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Throughout the developing world, smogs in many big cities are getting worse as more people use cars and more manufacturing firms are belching out pollution. Congestion is on the rise too: according to one estimate, a car in Bangkok now spends the equivalent of 40 days a year stuck in traffic. Environmentalists in the developed world also worry about air pollution in poorer countries, not just out of the goodness of their hearts but because they fear it may affect their own backyard. The biggest causes for concern are indoor air pollution, lead emissions and small particles.

ON COLD days in Delhi, the poor light bonfires of tyres, trees and rags whose fumes mix with the exhaust from the city's 2m vehicles to form a thick smog. On most days in Mexico city, a blanket of pollution cuts off views of the surrounding mountains. On one famous occasion it got so bad that birds fell dead out of the sky on to the Zocalo, the city's main square. Throughout the developing world, smogs in many big cities are getting worse as more people use cars and more manufacturing firms are belching out pollution. Congestion is on the rise too: according to one estimate, a car in Bangkok now spends the equivalent of 40 days a year stuck in traffic. The air in Asia's cities, like the water in its rivers, is particularly unhealthy, containing levels of dust and smoke several times higher than in the rich countries' cities.

Environmentalists in the developed world also worry about air pollution in poorer countries, not just out of the goodness of their hearts but because they fear it may affect their own backyard. Carbondioxide emissions, thought to be the cause of global warming, are growing particularly fast in developing countries. So are emissions of sulphur dioxide, blamed for acid rain which sometimes falls hundreds of miles from the source of the pollution.

But the harm that air pollution causes in the developing countries themselves is much more serious and immediate. The biggest causes for concern are indoor air pollution, lead emissions and small particles. Indoor air pollution in poor countries is not much talked about, but it is often as damaging to health as smoking cigarettes. Around a third of all energy consumed in developing countries comes from wood, crop residues and dung, which are often burnt in poorly designed stoves within ill-ventilated huts. Studies of women in India and Nepal exposed to smoke from such fuels show that their death rates from chronic respiratory disease are similar to those of heavy smokers.

Lead has long been known to be dangerous in large doses: some historians have argued that its use in piping and amphorae in ancient Rome caused many emperors to go mad, accelerating the collapse ofthe Roman empire. But only since the 197os have scientists been aware that relatively small quantities of lead in the bloodstream can be harmful to humans. In particular, many studies show a correlation between levels of lead in children's blood and lower IQ scores, hearing loss and hyperactivity. In cities in developing countries, people take in lead mostly by breathing in air polluted by burning leaded petrol.

But the kind of air pollution thought to cause the most damage to human health in developing countries is that from small particles (of less than lo microns in diameter, known as PM10). Caused by vehicle exhausts, coal-burning, smoke from factories and dust stirred up by vehicles, these particles easily find their way into people's lungs. Although doctors disagree about the precise mechanism by which they cause illnesses, studies the world over have shown a strong positive correlation between levels of PM10 in the air and death rates.