JAN C. VAN DER LEUN
describes what is known about the effects of ozone depletion on health, and shows how international action has avoided enormous rises in skin cancer
Ozone depletion increases the biologically harmful solar ultraviolet radiation reaching the Earth's surface. This leads to a variety of adverse effects, including an increase in the incidence of skin cancer. Research suggests that if the Montreal Protocol had not been adopted - and strengthened by the Parties in Copenhagen in 1992 - there would be a runaway increase in skin cancer in the next century.
UV-B radiation, which increases with ozone depletion, does not penetrate far into the body; most is absorbed in the superficial tissue layers of the skin. This limits the primary effects to the skin and the eyes. There are, however, also systemic effects, which start with a primary reaction on the superficial layers but have consequences throughout the body.
The sunlight that reaches us contains only 0.5 per cent UV-B radiation, in terms of radiant energy. This small fraction is responsible for most of the effects of sunlight on the body. It is the main cause of sunburn and tanning and of snow blindness, is probably one of the factors in the induction of cataracts, and has influences on the immune system. It contributes significantly to the ageing of the skin and eyes, and it is the UV-B range that is the most effective in causing skin cancer.
The first impression is that all these effects will grow with an increase of UV-B radiation - but closer study indicates that this is not necessarily so. There will be adverse effects on some diseases and conditions, but others will not be influenced appreciably and some will even decrease.
One early expectation was that there would be more sunburn with ozone depletion. On closer examination this has proved not to be well founded. People from northwestern Europe who settle in a Mediterranean country do not sunburn more than those who stay at home, apparently because their skin adapts to the higher UV-B exposures in their new environment. This suggests that the same will happen in northwestern Europe when the UV-B irradiance there gradually increases. In general, sunburn will not become more of a problem under a decreased ozone layer.
Patients with photodermatoses - skin diseases where lesions are caused by light - can often be treated effectively by regular exposures to low-dose UV-B radiation: loss of the skin's adaptation to light appears to be a predominant factor in them. So the increase in UV-B irradiation, especially in winter, from depletion of the ozone layer will to some extent improve the patients' conditions.
Snow blindness occurs typically when the eyes are exposed to UV radiation coming from unusual directions, such as in snow-covered mountains. It is very painful, and is sometimes described as having sandpaper in the eyes. The eyes usually heal spontaneously, but have no adaptation against this effect; they even tend to become more sensitive to the next exposure. There is little doubt that increased solar UV-B irradiance will lead to an increased incidence and severity of snow blindness, if behaviour remains unchanged.
Cataracts, opacities in the eyes which impair vision, mainly occur in elderly people and may ultimately lead to blindness. Surgery can usually prevent this in countries with good medical facilities, yet cataracts are still one of the main causes of impaired vision in the United States, and result in a much higher incidence of blindness in developing countries. It has been estimated that cataracts were responsible for 17 million cases - more than half of the blindness in the world - in 1985, and the problem is increasing with life expectancy.
It is becoming increasingly clear that sunlight, among other factors, plays a role in the formation of cataracts: both experimental and epidemiological work suggest an influence for UV-B radiation. It has been predicted that a sustained 10 per cent loss of ozone worldwide would - all other things being equal - lead in the long term to an additional 30,000 people going blind each year.
There is concern - arising from a growing number of observations showing influences of UV-B radiation on the immune system - that a higher level of irradiance might lead to an increase in some infectious diseases.
The answer is by no means obvious. The immune system is very complex, with several subsystems helping or suppressing each other. What UV-B radiation does to some of them is known for certain experimental conditions, but this is far from sufficient to predict the overall outcome for the entire system.
Infections that have a phase in the skin, such as malaria, are the most likely to be influenced. In animal experiments, leishmaniasis spread more fiercely through the body after exposure to UV-B, and the incidence of herpes increased. Experiments have also shown that the papilloma viruses and HIV-1 were activated by UV-B radiation. This would not lead to an increased rate of infection, but it might result in its taking a more rapid course or in the disease being more severe.
An open question
The question is still open. The pieces of knowledge available give good reasons for concern, but more investigations are clearly needed. One part of the answer is likely to be that the consequences will probably differ among diseases.
More research is also urgently needed into concerns that increased UV-B irradiance might interfere with the effectiveness of vaccinations, the main way of controlling infectious disease, especially in poor countries. There are observations showing that immunization through UV-B treated skin may make an individual more, rather than less, susceptible to the administered antigen - but even less can be said with certainty than about infectious diseases.
There is little doubt that the incidence of basal cell and squamous cell carcinomas, sometimes collectively called the 'non-melanoma skin cancers', will increase. These are some of the commonest cancers of all in Caucasians, though their mortality rate is comparatively low, about 1 per cent in countries with good medical care.
The incidence of cutaneous melanomas, cancers of the skin's pigment cells, is much lower, though for decades it has been one of the fastest rising of all types of cancer. The mortality rate is much higher; it has been brought down to 25 per cent in countries with good medical care, mainly by early diagnosis. The effect of UV-B radiation is not yet known, though research gives support to the idea that it may play a role in the formation of cutaneous melanomas.
Last year, investigators from Utrecht University (including myself), The Netherlands' National Institute of Public Health and the Environment and the United States National Oceanographic and Atmospheric Administration presented a new method of estimating future excess skin cancer risks in the journal Nature. We used it to compare the effects of three scenarios of ozone depletion - first where no restrictions on ozone-depleting substances had been agreed, second under the original Montreal Protocol, and third after the adoption of much stricter amendments to it in Copenhagen in 1992.
The first two produced a runaway increase in skin cancer incidence by the year 2100 - up to a doubling under the Montreal Protocol scenario and up to a quadrupling under the 'no restrictions' one. Under the Copenhagen Amendments scenario, however, there was a peak increase in the incidence of skin cancer of only 10 per cent, at around the year 2060, before it declined again.
We conservatively estimated 1.5 million excess cases of skin cancer a year in the United States, and 550,000 in northwest Europe, around the year 2100 under the 'no restrictions' scenario. Under the Montreal Protocol scenario, we estimated, again conservatively, there would then be 440,000 excess cases a year in the United States and 170,000 in northwest Europe. Under the Copenhagen Amendments scenario these fell to 8,000 a year in the United States and 4,000 in northwest Europe.
These projections demonstrate the important improvements that will be achieved if the measures agreed upon under the Vienna Convention are fully implemented.
Professor Jan C. van der Leun is Co-Chair of UNEP's Environmental Effects Panel, and is based at the University Hospital of Utrecht. He holds a UNEP Global 500 Award 1997.