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The heat is on

The heat is on IT IS only now that the unimaginable horrors of greenhouse gases and ozone-depleting chemicals are coming home to roost. Several recent studies indicate that changing climate and mutilation of the ozone layer have led to a wilder spread of infectious diseases, a lowering global cereal production and the aggravation of certain types of cancers (The Lancet, Vol 342, Nos 8878-8885).

Scientists say that an accumulation of greenhouse gases in the atmosphere -- particularly carbon dioxide and methane that are produced when fossil fuels are burnt -- will lead to an increase in the temperature of the earth"s surface and lower atmosphere. Scientists estimate that a doubling of carbon dioxide levels will increase temperatures by 1.3 to 2.30C.

Atmospheric pollution has not only warmed up the earth, it has also punched a gaping hole in the ozone layer, a sunscreen which envelopes the earth at a height of 15 to 25 km and shields it from harmful ultraviolet radiation. The damage to this lifesaving barrier has been caused by chlorofluorocarbons, used as a refrigerant in airconditioners and refrigerators, and as chemical solvents.

Raised atmospheric temperatures have boosted the infiltration of infectious diseases from their natural homes in the tropics to the warmer and wetter temperate regions. A warning comes from Andrew Dobson and Robin Carper of Princeton University, USA, that over the past 30 years, these bugs have developed a resistance to drugs and pesticides, making it difficult to control the germs in new areas.

It"s not a pleasant scenario. Among other things, global warming can alter the distribution map of malaria and viral encephalitides-carrying mosquito larvae, like those of the Anopheles gambiae. Under cooler conditions than the ones available, larval development is slow and restricted to stagnant ponds and canals. Warmth pushes larval development, allowing them to thrive in every puddle and water-filled container. A further increase in global temperatures, say health experts, will lead to a broader distribution of larvae and thus of malaria.

Studies of Culex tarsalis, the main carrier of Western equine encephalomyelitis and St Louis encephalitis in California, show that increasing temperatures cut short the egg incubation period. There is, however, a fortunate tradeoff -- an increase in temperature by 10C leads to a 1 per cent increase in mortality each day.

Global warming will undoubtedly have a major impact on life in the sea, increasing the risk of new diseases emerging. Scientists fear that with a hike in sea temperatures, the frequency and spread of coastal algal blooms -- a sudden overgrowth in phytoplankton and zooplankton -- is likely to increase. Not only do these algal blooms bear biotoxins that can poison fish and shellfish, they also harbour disease-causing bugs like the cholera bacteria (Vibrio cholerae), with zooplankton doing the most harm.

Already scientists have ample evidence to show that an increase in sea surface temperatures associated with the El Nino ***phenomenon -- a warm current that flows southward along the coasts of Ecuador and Peru, with its temperature rising every 2 to 7 years -- can lead to a catastrophic increase in algae. This warming decreases the amount of dissolved oxygen in the water and increases the rate of photosynthesis and metabolism of plankton, which release vast quantities of poisonous acids, like domoic acid. In 1987, in Canada, five people died after consuming mussels from a region of algal bloom.

Scientist Paul R Epstein of the Harvard Medical School and his colleagues fear that the recent spread of a modified bacteria known as V cholerae 0139, which emerged among coastal dwellers in India and is now spreading to other parts of Asia, may be linked indirectly to changes in sea surface temperatures.

Recent studies have also raised fears that global warming could seriously affect food production. According to the US Environment Protection Agency, if carbon dioxide levels in the atmosphere double, global cereal production will decrease by about 5 per cent. Ironically, say Martin L Parry of the University of Oxford and Cynthia Rosenzweig of Columbia University, climate change will increase cereal production in the developed world, but will reduce it in developing nations. Cereal prices and the population at risk from hunger in developing countries will increase. Physicians are also concerned about the possible increase in heat-related deaths of the elderly.

No resistance
The destruction of the ozone layer frees the entry of ultraviolet radiation, known as UV-B, now known to harm the skin and cause cataract. Amminkutty Jeevan and Margaret L Kipke of the department of immunology at the University of Texas say that ultraviolet radiation could knock the human immune system out of kilter. And some skin cancers are believed to result from UV radiation-induced modifications to the cell DNA.

Studies on mice over the past 15 years reveal that they lose the ability to mount an immune counterattack on UV-induced skin cancers. Scientists now know that this is due to the activation of certain white blood cells that suppress the ability of the body to recognise and fight the foreign proteins produced by the cancers. UV also raises the possibility of weakening the immune response to infection. Jeevan and his colleagues have found that if the herpes simplex virus is injected into a mouse whose skin has been exposed to UV radiation, the resistance of the mouse to the initial infection decreases, and UV goes on to induce the activity of suppressor white blood cells, or lymphocytes. Similar results have been obtained in case of leishmaniasis-causing Leishmania major.

A single high dose of UV radiation accelerated the rate of death from a Mycobacterium lepraemurium ***(?) and Candida albicans ***(?) infection. UV-irradiated mice infected with an AIDS-like virus died faster than those which were clean. Although data from studies on humans is still insufficient, lab studies suggest that a raised UV-B dosage could increase the incidence, severity or duration of infectious diseases.

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