Reproduced, with permission, from:
U.S. Department of Energy (DOE). 1993. UV-B critical issues
workshop. Workshop sponsored by the U.S. DOE Office of Health and
Environmental Research and organized by the Center for Global
Environmental Studies of Oak Ridge National Laboratory, 24-26
February 1993, Cocoa Beach, Florida. Washington, D.C.: Government
Printing Office.
Priority Human-Health Concerns
Of the human-health responses to UV-B studied thus far, the strongest case for a causal link to increased UV exposure is nonmelanoma skin cancer. For several other potentially important health effects, including cataract formation and cataract-related blindness and suppression of the immune system, gaps in our knowledge prevent the establishment of the requisite dose-response relations for forecasting. Acute biological effects from UV-B outbreaks may be significant and are also addressable research issues (probably more so than chronic effects ). Among these responses, immune-system suppression suggests consequences for human health and disease control that could be extremely significant and hence should be a high priority for future research. Important aspects of this issue are addressable with shorter-term (1-year-cycle) laboratory experiments, and these experiments should be started soon.
In designing experiments for human-health assessment and in forecasting UV-B-induced effects, the potential nonlinearity of responses was emphasized. The fact that an elicited response represents the balance between relative rates of repair and damage suggests that rapid increases in symptom expression may occur as the repair capacity of the system is exceeded. In human-health responses in general, the UV-A dose, and in particular the ratio of UV-A to UV-B, can be significant in determining the biologically effective dose. The reporting of spectral radiation and total UV-B dose enhancement was considered important in human-health-related studies so the diverse exposure conditions used in these studies could be extrapolated and compared.
Priority Concerns for Terrestrial Plants
Although more than 300 plant species have been included in screening studies to date, major gaps are still occurring in globally representative plant communities. Of greatest concern from a world economic perspective is the lack of adequate dose-response data for crop plants that make up a significant fraction of the world's food supply. Although some research is under way with soybeans and rice, apparently no information is available on the sensitivity of corn and wheat to enhanced UV-B. An important advantage of an early focus on such crops is that research would offer agronomists the time needed to breed UV-B-resistant strains and would increase the likelihood that agriculture could meet the challenge of this potential stress. The research that has been conducted to date indicates that even varieties of the same species can exhibit substantial variability in UV-B sensitivity. This variability increases the chances that breeders will have the genetic diversity available to increase plant resistance to UV damage.
Temperate-forest trees have also received relatively little emphasis in terms of research efforts. Yet, multiyear growth studies indicate that the potential for cumulative damage does exist. Recent results on the effects of UV-B on pollen germination and tube elongation also suggest that increased emphasis on the reproductive physiology of crops and trees is warranted.
A wide variety of experimental systems have been used in plant-effect studies and can provide useful information in the early stages of screening species and processes. However, an adequate supply of photosynthetically active light is needed to support normal repair processes. For this reason, reliable estimates of changes in production must be obtained under field conditions where ambient light is augmented with UV-B. A national "network'' of at least three well-equipped field-research facilities was suggested to support the screening of important species under an appropriate range of environmental conditions. Additionally, the inclusion of at least two levels of increased UV radiation in experimental protocols was seen as essential to establishing more meaningful dose-response curves.
Harvestable yield is the most obvious endpoint measurement in evaluating significant plant responses in this research strategy. If significant yield effects are found, knowledge about how yield is affected will ultimately be required to build the mechanistic understanding needed to project or model interactive stress effects on plant systems. Effects on carbon assimilation and allocation (particularly respiratory costs associated with repair) were identified as likely useful process-oriented indicators of a change in production capacity. The significance of recently detected DNA damage to the foliage of soybeans, the unknown rate of repair of the DNA damage induced by UV-B, and the heritability of DNA lesions represent areas of additional research need. Damage to DNA and the effects on pollen point to reproductive physiology as a critical research issue.
Priority Concerns for Aquatic Systems
Recent documentation of apparent shifts in phytoplankton productivity under an ozone hole in Antarctica highlight the sensitivity of this ecologically important area to levels of UV that are occurring now. Additional areas of high potential sensitivity are the oligotrophic freshwater lakes and streams at mid- and northern latitudes, where penetration of UV radiation at damaging levels can occur to depths of several meters. These ecosystems may provide early insights for addressing subtle changes over the broader expanse of midlatitude and tropical oceans if significant increases in UV-B are found there.
Three issues that should be addressed in aquatic research were identified. The primary one is that changes in the relative numbers and productivity of phytoplankton, if they occur over a significant area of the polar regions, have the potential to disrupt food-chain dynamics that are important for supporting significant world fisheries. In freshwater systems, the littoral zones that provide valuable habitats to organisms significant to aquatic food webs are susceptible to damage. At present, neither the distribution of effects among species in these populations nor the net loss in primary productivity of these systems is known. At issue is the need to provide an integrative assessment of how changes in species productivity translate into species diversity and system productivity. The highly productive surface waters of the oceans at higher latitudes provide important feedbacks with global climate through their exchange of radiatively active organic gases. If the impacts discussed in the preceding paragraph on biodiversity are projected to be significant, then the altered release of CO2, dimethyl sulfide, and other gases will be an issue in determining potential feedbacks in radiation and energy exchange in these areas.
Aquatic measurements require specialized instrumentation, which is not adequately developed and not routinely available. Instrumentation experts need to develop systems for the exposure and monitoring of the effective biological dose and radiation transfer in aquatic systems, both in the lab and in the field. An experimental approach that builds on experience in aquatic toxicology should initially use small test microcosms to identify which organisms and responses are most sensitive to UV-B and what the dose-response relations are for most critical responses. Multigenerational studies in which the chronic loss of biological function or genetic integrity is measured will be important in building an appropriate suite of indicators of UV aquatic effects. Such indicators should be applied to increasingly complex systems, including sensitive freshwater and marine models in the field.
Exposure Systems
Equipment that is currently used to simulate solar UV radiation in research on the effects of UV-B (exposure systems) is adequate for evaluating the mechanisms of biological response and for determining what species and what tissues are most susceptible to UV-B damage. However, existing exposure systems do not accurately simulate the UV-radiation quality of sunlight and are therefore of limited use for predicting the magnitude of responses to increased UV-B exposure produced by stratospheric-ozone depletion. Irradiance delivered by state-of-the-art exposure systems must be weighted with appropriate action spectra to arrive at a "biologically effective dose" (UV-Bbe). With the available action spectra, this procedure introduces a considerable amount of uncertainty into the results of the research. Also, many of the spectroradiometers do not accurately measure UV-B below 295 nm because of stray light from longer wavelengths.
Without accurate measurements across the entire UV-B range. UV-Bbe could not be calculated accurately even if the action spectra were known. For vegetation, the action-spectra problem is compounded by the fact that many species have different action spectra (i.e., different sensitivities to UV-B). However, the development of action spectra is very difficult and requires expert skills and equipment. As a result, the development of action spectra for a meaningful variety of species may require years.
Additional problems exist in field exposure systems. For example, the shade created by the UV-B lamps themselves and variable output along the length of the lamps result in an uneven distribution of UV-B. Also, UV-B dose decreases vertically with increasing distance from the lamps, causing plants to receive gradients of UV-B exposure along the plant-stem axis.
To take full advantage of today's exposure systems, the following approach is recommended:
To improve our ability to predict the magnitude of organismal responses to the expected enhancement of UV-B, the following approach is recommended:
Improved integration of knowledge about UV-B climatology and reliable projections of its change will be extremely important in designing experiments and interpreting their results to realistically forecast future UV effects. Questions concerning expected UV-B dose (including the upper limits of future exposure; spectral shifts in wavelengths at the shorter, more biologically active end of the UV-B spectrum; and periodicity of UV pulses) formed valuable points of interaction among biological and physical scientists at this conference. This need for interaction within the technical community is crucial to facilitating the discovery of timely and scientifically defensible answers. Such answers are important because many groups outside the scientific research community (risk analysts, policymakers, medical practitioners, economists, business leaders, and more) will feel the effects of and have to grapple with the implications of any increase in UV-B.
The approaches and assessment methods that are appropriate to the UV-B problem are myriad and cannot be encapsulated in any one workshop or report. This workshop built upon and extended the prior efforts in this field while introducing new information and perspectives to the discussion. It sought to bring about an integration of scientific approaches to the problem of studying and understanding the potential effects of increased UV-B irradiance of the Earth's surface. In retrospect, it achieved this goal to a creditable extent, and that achievement is summed up in the directions and solutions suggested at the workshop and in the four critical issues that it identified:
Prior Workshops and Conferences
The UV-B Critical Issues Workshop was not the first occasion for the consideration of this topic. Prior to 1993, several publications and workshops had reviewed the situation and revealed new information about the effects (observed and potential) of ultraviolet type-B radiation (UV-B) and the hazards associated with increased exposure to it.
WMO/UNEP
Three major reports, the 1989 World Meteorological Organization (WMO) and United Nations Environment Programme (UNEP) scientific assessments of ozone depletion,[1,2] and the 1991 update of the Environmental Effects of Ozone Depletion,[3] summarized the scientific information available on UV-B radiation, and this information was reviewed in the proceedings of the 1992 UV-B Monitoring Workshop.[4] The WMO/UNEP reports drew the following picture.
Although spectral data have shown stratospheric ozone levels to be decreasing in both hemispheres, increases in UV-B effects are more likely to appear first in the Southern Hemisphere. There, stratospheric ozone losses have been more severe, tropospheric ozone has not increased, and aerosol concentrations are lower. Indeed, satellite data indicate an increase of annual DNA-damage weighted UV-B over large geographical areas of the Earth, especially in the Southern Hemisphere: 5% per decade at 30deg.N, 11% per decade in the northern polar region, 5% per decade at 30deg.S. and 40% per decade at 85deg.S.
Unfortunately, models in use today cannot be used to accurately predict future changes in UV radiation reaching the Earth's surface because of the complexity of heterogeneous chemistry. Many influences affect the amount of UV-B radiation reaching the ground. Studies indicate that a decrease in UV-B of 2% per decade can be expected with current tropospheric ozone trends. In some locations, this reduction may be as large as 18% because of the combined effects of tropospheric ozone and sulfuric aerosols. Ironically, efforts to improve air quality may therefore accentuate the potential increases in UV-B associated with the depletion of stratospheric ozone.
From a health perspective this balance between increased UV-B from thinning of the stratospheric ozone and decreased UV-B from higher concentrations of tropospheric ozone is important. UV-B exposure can affect the incidence of skin cancer and ocular damage and could potentially affect the incidence and severity of certain infectious diseases. A sustained 10% reduction in stratospheric ozone could result in a 26% increase in nonmelanoma skin cancer worldwide (or 300,000 new cases per year) and would cause the cancer to occur at a younger age. (It should be noted that the data are much less certain for melanoma skin cancer.) Moreover, cataracts are responsible for 17 million cases of blindness each year. A 1% stratospheric ozone loss is predicted to result in a 0.6% increase in cataract incidence or 100,000 new cases per year. And animal models suggest that UV-B exposure impairs skin immune functions, some systemic immune responses, phagocytosis, and immunity to infectious diseases (e.g.. herpes simplex virus. Leishmania, Mycobacteria. and Candida).
Studies have shown that plants exhibit a tremendous variability in their sensitivity to UV-B radiation. Responses that do occur include changes in leaf secondary chemistry (flavonoid accumulation), alterations in leaf anatomy and morphology. reductions in net carbon assimilation capacity (photosynthesis), and changes in biomass allocation and growth.
Laboratory studies indicate that UV-B radiation impacts marine organisms by affecting their adaptive strategies, impairing important physiological functions, and influencing the developmental stages. In addition to damaging DNA, UV-B radiation affects enzymes and other proteins. eliciting photodynamic responses that can potentially result in reduced biomass production, altered species composition and biodiversity, and decreased nitrogen assimilation.
1991 USDA Workshop
In January 1991, the U.S. Department of Agriculture (USDA) sponsored its first UV-B workshop in Denver.[5] That workshop identified the development of a cost-effective monitoring network to obtain UV-B climatology over wide geographic areas of the United States as a major research need in the effort to understand UV-B effects. Such a widespread network would overcome at least some of the factors contributing to the imprecision of current measurements (for example, variations in cloudiness, tropospheric ozone, air pollution, surface reflectance, and annual variations). Several characteristics of the proposed network were identified:
At that meeting, specifications were drawn up for a scanning spectroradiometer capable of producing the needed data. However, those specifications were more stringent than the capabilities of any readily available instrument. As a result, the USDA Cooperative State Research Service (CSRS) awarded a grant to design and build an instrument that would meet the recommended specifications. But both the initial and operating costs of the instruments developed were too great to allow the instruments' deployment to provide broad regional coverage in an effort to gain an understanding of UV-B climatology.
1992 USDA/AFEAS Workshop
In March 1992, the USDA conducted another workshop[4,6] in conjunction with the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) to address issues related to the affordable acquisition of data from a greater number of sites able to provide a realistic representation of the UV-B climatology and to determine the status and trends of UV-B exposure over large geographic areas.
At the workshop, many researchers presented findings in a variety of disciplines related to the study of UV-B. The presentations described
These findings repeatedly supported the general assertions presented in the earlier WMO/UNEP reports and underscored the need for status-and-trends data to
At the end of the 1992 workshop, five major recommendations were made to guide the USDA/CSRS in the development of future monitoring efforts.
Although many workshop participants were not enthusiastic about the prospect of deploying available broadband instruments, it was generally recognized that this might be the only alternative. given the immediate urgency of establishing a network. The agreement was tempered with a commitment to seek and deploy better-qualified alternative instrumentation as soon as possible.
SCOPE Workshop
The Scientific Committee on Problems of the Environment (SCOPE) held a workshop on the effects of increased UV radiation on biological systems in Budapest in February 1992. The report from that meeting[7] summarized the current state of knowledge of UV-B effects and monitoring instrumentation and set a research agenda.
Laboratory studies of temperate crops show many plant species to be detrimentally affected by UV-B, but few studies have been conducted under field conditions or with nontemperate crops. Temperate-latitude crop species may not be representative of species from low latitudes, where UV-B flux is much higher.
Laboratory and microcosm research has consistently shown that increases in UV-B are associated with decreases in phytoplankton productivity. Recent fieldwork confirms that enhanced levels of UV-B can reduce the productivity of natural populations of phytoplankton. But the magnitude of change in productivity is not known, and the decrease may portend a serious decrease in available food sources.
Increased UV-B exposure is expected to lead to an increase of cataracts, skin cancers, and specific suppressions of the immune system. Immune suppression might lead to increases in the incidence or severity of certain infectious diseases and to decreased effectiveness of vaccination programs.
Increased UV may alter the rates of exchange of trace gases between the ocean and the atmosphere, affecting, in turn, the atmospheric concentrations of greenhouse gases, such as CO2 and N2O. These gases affect the self-cleaning capacity of the atmosphere and the formation and optical properties of aerosols that are important in cloud production.
A reliable long-term global UV monitoring network, supplemented by radiative-transfer modeling and satellite observation, is needed to provide interpolation among the monitoring sites. Although current instrumentation cannot provide precise data at an affordable cost, UV sources and monitoring instrumentation used in laboratories should be applicable to measuring environmental (outdoor) UV irradiances. Some of the research priorities identified at the workshop were to
CNIE Seminar
Concerned that the increases in UV-B associated with ozone depletion will persist well into the next century, threatening global food supplies and jeopardizing the oceans' ability to absorb carbon dioxide, the Committee for the National Institutes for the Environment (CNIE) convened a seminar of experts in October 1992. In a draft version of a report on the seminar,[8] the sponsors note the following observations.
Field studies have been conducted on only two crucial food crops, rice and soybeans. The rice data are not yet available. But one soybean strain has been shown to suffer 20% yield declines under heightened exposure to UV-B, although another variety experiences no effect or even small increases in yield. Laboratory studies appear to indicate that one-half to two-thirds of all plants are impacted by heightened levels of UV-B, which points to the importance and possibility of developing new UV-B-resistant agricultural species.
Preliminary measurements of phytoplankton populations in Antarctic waters show losses from 3 to 25% in the top of the water column. Phytoplankton are especially vulnerable to UV exposure because they lack the protective epidural UV-absorbing layers characteristic of higher plants and animals. Because they form the base of the marine food web, the decline of phytoplankton has grave implications for ocean fisheries.
Phytoplankton's absorption of carbon dioxide is the mechanism by which the ocean acts as a carbon sink. Even a 5% loss in phytoplankton productivity could lead to an increase in atmospheric carbon equal to the annual global increase from industrial activity.
Given these findings, the seminar participants recommended the following actions:
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