IT MAY have been a mild winter so far, but it has been a hard one - and a hard year - for the millions of Britons who now suffer from asthma and other breathing difficulties. For smog has returned to our cities.

After 30 years of clear skies, murky brown photochemical smog has succeeded the pea-soupers that cloaked London for generations before the Clean Air Act took effect. The big picture of the capital on the right was taken on 9 December, at the height of the smog season (contrasting with the clear conditions, above, of l e ss polluted days). This picture marks two grisly anniversaries. Almost exactly 42 years before, from 5-8 December 1952, the Great London Smog - the worst of the pea-soupers, formed by a mixture of soot and dilute sulphuric acid from burning coal - killed 4,000 people over a weekend. And from 12 to 15 December 1991 a blanket of photochemical smog, thick with nitrogen dioxide from vehicle exhausts, killed 160 people in the capital.

This year, a fortnight after the picture was taken, the worst smog since the 1991 disaster settled over London, Birmingham, Walsall, Leeds, Manchester and other towns and cities. Nobody yet knows how many died or became ill over the black Christmas.

Winter smogs occur in still weather when pollution is caught in a "temperature inversion": cold air is trapped near ground level by a layer of warmer air higher up - conditions that are particularly likely to occur in December. But recently there has been no escape in summer either. During last year's heatwave, high levels of nitrogen dioxide plagued British cities. And sunlight converted the gas into the equally dangerous ozone, which drifted on the breeze to take the smog deep into the countryside. InJune, southern and central England suffered the worst outbreak of asthma ever recorded anywhere in the world .

"The fogs," said the Times, in a whimsical but wrong-headed fourth leader at the outset of the 1952 calamity, "are ancient Britons ... They roam about on their little cat feet as freely as they did before anyone had heard of smoke abatement." The smogs -

and official complacency about them - certainly have a long history. The first commission to investigate air pollution from coal-burning met in 1285, and sat fruitlessly for a quarter of a century. Elizabeth I, "greatly grieved and annoyed with the taste and smoke", banned coal-burning in London for a while - but only when Parliament was sitting. John Evelyn, the diarist, raged against the pollution which made "Catharrs, Phthisicks, Coughs and Consumptions rage more in this one City than in the whole Earth besides", and submitted a clean air plan - in vain - to Charles II. The smogs survived to swirl through the pages of Dickens and Conan Doyle, and to haunt the canvases of Monet and Turner.

Just before the Great London Smog, Harold Macmillan (then the equivalent of today's Environment Secretary) twice rejected clean-up measures, saying, in words that echo those of today's ministers, that governments cannot "solve every problem". Despite hi m , the 1956 Clean Air Act was passed, and the amount of December sunshine in London increased by 70 per cent. But the old complacency soon returned. Mrs Thatcher's first government scrapped the official air pollution research teams, on the grounds that th e problem was solved. A decade of research was lost - the very decade in which the new photochemical pollution was building up. The smog will no doubt return this summer. !