight lamp uses
a nickel/cobalt-oxide (Woods glass) envelope that is almost entirely
opaque to light. The phosphor chosen for this type of lamp emits
around 370 nm in the UVA. Such lamps are used for a number of
commercial, scientific and industrial fluorescence purposes as well
as for display and entertainment. Three types of fluorescent lamps
that emit UVA in printing and copying merit mention viz; lamps
suitable for diazo printing with a principal emission around 360 nm;
those intended for modern fast diazo printing with a main emission
around 420 nm; and those used for photocopying with predominantly
green emissions.
A medical treatment for hyperbilirubinaemia in neonates
(neonatal jaundice) consists of irradiating the newborn child with
phototherapy lamps emitting in the wavelength range approximately
400 to 470 nm. Some lamps used for this purpose have emission
spectra that extend into the UV region. Fluorescent lamps in
printing and copying include lamps for diazo printing with a
principal emission around 360 nm.
The development of a range of phosphors with enhanced UVA
emissions has led to the widespread use of fluorescent lamps in
sunbeds, solaria and PUVA (Psoralen + UVA) treatment cabinets.
High pressure discharge lamps
The designation "high pressure discharge" lamps is taken in
this monograph to include the families of lamps often called high
intensity discharge (HID), such as mercury vapour or metal halide
lamps, where an electric arc is not created. At still higher
pressures, arcs may be produced, e.g., the xenon or mercury compact
(short-arc) lamps.
Mercury and metal halide lamps
High-pressure mercury vapour lamps are widely used for lighting
in commerce, streets, displays, floodlighting and a large number of
printing, curing and other industrial applications. The spectral
emissions of the discharge are in the blue, green and yellow regions
of the spectrum and a large amount of UV is also generated. The
general construction of high-pressure mercury lamps is a fused
silica (quartz) discharge tube containing the mercury/argon vapour
discharge mounted inside an outer envelope of soda-lime or
borosilicate glass.
The outer glass envelope effectively absorbs most residual UV.
Consequently the quantity of potentially harmful UV emitted by such
lamps depends critically on the integrity of this envelope. In the
USA, but apparently not in Europe, it is a legally enforceable
manufacturing requirement that breakage of the outer envelope must
either cause the lamp to fail to operate, in which case the lamps
are described as "self-extinguishing" and are marked with the letter
"T", or if not "self-extinguishing" they should be marked with the
letter "R". In the latter case a warning notice must be included
with the packaging of the lamp (FDA., 1988). Data from measurements
made at 2 m from a mercury HID lamp with the outer envelope removed
illustrate the importance of this aspect of safety design, as shown
in table 3.7.
Table 3.7 UV emissions from HID mercury vapour general lighting (USA) lamps; effective irradiance
in mW m-2effective, (ACGIH), (Piltingsrud et al., 1978).
Test condition A B C D
Lamp type With outer Without With outer Without With outer Without With outer Without
bulb outer bulb bulb outer bulb bulb outer bulb bulb outer bulb
General Electric 2.5 110* < 0.1 160 12.5 3640 0.2 0.2
H400 A33-1
Clear
Westinghouse < 0.1 1040 < 0.1 190 0.2 3680 < 0.1 0.3
H33 GL 400/DX
White
General Electric < 0.2 75* < 0.1 180 0.5 3900* < 0.1 < 0.1*
H400D x 33-1
White
General Electric < 0.3 122* < 0.1 6* 1 510* < 0.1 < 0.1*
MV400/BUH
[Metal halide]
Test Conditions:
A: Lamp mounted vertically - measurements at 2 m on mid-line axis of lamp.
B: Lamp mounted horizontally - measurements at 2 m on central axis of lamp.
C: Lamp mounted horizontally in reflector shield with no face plate - measurements at 2 m on central axis of lamp.
D: Lamp mounted horizontally in reflector shield with glass face fitted - measurements
at 2 m on central axis of lamp.
* Lamps did not operate at normal intensity.
The family of metal halide lamps encompasses a number of
different types of high pressure mercury lamps whose discharges all
contain additives. The additives are most typically metal halides
chosen to produce either a strongly coloured emission (usually a
single halide), to produce a more broadly spectrally uniform
emission (multi-halide) or to enhance the UV (most often UVA)
emission. Compared with ordinary high pressure mercury lamps the
luminous efficacies of metal halide lamps are high. They are used
for a range of industrial and commercial applications that include
photochemical processing, graphic and photographic illumination,
studio lighting, reprography and are also used for UVA cosmetic
tanning equipment, for some medical applications and for solar
radiation simulation. The UV irradiances of some metal halide lamps
used in filtered industrial applications requiring an activating
range of wavelengths between 320 and 440 nm, e.g. lithographic
platemaking and printed circuit photo-resist etching, are shown in
table 3.8 (McKinlay et al., 1988).
Table 3.8 UV irradiances measured at 1 m from typical graphics arts
metal halide mercury lamps (McKinlay et al., 1988)
Lamp type Power UVC UVB UVA
[W] [W m-2] [W m-2] [W m-2]
HPA 400 400 0.5 3.2 9.0
HPA 1000 930 2.3 9.0 23.0
HPA 2000 1750 4.5 19.0 48.0
The emission of a device incorporating iron additive halide
lamps for phototherapy is illustrated in figure 3.5. The importance
of incorporating filtration in such devices in order to remove
unwanted components of the UV spectrum is illustrated.
Xenon, compact and linear arcs
Where an optical source of very high radiance is required and
of small size a very high pressure arc lamp may be used. These have
a filling gas of mercury vapour, mercury vapour plus xenon gas or
xenon gas. Metal halide types are also available. Two physical types
are commonly used; the compact (short) arc and the linear arc. The
spectral emission of xenon lamps, which at wavelengths shorter than
infra-red, closely matches that of a black-body radiator at about
6000 K. This enables their use in photography and as solar radiation
simulators.
The spectral emission of xenon lamps, which at wavelengths
shorter than infrared closely matches that of a black body radiator
at about 6000 K, enables their use as solar radiation simulators.
Their emission spectrum is continuous from the UV through to the IR
regions. Large amounts of UVA, UVB and UVC are emitted by unfiltered
lamps to the extent that they can present a significant health
hazard if incorrectly used. The luminance of compact xenon arcs may
approach that of the sun and in some lamps with greater than 10 kW
rating the luminance may exceed that of the sun. They therefore
present a potentially severe retinal hazard if viewed.
3.2.3 Gas welding
Oil, coal and gas flames normally operate at temperatures below
about 2000 K and consequently emit virtually no UV. Oxyacetylene and
oxyhydrogen flames burn at much higher temperatures and emit UV
mostly in the UVA region.
3.2.4 Arc welding
By comparison with gas flame processes, the emissions of UV
from arc welding are very high (see figure 3.6) and many data on the
optical radiation emissions associated with a variety of electric
arc welding processes have been published (e.g., Sliney & Wolbarsht,
1980).
3.2.5 Lasers
All lasers have three basic components (see figure 3.7): (a) a
laser (active) medium; (b) an energy source (pumping system) and;
(c) a resonant optical cavity. A pumping system is necessary to
provide energy to electrons to raise them to excited states and
achieve population inversion. Optical pumping, using an intense
source of light such as a xenon flashtube; electron-collision
pumping, using an electrical discharge and; chemical pumping, using
the energy released from making and breaking chemical bonds, are all
used for this purpose. A resonant optical cavity is formed by
mirrors placed at each end of the laser medium. The construction is
such that the beam passes through the laser medium several times and
the number of emitted photons is amplified during each transit. One
of the mirrors is chosen to be partially transmitting thus enabling
part of the beam to be emitted from the cavity (figure 3.7).
Examples of lasers that emit UV are presented in table 3.9.
Table 3.9 UV laser emissions and characteristics,
(McKinlay, 1992)
Type Name Spectral emissions
Excimer Argon fluoride [(ArF) 193 nm
Krypton fluoride (KrF) 248 nm
Xenon chloride (XeCl) 308 nm
Xenon bromide (XeBr) 282 nm
Xenon fluoride (XeF) 351 nm
Krypton chloride (KrCl) 222 nm
Dye Excimer-, Nitrogen-, 300 nm-
Flash lamp pumped 340 nm-
Gas Nitrogen (N2) 337 nm
Helium cadmium (He-Cd) 325 nm
Gas ion Argon (Ar+0 333 nm-
357 nm
363 nm
Krypton (Kr+) 337.5 nm
350.7 nm
356.4 nm
A recent generation of UV lasers are the excimer lasers. The
active media used are rare gas halides such as ArF, KrCl, XeBr or
XeF. The molecules of those gases have only a short lifetime in the
ground state, however they are very stable in the excited state. As
a buffer medium, helium is often used. Population inversion is
easily achievable because the ground state is very unstable.
3.2.6 Sunbeds
Sunbeds are broadly used for cosmetic tanning purposes. The
expression "sunbed" includes tanning equipment consisting of a UV
emitting lamp or a number of such lamps incorporated in a bed,
canopy or panel, or any combination thereof. There are four distinct
types of lamps in use, each with different UV emission
characteristics. Those are UVA, low-pressure fluorescent tubes; UVA,
filtered high-intensity discharge lamps; UVB, low-pressure
fluorescent tubes; UVB, filtered high-intensity discharge lamps.
The emission characteristics and the health risks associated
with the use of each type of lamp are different. The last two lamp
types are associated with high levels of UVB and are now little
used. They have been almost universally replaced by the
predominantly UVA emitting lamp types.
4. HUMAN EXPOSURE
4.1 Sunlight
Outdoors, exposure to UV constantly changes during the day.
People are largely unaware of the degree of these changes. At noon,
when the sun is overhead, the level of UV at a wavelength of 300 nm
is ten times greater than at either three hours before (9 am) or
three hours after noon (3 pm). An untanned person with fair skin may
receive a mild sunburn in as little as 25 minutes at noon (depending
on the time of year and the latitude) but would have to lie in the
sun for at least two hours to receive the same dose after 3 pm. The
global biologically effective UV falling on a horizontal surface
occurs primarily during the midday hours, about 50% during the four
hours centred on noon-time zenith (Sliney, 1987).
Scattering of sunlight by air molecules (due to Raleigh
scattering) favours UV and blue light (hence the blue sky). For
longer pathlengths through the atmosphere when the sun is low in the
sky, more UV and sunlight is scattered. The sun which is white at
noonday becomes yellow and then orange as less UV and blue light are
present in the direct rays. When the sun is overhead staring at the
sun for 90 seconds would cause solar retinitis. A few hours later,
with the sun much lower in the sky, it would take many minutes to
reach a hazardous retinal dose, and it is virtually impossible to
cause any eye damage at sunset. Thus, the geometry of exposure as
well as the spectrum (hue) plays a major role in determining the
hazards from direct exposure from the sun (Sliney, 1983, 1986).
Estimation of an individual's lifetime UV exposure requires
knowledge of the ambient solar UV levels, history of outdoor
exposure and the relative exposures at the different anatomical
sites. Studies on the anatomical distribution of solar UV have been
reported (Diffey et al 1979; Rosenthal et al., 1985; Holman et
al., 1983; Gies et al 1992a; Roy et al., 1988). The relative
doses at various body sites have been determined using UV sensitive
polysulphone film on rotating manikins and headforms. It was found
that even though the relative doses to the face and eyes are higher
in winter, due to the lower solar elevation, the absolute doses are
higher in summer. The presence of a brimmed hat reduced the face
exposure by a factor of at least two and the eye exposure was
reduced by a factor of 4 to 5 (Diffey et al., 1979, Roy et al
1988).
Polysulphone film badges have also been used to quantify the
solar UV exposure received by different subjects and results
compared to those calculated from personal diaries and measured
ambient solar UVB (Gies et al., 1992a). In general, when UV
exposure activities took place under close supervision, good
correlations were obtained between the polysulphone badges and the
ambient/diaries approach. Results from a recent study (Roy and Gies,
1993), of indoor, outdoor and retired workers indicated that
exposures to badge locations of up to 30% of ambient were recorded.
Through studies of this type, knowledge is gained on the amount and
pattern of exposure from routine activities and this can then be
applied in the design of educational campaigns to modify outdoor
behaviour and reduce UV exposure. The use of polysulphone or CR39
plastics (Wong et al., 1989; Sydenham et al., 1991) as
contact-lens dosimeters have been proposed, but lack of sensitivity
and personal comfort has discouraged their use in field studies.
4.2 Skin Exposure Geometry
UV incident on human skin can follow one of three courses, it
can undergo absorption, reflection, or scattering. Thus, the actual
radiant exposure received by the various layers of the skin will be
lower than the incident exposure. Reflection not only occurs at the
surface of the stratum corneum, but at all interfaces changing in
refractive index. Scattering occurs because of the different
structural elements, such as hair follicles and sebaceous glands,
and also by cellular components, such as mitochondria and ribosomes.
The remaining UV can penetrate into deeper skin layers.
UV penetrates into the dermis exposing a variety of cells and
structures, depending in part on the thickness of the human stratum
corneum and epidermis. The depth of penetration is wavelength
dependent the longer the wavelength the deeper the penetration
(Bruls et al., 1984). From figure 4.1 it is seen that the same
incident exposure of UVA or UVB radiation will result in a higher
actual exposure of UVA than UVB at a given depth. For example, if
the incident UV exposure was 100 kJ m-2 then 50 kJ m-2 of 365 nm
radiation would be present at a depth of 30 µm and only 19 kJ m-2
at a depth of 70 µm. For 313 nm radiation only 33 kJ m-2 would be
present at 30 µm and 9.5 kJ m-2 at 70 µm. While less than 1% of
the UVC wavelengths can barely penetrate the epidermal layer (Bruls
et al., 1984), about 1% of the incident UVA dose can penetrate
into the subcutaneous tissue (Parrish et al., 1978). The
distribution and size of melanin particles also plays an important
role in protecting epidermal cells. Melanin particles have a
distribution within the stratum corneum and epidermal cells
depending upon skin type. In dark skin types (5 and 6) these
particles are positioned within cells to provide optimum optical
protection for the cell nuclei and in adequate size in the stratum
corneum (Kollias et al., 1991).
4.3 Ocular Exposure Geometry
People seldom look directly at the sun when it is overhead and
very hazardous to view. It is not very hazardous to view when the
sun is low in the sky and falls within the normal field-of-view.
When the sun is more than 10° above the horizon, the natural
tendency is to partially close the eyelids or squint (called squint
reflex), thus shielding the retina from direct exposure. These
factors reduce the exposure to the cornea to a maximum of about 5%
of that falling upon the exposed top of head (Sliney, 1986). If the
squint and other behavioural factors are not considered, the dose to
the eyelid would be approximately 20% of the dose falling on a
horizontal surface.
Although the cornea is more sensitive to UV injury than the
skin, people seldom experiences a corneal burn when in sunlight.
Using the action spectrum for human photokeratitis and
mathematically weighting this with the midday solar spectrum, the
time to achieve the threshold for photokeratitis is about 100
seconds (Sliney, 1987, Rosenthal et al., 1988). Again, the
geometry of exposure precludes photokeratitis except when ground
reflectance exceeds approximately 10%. When the sun is overhead and
UV exposure is most severe, the brow ridge and upper lid shield the
cornea, and if the eye is turned away from the sun, the more intense
scattered UV from overhead strikes the cornea at a grazing angle of
incidence where most is reflected and little is absorbed (Sliney,
1983). Only when the incident UV rays are parallel to the pupillary
axis are most rays (approx. 98%) absorbed.
When looking at snow, UV is reflected directly into the eye;
hence, the traditional eye protector of the Inuit or Eskimo, the
slit, in whalebone or in a seal-skin mask, provided geometrical
rather than spectral protection against UV exposure (Hedblom, 1961).
The lack of protection above and to the sides of sunglasses is a
serious shortcoming. However, to obtain a quantitative idea of this
component of exposure to the eye, measurements were made using a
simulated ocular geometry in sunlight (Sliney, 1986). The human eye
received 10 to 25 % of the UV dose when wearing glasses with lenses
opaque to UV compared to no lens in the glasses. Therefore, unless
goggles with side-shields are used, UV transmission factors in
lenses much less than 2-5 % do not provide the eye protection
suggested by the transmission factor (Sliney, 1986).
The strong dependence of reflectance with angle of incidence is
termed Fresnel's Law of Reflection. This law not only explains the
survival of the cornea exposed overhead to UV, but also the glare
experienced over water. When the sun is overhead, water reflects
about 2% of the UV upward (Sliney, 1986). When low in the sky, much
of the sunlight is reflected while the UV and blue light are
filtered by the atmosphere. Nevertheless, the strong reflections
from water at these low sun angles create discomfort glare and UV
exposure of the cornea is further reduced because of the squint.
If dark lenses are placed over the eyes, the natural aversion
to bright light, which leads to the squint reflex (that greatly
lowers retinal UV or exposure to the eye), would be disabled. This
may appear to be an unusual way to consider the comfort that shaded
lenses provide. However, poor sunglasses may actually lead to a
higher UV exposure (Sliney, 1983).
However, quantifying the protective value of the upper and
lower eye lids when they close to squint is difficult. In terms of
UV exposure, at least a twenty-fold reduction is likely. For
shielding the retina from the direct image of the sun, the upper lid
probably provides a protection factor exceeding a thousand (Sliney -
personal communication). If a brimmed hat is worn, the direct image
of the sun on the retina is rare and overhead UV exposure is
virtually eliminated. However, while using a hat the lid opens
further and ground reflection of UV could become important.
On an overcast day, the lids open wider, and although the UVB
irradiance is reduced by cloud cover, the actual UVB dose rate to
the eye from atmospheric scattering near the horizon may be reduced
by a factor of only two (Sliney, 1983). Hence, on a cloudy day the
eye may receive a greater UVB dose than on a bright sunny day.
However, a heavy overcast may attenuate the UVB sufficiently, that
this observation may not be true. As sunglasses are not typically
worn on an overcast day, one could argue that the concern over
sunglasses increasing total ocular exposure is unimportant. However,
sunglasses should have sufficient UV filtration so that ocular
exposure does not actually increase when they are worn on a sunny
day.
Eye and head movements can further reduce UV exposure. Most
humans in bright sunlight squint or avoid looking into the sun
sector of the sky. These behavioural and physiological factors are
not taken into account by simple UV measurements. There is an
obvious need to determine accurately the corneal and lenticular
exposure to ambient UV. Indeed, the results of previous
epidemiological studies of cataract may be questioned because of
inadequate dosimetry. Some epidemiological studies may have reached
incorrect conclusions regarding risk from UV or sunlight exposure by
assigning inaccurate exposure levels to population groups, because
they assumed that overhead UV exposure accurately predicts corneal
exposure.
4.4 Workplace
4.4.1 Outdoor work
Although humans have adapted and acclimatized to solar UV it
nevertheless represents the most hazardous source of optical
radiation likely to be encountered by the average person and with a
few exceptions by the worker. People who work outdoors will be
subjected to involuntary UV exposure. The highest exp