One in three cars in Britain runs on diesel, and 95% of diesel cars emit more nitrogen oxides than is legally allowed, according to tests done by consumer group Which? (Fifteen 15 times more in the case of the Jeep Grand Cherokee, 47 of which I found for sale secondhand within 15 miles of my home when I searched.)

Two-thirds of petrol cars break the carbon monoxide limit, and 10% break the nitrogen oxides one too. In other words, forgetting for now the other problems caused by cars – road crashes, climate change – most of the vehicles that drive past me and my children on our walk to school and back are poisonous.

Why bring my children into this? Because air pollution – in the memorable phrase of Prof Chris Griffiths, who shared the preliminary findings of a study with Channel 4’s The Great Car Con last week – causes children growing up in polluted areas (of which my London borough is one) to develop “smaller, stunted lungs”.

It also causes people to die from heart attacks, strokes and lung cancer, and exacerbates other lung diseases and asthma. Usually it’s not the only cause, but air pollution is a factor in at least 30,000 deaths each year in the UK, although scientists are struggling to disentangle the damage caused by nitrogen dioxide from that caused by particulates, or soot. By way of comparison, in the UK there are up to 100,000 smoking-related deaths each year, nearly 9,000 alcohol-related ones and about 1,800 people killed in car crashes.

Why are these theoretically illegal cars on the roads? Because even when manufacturers don’t actually cheat with “defeat device” software, as Volkswagen did, European emissions tests are inadequte and don’t measure performance in the “real world”.

Change is on the way, though a victory by car manufacturers this week means new standards have already been diluted. But there’s another reason why our air is so filthy, and recognised as such by the government, the supreme court and the European commission (both of which have ordered the UK to take action), London’s mayoral candidates, thinktanks, and everyone else who knows anything about it. It’s because people carry on driving.

I used to own a car and had a brilliant time driving around Devon in a hired one last summer. I know that if you live in an area with poor public transport, or with old or disabled family members, a car can be not just useful but necessary. But why is it that when we are told to cut down on bacon, wine and sugar, we aren’t told to cut down on cars?

Last week newspaper columnists across the land, along with Jeremy Corbyn and the BBC’s Question Time, weighed in on the question of whether it’s OK to take your kids to school wearing pyjamas. Last month, when it was reported that almost all diesel cars breach limits and the World Health Organisation called air pollution a global “health emergency”, no senior figure (no leading doctor, government scientist, minister or the London mayor) saw fit to ask: is it OK to drive to school?

In October it became illegal to smoke in a car with a child, while Dame Tessa Jowell, runner-up in the Labour mayoral selection contest, made a big thing of her plan to ban smoking in parks. Yet when there is a bad pollution episode no one tells people to stop driving, as the mayor of Paris did last year. Instead, people with asthma are warned not to go out. It’s as if, rather than banning smoking in public places in England in 2007, the government had advised people wishing to avoid lung cancer to stay away from pubs.

If this makes a kind of sense for rightwing haters of the so-called nanny state, for whom cars are totems of individual freedom, it makes much less for social democrats. Policy Exchange’s head of environment, Richard Howard, says it all comes down to “political acceptability”. At 54% even in London, car-owning households are in the majority, and while the rules are to be tightened in cities including Birmingham and Leeds as well as the capital, for now it seems that telling unemployed parents off for being badly presented, or pregnant women for drinking, or fat people for being fat, are all more acceptable than asking the drivers of Britain’s 30m cars if sometimes, instead of driving, they might get on their bikes.