Smog: A Problem the Chinese Will Find Hard to Eradicate

China's air-quality crisis can be compared in some respects to Britain's more than half a century ago, before the Clean Air Act of 1956. But there are important differences that could make China's problem more difficult to tackle.

China, like Britain in the past, is heavily reliant on coal-burning power stations for generating electrical power. However, Beijing has more traffic than London in the 1950s and its vehicles are powered by low-grade fuel that produces a toxic mixture of sulphur and nitrogen oxides as exhaust emissions.

In Britain, the Clean Air Act came about in response to the notorious "pea-souper" fog that had periodically plagued London since the Industrial Revolution. The great smog of 1952, which precipitated the Act, is estimated to have killed up to 12,000 people and became a national scandal.

In Britain's case, the problem of burning coal in cities was solved relatively easily with help of North Sea natural gas and a policy of moving coal-based industries away from heavily populated areas. China is trying to do something similar, but it still has to deal with the exhausts of millions of cars and lorries that are burning fuel with a high sulphur content.

Smog, which originally meant an amalgam of smoke and fog but now includes the exhaust emissions from cars, is made worse by the kind of weather conditions seen in Beijing this week with cold, windless days allowing pollutants to build up to dangerous concentrations.

On top of this sits a temperature inversion, where instead of getting colder it gets warmer above ground level. This creates an invisible "lid" that prevents the pollutants from escaping into the upper atmosphere where they can disperse.

Diesel engines in particular are prone to producing fine particles of "black carbon", or soot, less than 10 micrometres in size, which can be small enough to enter deep into the respiratory pathways. Particulate matter smaller than 2.5 micrometres is particularly hazardous as it can carry toxic compounds deep into the lungs, making breathing difficult and increasing the risk of heart problems.

Coal-burning power stations on the fringes of Beijing and vehicle exhausts within the city increase concentrations of nitrogen oxides that are linked with emphysema and bronchitis. They also produce sulphur dioxide, which is known to trigger asthmatic attacks.

Environmental measures designed to curb traffic emissions, such as exhaust fitted with catalytic converters, have had a great impact on air pollution in the West. But even in California, the richest state in America, smog is still a hazard.