article: 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,
summary: The former head of the Downing Street policy unit, Camilla Cavendish, assesses government efforts to control traffic pollution.