Rapidly growing cities, more traffic on roads, use of dirtier fuels, reliance on outdated industrial processes, growing energy consumption, and the lack of industrial zoning and environmental regulations are continuing to contribute to falling urban air quality and deteriorating public health. Much of the world’s population lives in cities where pollution levels exceed recommended WHO guidelines, thereby exposing inhabitants to sub­stantial health threats. More than 1200 million people may be exposed to excessive levels of sulphur dioxide; 1400 million people to excessive levels of SPM and smoke; and 15-20 per cent of North American and European urban residents could be exposed to excessive levels of nitrogen dioxide. The air in half of the world’s cities contains excessive levels of carbon monoxide; and people in up to a third of all cities may be exposed to excessive lead levels. There are many different pollution control options available: using clean­er fuels, cleaning fuels before and during combustion, purifying emissions, encouraging energy conservation and, perhaps most important, planning carefully. But controlling pollution is not only expensive but also a challeng­ing task: how to encourage industrial development while imposing tighter pollution control regulations? The following conclusions have emerged from recent studies: 1.

Pollution-control strategies are beginning to take effect in many indus­trialized countries, where trends in S02, SPM and lead emissions have been generally downward; 2. Controls on CO and NOx have been less effective, mainly because both pollutants are produced by road traffic, which is increasing everywhere; 3. Data from developing countries are incomplete, but indicate that emissions of all five pollutants are growing; 4. There are many cities in developing countries where pollution levels still exceed WHO guidelines. By the year 2005, every second person on Earth will be an urban resident, and by 2025, the total urban population in developing countries will have more than doubled to 4050 million. Against a prevailing background of rapid urban growth and variable treatment of pollution problems, the future health of the majority of the world’s urban residents is likely to be in jeopardy. Sulphur compounds give the earth’s atmosphere a kind of built-in ther­mostat. They scatter sunlight back into space before it can contribute to global warming.

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However, the problem is complicated by the action of sulphate aerosols which are distributed unevenly in the world and which have no effects during the night. Eliminating sulphur emissions can be effective in accelerating the warming by greenhouse gases.