Last week, the Mayor of London issued his first "black alert" warning about air pollution. With pollution levels as high as they've been for five years, Londoners were advised to limit the amount of time they spent exercising outside.
Today's pollution is usually invisible. But standing outside Sir John Cass Foundation School in the City of London, where some younger pupils had restricted time in the playground during the high pollution episode, we found parents who feel they can smell and taste it.
"It's a sad state of affairs when you've got to tell your kids not to breathe too deeply," said one father of two. "Sadiq Khan (London's Mayor) really does need to ban diesel cars."
Another father drives a black cab: "I drove in towards the school, and looking towards the city you couldn't see the top of the Gherkin. I was thinking that can't be fog, must be pollution.
"I can't wait until they bring out hybrids or something because I feel I'm contributing to the pollution obviously. So I'm doubly worried about it."
I took a close interest in this problem when I was head of the Downing Street Policy Unit under David Cameron. One of my children is asthmatic, so I have followed the growing health evidence  - and the court battle.
In November, after a series of challenges from an environmental law firm, the High Court ruled that the government must produce a credible plan to stop breaching air quality standards, by July of this year.
Many things contribute to air pollution: central heating, wood burners, generators. But there is growing concern about diesel emissions from traffic.
The government wants five major cities - Birmingham, Leeds, Southampton, Derby and Nottingham - to create clean zones which would charge some of the most polluting vehicles. In addition, London is creating an ultra-low emissions zone. But will this work? And will it be enough?
In 2012, the World Health Organization classified outdoor air pollution as a cause of cancer.
The UK government estimates that the harm done by particulates, tiny particles we can't see, in shortening our lives is equivalent to 29,000 premature deaths a year. Add in nitrogen dioxide emissions, which, like particulates, are partly generated by diesel traffic, and that figure rises to about 40,000. This would make air pollution more deadly than obesity and alcoholism combined.
We asked Frank Kelly, prof of environmental health at King's College London, what 29,000 premature deaths means. "Everyone in the UK from birth is losing an average of six months off their life," he says. In practice, he explained, some individuals will suffer more than that, and some less.
Prof Kelly also described the developing body of science. We have known for a long time that particulates could enter the lung. But new evidence links pollution to heart attacks, strokes, even dementia.
There is also concern about the impact on children's development. Dr Satish Rao, consultant in respiratory medicine at Birmingham Children's Hospital, told us that traffic pollution can aggravate the symptoms of children who already have respiratory problems, but that it can also affect the lung growth of those who are healthy.
In 1952, worries about pollution reached a peak with the Great Smog which is said to have killed more than 4,000 Londoners. In 1956, Parliament passed the Clean Air Act to tackle smoke and sulphur dioxide. The Act required everyone to do their bit: factories had to make their chimneys higher, householders had to burn smokeless fuels, and the government moved some power stations out of cities.
Today, many campaigners believe that we need a modern Clean Air Act with a similar combination of carrots and sticks, to tackle particulates and nitrogen dioxide.
Such an Act could clean up trains, taxis and buses, widen the network of emissions zones, and subsidise drivers to scrap older diesel vehicles in favour of electric.
We asked every campaigner we interviewed what this might cost. None would name a figure. But it would be expensive. Few people can afford to swap their diesel car or van for an electric model without subsidy.
The Mayor of London has no legal powers to ban diesel cars from the city but he hopes to introduce a £10-a-day T-charge for the most polluting vehicles - broadly those registered before 2005 - on top of the congestion charge.
Apart from charges, where else could the money been found? One option might be to unfreeze fuel duty. Last year, the Chancellor said this year's freeze is equivalent to a tax cut of £850m.
The Department for Environment, Food and Rural Affairs (Defra) has said that it will force the five cities (not London) to charge drivers of most buses and coaches and most diesel taxis, but not private cars.
However, councillor Lisa Trickett, cabinet member for air quality at Birmingham City Council, told us she thinks Birmingham may have to restrict private car use too, if the targets are to be met. That would be hugely controversial. Millions of us drive diesel cars, having been encouraged to buy them by successive governments on the basis that they have lower carbon emissions than petrol.
The headache for government is that the UK is a leading manufacturer of diesel engines so bans on diesel could have an impact on the economy. And 95% of vans are currently diesel-powered.
Mike Hawes, chief executive of the Society of Motor Manufacturers and Traders, does not think that diesel has had its day.
"Diesel is the power trend, the engine for all the commercial vehicles; it will be around for some time," he says.
"And the diesel engines of today have got immeasurably cleaner than those of yesterday. What we need to do is continue to improve them so that you can address both the NOx (nitrogen oxide) emissions, in the same way that we've addressed the particulate emissions, sulphur emissions, lead and petrol and so forth. It's an evolution of technology."
Jim Farley, chief executive of Ford Europe, told us that the company produces both petrol and diesel engines in the UK, and that as long as we have flexible manufacturing, his operations can switch between both if customers demand it.
Mr Farley also has his eye on the electric market. The UK already makes a quarter of Europe's electric cars, and the prime minister recently announced a new institute which will do groundbreaking research into batteries.
So there is hope. But the stakes are rising. Today's pollution is largely invisible. But just because we can't see it, doesn't mean we should ignore it.
Camilla Cavendish took a close interest in the problem when she was head of the Downing Street Policy Unit under David Cameron. She's been looking at the issue of air pollution for BBC Radio 4's Today programme.
The former head of the Downing Street policy unit, Camilla Cavendish, assesses government efforts to control traffic pollution.