Such talk is alarmist, and probably unwarranted. After all, compared with London in the old days, before the Clean Air Act banned coal smoke and pea soup was something we breathed rather than something we ate, we are enjoying air of lavender-scented purity. Yet there's no doubt that the air in major British cities could be much sweeter. I got thinking about 10 easy ways to make breathing better:
1. If building more roads encourages more cars, why shouldn't the same rule apply for bicycles? More cycle paths should mean more bicycles - and we're talking about bikes being used by ordinary people, rather than the Lycra-clad, multi-coloured health fetishists who jump red lights and terrorise innocent motorists and who use cycling as a jogging substitute rather than as transport.
It's reckoned that about 60 per cent of all car journeys in Britain are under six miles long. The bicycle is an efficient, convenient mode of transport for such journeys - just ask the Dutch. Bespoke cycle paths, in some cases built by shaving the width of roads, would encourage confidence in the safety of cycling, the current major bugbear.
2. Encourage car pooling. In many foreign cities, cars with three or four people on board can use bus lanes during the peak hour. Small cars, filled to capacity, are very energy-efficient modes of transport.
3. Dissuade businesses from issuing company cars, the biggest single culprit in the peak-hour traffic gridlock. Company car drivers like to enjoy their perks. If they didn't have the perk, they'd probably go by train.
4. New, or newish, cars fitted with catalytic converters are much cleaner than old cars without. There should be tax incentives to encourage people to buy catalyst-equipped cars. Increasing the road tax on non-cat cars (possibly to £200), and cutting it for cat cars (to £100) would be one way.
Diesel cars should also be targeted, as diesels spew out higher levels of oxides of nitrogen and particulates (minuscule particles of soot, said by some to be carcinogenic), both of which are closely linked to increased levels of asthma. Old diesels are particularly noisome.
5. Get old "smokers" off the road. The French and Spanish governments offer incentives for people to trade in their 10-year-old bangers for much cleaner new cars. In France, owners of cars more than 10 years old who buy new cars get a 5,000-franc (£625) rebate from the government. These old cars must not be re-registered. Classic car owners need not fear. Dealers aren't stupid: they won't scrap valuable old cars.
6. Get tough on owners who fail to maintain their cars properly. There are some roadside pollution checks in Britain, but it's still small scale. If hefty fines greeted owners of filthy cars, the crackdown would soon be self-financing, possibly even profitable. At the same time, make the exhaust emission MoT test tougher. It's currently a joke.
7. Buses are not always greener than cars. A belching bus is one of the biggest environmental disasters on the road, as your eyes and nose will testify. Our urban buses almost invariably use old-technology diesel engines, and spew out more toxins (particularly those that lead to respiratory problems) than scores of new catalyst-equipped cars combined.
When its characterful old Routemasters were recently fitted with "new" Iveco engines, London Transport chose old-tech units, which failed to meet the latest EU diesel regulations. New-tech diesel engines should be phased in soon. Equally, taxis are much dirtier than they should be.
8. Crack down on poorly maintained lorries. They are health risks, and should be treated a such. Trucks that fail to meet the latest (1993) pollution regulations should be forbidden from entering major conurbations.
9. Americans already have cleaner burning petrol that has been specially formulated for smelly cities. Why can't we have it?
10. Encourage people to live in cities (again) instead of merely driving through them. The trend for people to live in the country but work in the city is one of the most environmentally damaging facets of the last 20 years - whether they commute by car or train. It leads to city degeneration and massive damage to the countryside because of new housing and transport requirements. Personal tax rates could alter depending on how close you live to your place of work. The closer, the less you pay. This might help cut down on the massive jams into, and out of, major cities every day - both on the motorways and in the trains.
Finally, let's be realistic about the potential for public transport. Everybody wants more spending on buses and trains so somebody else can use them. Cleaner, cheaper, more regular public transport may tempt some people out of their cars - particularly for commuting to work and for journeys within city centres. But most of us will stick to private transport for many of our town journeys - particularly from suburb to suburb. It is, after all, often more convenient and invariably more comfortable, as well as being frequently more energy efficient.
The car - or in the long term some other form of public transport - will always figure in the urban transport picture. Town planners who gainsay that will doom great cities such as London to economic bankruptcy (just ask any shopkeeper) and tedium. And then we'll all move out to the country and use out-of-town shopping malls, and environmental Armageddon will finally be with us.