Effects of UV radiation
UV radiation emits more photonic energy than visible light and infrared radiation, thus having a much stronger influence on materials, flora, fauna and people. Short wavelength UVB radiation has more energy per photon than longer wavelength UVA radiation and is potentially more harmful.
UVB is only a small proportion of the UV energy from the sun reaching the earth’s surface, UVA is 15-20 times greater in intensity, but the amount is highly dependent upon the concentration of Ozone in the stratosphere. ‘Holes in the Ozone layer’, with large reductions in concentration of 25% or more, are mainly confined to the North and South Poles. However, areas of up to 10% Ozone depletion are spread more widely. A reduction in Ozone means that more UVB reaches the ground.
To determine the concentration of atmospheric Ozone (not ground level Ozone) the spectrum of the UV radiation from the sun is measured by the Brewer spectrophotometer, and the data analysed to calculate the ‘Total Ozone Column’ in the atmosphere between the instrument and the sun in Dobson Units (DU). The global average value is about 300 DU and the boundary of an Ozone hole is normally defined as 220 DU.
Monitoring UV radiation is becoming more and more important, especially in view of the continuing changes to processes in the ozone layers of the atmosphere and the consequential effects on the environment and physical health.
Increased UV flux to the earth’s surface has heightened concerns over the impact on human health, as the potential health impacts are serious.
UV radiation has some beneficial effects for people. Vitamin D is important for health and one of the main sources is production within the skin stimulated by UV radiation. UV also helps with skin conditions such as psoriasis and acne and promotes general health. However, too much UV can burn the skin and cause cancers, melanoma, cataracts and DNA damage.
There is continuing, and expanding, research into the epidemiology and pathology of UV exposure and the preventative measures that can be taken.
Most UV effects research has centred on skin cancer and eye damage, but only minimal effort has been devoted to studies of immune suppression and infectious diseases.
- Recent Australian and Canadian research has shown that members of lower socio-economic groups with substantial outdoor exposure have a lower risk of malignant melanoma and basal cell carcinoma than those engaged in indoor, professional, managerial, and technical occupations, who receive only intermittent exposure to the sun.
- Squamous cell carcinoma appears to be related to cumulative exposure.
- Acquired (non-congenital) skin moles (nevi) are an indicator of elevated risk for malignant melanoma. Australian and Canadian researchers have shown a direct relationship between mole density and solar UV exposure. Generally, the risk of melanoma increases with mole density.
- Persons with light skin colour, red hair, and a propensity to burn in the sun are at greater risk of basal and squamous cell carcinoma.
- UV radiation may suppress the human immune response, increasing the risk of skin cancer and bacterial and viral infection (including the re-activating of smallpox and herpes lesions).
- Acute, long-term, and chronic eye conditions may become more prevalent if UV exposure increases.
- Avoidance of the sun, use of sun screens, suitable clothing and wearing UV protective glasses are important preventative measures.
In order to make it easier to understand the risk of UV exposure the Global Solar UV Index was created under the auspices of the United Nations, World Meteorological Organisation and World Health Organisation. This UV Index is composed of weighted measurement values (erythemal action spectrum as per ISO 17166:1999, CIE/S 007/E-1999) and describes the effects of UV radiation on the human skin, the exposure risk and the protective precautions to be taken depending upon the radiation intensity.
Today, the Global Solar UV Index is internationally recognised as the standard for evaluation of the sunburn risk and runs from UVI of 1 to UVI of 11+, where higher UV Index represents higher risk of sunburn and skin damage. The scale is shown below.
UV irradiance is measured in W/m2. The Global Solar UV Index can be calculated by multiplying the UVE radiation value by 40 m2/W. For example, 0.25 W/m2 of UVE represents a UV Index of 10 and this is the value used for public health information.
Humans have to be exposed to the sun for quite a while to get sunburn. However, when the skin starts reddening it means that the maximum ultraviolet radiation dose for that skin has been exceeded. This may result in sunburn. The maximum dose depends upon the individual skin type which has been classified into six categories in the UV index, starting from the sensitive type (extremely light-skinned) to the dark-skinned type with natural protection.
Because of the variation in the effect of UV radiation on the human skin; depending on skin type, altitude, UV spectrum and any protection applied, the quoting of ‘burn times’ is actively discouraged. People who have a tan already may stay in the sun about twice as long as people with no tan at all.
Note that the human skin can protect itself against UV radiation to a certain degree, but the human eye cannot. UV radiation on the human eye may lead to temporary ‘snow-blindness’, keratitis and even promote cataracts. Eye protection with full UVA and UVB absorption should be worn when necessary.
In general, UV monitoring has not been the historical interest or responsibility of meteorological organisations. As it is largely a public health issue it normally falls within the parameters of environmental protection or pollution monitoring authorities.
Ecosytems and biomes have adapted over long periods to a particular range of UV intensities and a significant change in the situation will influence their stability, geographical range, and possibly survival. Whether changes wrought by an increase in UV flux will be profound or subtle, minor or dramatic, beneficial or inconsequential, depends on many factors, most of which are poorly understood.
There is, therefore, a pressing need to measure, monitor, and understand the effects of enhanced UVB irradiance on biological communities if we are to be able to predict the impacts that will occur in the next 30–40 years and to manage the consequences.
Alterations in UV irradiance can affect primary production in all ecosystems, terrestrial and aquatic, natural, managed, or exploited with a potential cascade of effects. Current understanding of these processes does not enable confident prediction of the impacts.
There has been little systematic research on the impact of enhanced levels of UVB on flora and fauna. A few studies of some agricultural and commercial forest species provide limited insight into the problem, but it is difficult to extrapolate from these to predict impacts on whole ecosystems.
It is important that the effects of both chronic increases in irradiance leading to cumulative doses, and also episodic peaks or events that may coincide with critically vulnerable stages in life cycles, be evaluated. Further, the influence of ozone depletion must also be considered in conjunction with the effects of other stressors such as climate change, acidification, and the presence of toxic chemicals, making it essential that UVB impact studies be integrated with existing ecological research, monitoring, and assessment programs.
Ultraviolet radiation damages DNA, cell membranes, and organelles (e.g., chloroplasts) in plants. Plants have a capacity to repair damaged DNA, and some can protect themselves by synthesizing UV absorbing pigments and by modifying key metabolic enzymes. Harm occurs when the radiation dose causes damage beyond a plant’s capacity for repair and protection.
Crop damage is manifest as a decline in yield, reduced fertility with fall in seed and fruit production, drop in marketable quality, and ecological effects such as changes in crop-weed interactions and pasture mixtures. While the extrapolation from controlled environments to field conditions remains an issue, research is consistent in identifying that individual varieties differ in their sensitivity to UVB, as measured by changes in photosynthesis, growth, yield, and reproduction.
Impacts on forage and vegetable crops in Canada have been equivocal. Of over 100 varieties of 12 important crop species, 40% were unaffected by UVB equivalent to a 20% decrease in the ozone layer and 60% were affected in some fashion. Soybean, tomato, and canola losses may be expected, possibly totalling hundreds of millions of dollars annually. Maize does not appear to be vulnerable to anticipated increases in UV.
UVB may accelerate the rate of decomposition of straw and chaff, with potentially beneficial effects in arid environments and under minimum-till conditions.
Although forests are of major economic, social, and natural importance, little is known about their susceptibility to enhanced UVB radiation. Trees are long-lived and will be exposed to increased UVB over decades. Short-term effects have been reported, and there is some indication of cumulative, chronic impacts. Multi-year experiments with long-lived species are desirable. The impacts of UV radiation appear to be less serious on species native to high elevations, which tend to be adapted to greater irradiance.
Impacts of enhanced UV radiation should be considered in conjunction with the effects of other stressors such as climate change and acidification. Studies of the interactive effects of UVB and CO2 enrichment on the growth and physiology of conifer seedlings suggest that future conifer seedling growth and competitive ability will be altered by the changing environment.
A field study on UVB effects on trees in North America attributed sun-scalding of white pine foliage in Ohio and Ontario to elevated ambient UVB levels. Recent research has also shown that epicuticular wax chemical composition in certain conifer species is affected by UVB exposure in a manner that inhibits photosynthesis.
The 50-year window of significant ozone depletion has sufficient potential to affect North America forest productivity on a large scale, with far-reaching consequences that are not yet apparent.
Freshwater and wetlands
Shallow freshwater ecosystems are particularly vulnerable to enhanced levels of UV radiation, showing changes in primary productivity, nutrient cycling, community structure, and modification to the transport and speciation of toxic chemicals in the food-chain.
UV penetration of surface waters is attenuated by the presence of dissolved organic carbon (DOC), and changes in DOC levels have a greater effect on the vulnerability of freshwaters to UV than changes in stratospheric ozone. In boreal lakes, where global warming and lake acidification have caused a sharp decline in DOC, UVB penetration into the water column has increased between 22% and 60%.
Even at current ambient levels, UVB inhibits some species of zooplankton from frequenting their preferred position in the water column. Under experimental conditions, enhanced levels of UV radiation depressed algal production but affected the zooplankton that fed on the algae to a proportionately greater extent. As a result, the accumulated algal biomass increased, even though primary production was reduced. Because a key link in the food chain had been weakened, less production was transferred to higher trophic levels. The implications for aquatic ecosystems are disturbing.
Concern has been expressed over synergisms between UVB, climate change, lake acidification, fungal infections, and toxic chemicals that could affect amphibia. However, Ontario amphibians do not seem to have been affected by current levels of radiation, despite the apparent vulnerability of eggs and larvae.
Direct impacts of UVB on microbial processes and higher trophic levels have been demonstrated, but the cumulative effects of ozone depletion on marine ecosystems are, as yet, unpredictable. Potential responses include changes in species composition, shifts in food webs (with collateral impacts on fisheries), and possibly climatic changes.
Increases in UVB alter the growth, survival, and biogeochemical activities of many microbes, plants, and animals in the sea. Damage to DNA directly influences survival, but UVB also affects spatial orientation (and hence vertical migration) of phytoplankton, nitrogen metabolism, photosynthesis, larval mortality, and processes ranging from viral infection to hatching success in fish eggs.
The sensitivity of physiological processes to solar UVB requires a biological weighting function (BWF, also called an action spectrum) to quantify the effective irradiance. BWFs need to be determined for a greater number of biological functions and estimates need to be made of the range in variation of BWFs for each process, as these appear to be highly variable between species.
Biological effects of UVB have been detected tens of metres into the water column, most notably the inhibition of short-term photosynthesis in Antarctic phytoplankton. Quantitative estimates of the inhibition of photosynthesis by UV radiation are converging as research continues.
Some toxic dinoflagellates show UV photo-protective mechanisms that might give them a competitive edge, perhaps leading to a greater dominance of toxic or nuisance algae. Increases in UVB may favour species of phytoplankton that produce dimethyl sulphide (DMS), a gas involved in cloud formation, and thus contribute to climate change.
Interpretations of UV effects in oceans require recognition of the fundamental differences that exist between the Antarctic and waters of the northern hemisphere, including the Arctic. Consequently, basic research on marine ecosystems is essential.
UV radiation causes significant and deleterious changes to many materials used in outdoor applications. Any increase in UV flux to the earth’s surface will degrade infrastructure more quickly and so generate significant costs for repair and replacement.
Canadian research has addressed the effects of UV on polymers, wood and paper, building materials, paints and coatings, and textiles and clothing, although the main thrust has been on the evaluation of radiation resistance of materials used in outer space and of clothing materials.
UV-B radiation damages synthetic polymers and other materials, but the mechanisms are not well understood at the molecular level and the combined impacts of both short and long wavelength radiation, and other environmental variables, adds further complexity to the issue.
Bleached pulp and paper products made by inexpensive processes are discoloured by UV radiation. Canadian researchers have made significant advances in understanding how this occurs. The ability to reduce discolouration could greatly expand the market for this class of paper.
Non-plastic building materials such as roofing membranes and outdoor sealants are currently being studied with respect to their resistance to UV but not specifically in the context of enhanced, ozone-related irradiance.
Extracted from material of Environmental Canada 1997, edited by D.I Wardle, J.B. Kerr, C.T. McElroy and D.R. Francis.