Edited by James G. Titus U.S. Environmental Protection Agency
The adverse biological and environmental effects due to any inadvertent ozone modification have received considerable scientific attention but have been largely concerned with ultraviolet radiation damage to human skin in specific relation to protection of deficient white skin. In this respect, the ozone problem somehow has not appeared to be of serious interest to the people of developing countries living in the tropical/equatorial geographical belt. However, in view of the manifold increase in the ultraviolet radiation received at the lower latitudes compared to the mid and high latitudes, it seems reasonable that the ozone layer issue should be of serious interest in the tropics. In fact, in these regions, more serious kinds of solar UV radiation-induced health effects such as cataracts, viral infections, and immune system damage may be connected to the "ozone-UV" issue. These aspects need to be studied under harsher tropical conditions, but the understanding that has been achieved so far (for mid-latitude conditions) can provide a valuable base from which the specific situation for tropical countries can be tackled as an extension.
For several decades, atmospheric ozone, especially the stratospheric layer, has been known for its important protective role against the incoming solar ultraviolet radiation. In general, there is a natural balance between the ozone's production and its destruction with a net ozone surplus. Under the natural equilibrium, there are small dosages of solar ultraviolet radiation to which populations, living organisms, plants, and aquatic systems have generally become adapted. The harmful effects are thus generally very small except perhaps in the tropical/equatorial region. The surface dosage of natural ultraviolet radiation increases by many folds in going from high/mid latitudes to the equator. Because people living in the high dosage tropical belt have a darker skin tan which provides greater protection to the skin against ultraviolet radiation, this high UV radiation situation has not been considered particularly seriously in terms of adverse health and environmental effects that might occur in these regions.
The ozone layer suddenly came into scientific prominence about 12 years ago when it was realized that certain human activities may lead to significant ozone depletion from the naturally balanced level, thereby increasing the surface dosage of ultraviolet radiation leading to increased skin cancer and many other adverse biological, environmental, and climatic effects. Since any such ozone destruction would spread out globally, the net UV radiation effect would be most serious in the equatorial and tropical belt when already the ozone column thickness is minimum and ultraviolet radiation penetration is maximum. But perhaps even a more fundamental issue is to ascertain whether the already high UV radiation dosages received at the lower latitudes are contributing to tropical diseases and medical problems. Whether the 5 to 10 times higher dosages (in absolute terms) that would be received in the low latitude region in comparison to the higher latitudes (if a certain fraction of ozone column is reduced) would bring the radiation level above a certain threshold is another aspect relevant to the tropics. In other words, the "ozone modification" issue is of serious importance to the low latitude countries. The whole issue of ozone layer protection is thus indeed of global dimension. Yet, there is very little apparent involvement of scientists from the developing countries in the "ozone layer-ultraviolet radiation" work. While financial constraints may account for some of this non-involvement, lack of proper understanding of the effects at the public, scientific, and political levels is also a serious factor. Nevertheless, in the coming years, it should be appropriate to examine the "ozone layer-UV-B" matter in the specific context of tropical countries.
In order to examine the relevance of the ozone layer issue for the developing countries in the equatorial/tropical region, it would be helpful to summarize the atmospheric ozone and solar ultraviolet radiation influx globally. The seasonal distribution of vertical ozone column for different latitudes is shown in Figure 1. It is clear that at the lower latitudes, not only the ozone column thickness is significantly small but it also does not vary much seasonally. The solar ultraviolet radiation penetration is thus maximum at the lower latitudes throughout the year. The overall effect of the low ozone content coupled with smaller seasonal change in the solar declination results in the latitudinal distribution of annual erythemal dosage (incoming radiation weighted according to skin erythemal action spectrum) as shown in Figure 2. This diagram illustrates the manifold increase in the damaging UV-B dosage from high latitudes to the equator. Some towns are also marked on the curve against their respective latitudes. Figure 2 is based on calculated data for clear sky conditions (Mattingly 1976; Ilyas 1979).
In the equatorial/tropical region, there is a general lack of observational data pertaining to the atmospheric ozone and solar UV radiation. With this consideration in mind, about 10 years ago we initiated a comprehensive program of measurements involving ozone soundings, erythemal UV-B, UV-A, total solar radiation, surface ozone, and a series of relevant meteorological parameters at our equatorial place, Penang (5.5deg.N). This is perhaps one of the most rare comprehensive programs in the equatorial/tropical belt. Some of the initial results from this study have become available and more refined data should follow (Ilyas 1984a, 1984b; Ilyas and Barton 1983).
One of our immediate interests from this study is to see if the observational data obtained at Penang can be used in a general way for the equatorial/lower tropical region. For this, the most important and directly usable data in the context of ozone layer effects study are the directly involved ultraviolet radiation data. The input radiation flux does not change much over the entire equatorial/lower tropical belt. It would, however, be modified differently at different places depending upon the sky conditions (cloud cover). The long-term cloud cover data at Penang indicate the cloud cover to be close to 85% (Ilyas et al. 1981) which reduces the incoming UV-B radiation to about half. This is consistent with the theoretical relationship of cloud cover effect on ultraviolet radiation (Johnson et al. 1976). In a more comprehensive study of cloud cover effect, we used 5-year-long observed UV-A data (Ilyas et al. 1986), together with the calculated data for clear sky conditions (Johnson et al. 1976). The observed data were found to be in excellent agreement with the computed data modified for the average cloud cover (Ilyas 1986). This is shown in Figure 3.
The excellent agreement between observational data and calculated data indicates that the cloud cover effect for other locations may be easily incorporated into the clear sky calculations of ultraviolet radiation. The radiation conditions for evaluating the adverse biological/medical and environmental effects can thus be ascertained. In any case, because of the relatively high cloud cover conditions at Penang (5deg.N), the observed UV flux represents a lower limit of UV dosage that would be received anywhere in the entire equatorial belt. A summary of the erythemal UV-B dosage together with some meteorological conditions at Penang is presented in Figure 4. The information in Figures 3 and 4 thus represents lower tropical model conditions which must be simulated for the photobiological effect studies for this region.
Figures 12, 3-4 provide a good summary of low ozone content and high ultraviolet flux conditions prevailing at the lower latitudes against the high ozone content and low ultraviolet flux at mid and high latitudes. Besides, the high (air) temperatures and humidities in the tropics prevail throughout the year and may couple together with the UV radiation in producing more severe biological and physiological effects. Lack of education and awareness of UV-induced damages, skin's false sense of protection, increased outdoor occupations including young school children being exposed to relatively large amounts of ultraviolet radiation (under hot and humid conditions) due to inadequate clothing and/or outdoor work (like P.E. lessons and games) at wrong times of the day may all add up to the seriousness of the situation. Unfortunately, there is not much basic data available let alone studies of adverse radiation effects under specific conditions. Although skin cancer and other related problems, hitherto the prime concern in Western populations, may not be of sufficient importance in the tropics, more serious effects on humans such as viral infections (herpes, hepatitis), eye damage (cataracts), damage to the immune system, and life expectancy (perhaps we don't record many skin cancer cases in the tropical countries because most of such people don't live long enough for the effects to show up) are important avenues for future work. Also, what adverse effects the very high dosages may have on tropical plants and aquatic organisms and the exposed materials would also need to be studied. Finally, whether any further increase in UV radiation, as a result of ozone reduction due to human activities, would affect any threshold limit--to which the tropical systems may have become adapted--would be important to examine. Side by side, populations would need to be educated on these effects and some simple protection methods, whereas scientists in these regions should be increasingly involved in this program with the realization of local relevance and overall importance. The exercise of protecting humans globally by protecting the ozone layer would then become a very involved matter. An organization such as UNEP is well suited to make a move in this direction.
A UNEP Fellowship which enabled my participation in the conference is gratefully acknowledged. I benefited greatly from my discussions with many persons at the meeting which has helped in preparing this paper.
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