Once, there was a time when you could see air pollution. More than 300 years ago, the diarist John Evelyn wrote of London's envelopment in "such a cloud of sea coal, as if there be a resemblance of hell on earth". The Great Smog that killed 4,000 people in the capital in 1952 was all too visible. But the air pollution of the Nineties is more subtle and less conspicuous.
After a brief interlude in which the conquest of the old pea-soup fogs following the 1956 Clean Air Act held out the prospect of cleaner air in our cities, urban air quality is worsening again. This week's report from the London air quality network found pollution levels increased last year and that poor air quality is now a routine feature of life in the capital. The same is surely true of other British cities - London merely has the monitoring to prove it. There is equally little doubt about the culprit: road traffic.
The Clean Air Act cut smoke emissions by 85 per cent: winter visibility in London trebled, from around two to six kilometres. The chief ingredient of the pea-soupers - sulphur dioxide from the burning of coal and oil - has been drastically reduced. But as the great coal economy drew to a close, what Mrs Thatcher called the great car economy was dawning, bringing with it a cocktail of toxic gases.
Baroness Thatcher's phrase may well come to haunt her successors. Given the growing evidence about the links between exhaust fumes and heart and lung disease, the forecast doubling of traffic over the next 30 years presents a serious threat to public health, requiring urgent political action.
Traffic is the fastest-growing source of air pollution and also one of the largest. Half of the oxides of nitrogen and also of the fine black smoke known as particulates come from road transport, nine-tenths of the carbon monoxide. In cities, congestion and idling engines mean the concentrations are much higher. In London, road traffic is the source of 96 per cent of particulates, 99 per cent of carbon monoxide and three-quarters of the nitrogen oxides.
Like the ozone these chemicals generate - which produces the Los Angeles- type photochemical smogs virtually unknown in this country a decade ago - many of these "new" pollutants are scarcely visible. Yet their health effects may be as great as the pea-soupers. Most of them damage the lungs and cardiovascular system; some, such as benzene, are carcinogenic.
US studies suggest that 10,000 people in the UK may die in Britain each year as a result of particulate pollution. The worst air pollution episode in London in 20 years - in December, 1991, when levels of nitrogen dioxide reached twice the World Health Organisation's "safe" limit - probably killed about 150 people in one week, according to a Department of Health study.
Asthma is the only treatable disease in the West which is increasing. It affects one in seven British children. No one can say with certainty that the asthma epidemic is caused by rising traffic volumes but the link is strong: pollution is thought to act as a "trigger' for the disease. The tenfold increase in asthma cases last June, reported by hospitals in London and southern England, was probably due to an abnormal release of allergens produced by thunderstorms, coupled with high levels of pollution.
Yet most of us only learn of such incidents after they have happened. The 1991 episode was only publicly acknowledged by the DoH last year. Back in 1991, the department put out a statement that air quality was "very poor" but it was not expected that many people would be seriously affected.
Our system of monitoring air pollution is sparse and fragmented; standards on what constitute safe limits vary widely and the information disseminated by the Government - the Department of the Environment's free phoneline, for example - often confuses people further.
There are, for example, only three official sites monitoring nitrogen dioxide for London, one of them bizarrely situated in one of the few quiet backstreets of Victoria. Nationally, there are seven such sites, compared with 200 in Germany. And Britain's idea of what constitutes good air quality differs radically from, say, that of the WHO. Last year, the 31 official monitoring stations recorded 358 occasions when the WHO's safe limit for ozone - 50 parts per billion over eight hours - was breached. But the DoE's limit is much laxer - 90 ppb. According to the DoE, on only 30 of those 358 occasions was air quality poor.
This is why this week's London air quality study, which relies on monitoring stations set up by local authorities, says government information "lacks credibility" with the public. A welter of terms such as PM10s (particulates) or 1,3-butadiene (a hydrocarbon in car exhausts suspected of causing cancer) does little to help.
The Government announced an "air quality management strategy" earlier this year, has launched a £5m study into the links between air pollution and health and has also embarked upon highly publicised initiatives such as its "belcher blitz" on smoky exhausts. But according to groups such as Friends of the Earth, what is really needed are mandatory, and tighter, air quality standards, curbs on cars, investment in walking, cycling and public transport, and short-term powers to ban traffic during acute pollution episodes. For all their much-vaunted rethink on transport, as yet, ministers show little sign of wanting to go anything like so far.