The number of London schools with illegally toxic air has dropped dramatically since 2016 – down 95 per cent in four years.

More than two million Londoners lived in areas over the legal limit for air pollution four years ago, but that figure is down 94 per cent to 119,000 people.

Just 25 schools had illegal levels of dirty air last year – dropping from 671 in 2016 – and there are now no outer London schools in highly toxic areas.

Six inner London boroughs – Lambeth, Westminster, Tower Hamlets, Camden, Hammersmith and Fulham, and Islington – still have state schools that breach the limits.

The main pollutants in the capital are nitrogen dioxide, a byproduct of diesel engines, and particulate matter, micro-dust that can penetrate the lungs and enter the bloodstream.

Both can cause stunted lung growth in children and worsen chronic illnesses like asthma and heart disease.

By law, nitrogen dioxide must not be more than 40 micrograms per cubic metre on average each year, or spike above 200 micrograms more than 18 times.

And the tiniest particulates – known as PM2.5s – cannot be above 25 micrograms per cubic metre.

But the World Health Organization believes any particulate dust above 10 micrograms per cubic metre is a health risk – and 99 per cent of London still breaches this limit.

Meanwhile almost a quarter (24 per cent) of inner London roads still have illegal levels of nitrogen dioxide.

Pollution is 16 to 19 per cent higher in areas where black, Asian and minority ethnic Londoners are most likely to live, according to a study by air quality consultants Aether for City Hall last year.

Dr Gary Fuller, a senior air quality academic at Imperial College London said dirty air has had an “intolerable” effect on Londoners “for far too long”.

But since 2016 Dr Fuller has seen a “dramatic” improvement in air quality in the capital.

“The changes in nitrogen dioxide in central London and along main bus routes before Covid-19 were some of the fastest that we’ve ever measured,” he said.

The Mayor says expanding the Ultra Low Emission Zone – the £12.50 daily central London charge on polluting vehicles – to the north and south circular roads next year will drive more progress.

Almost four million people will live within the expanded zone, and City Hall believes air quality benefits will save the NHS £5 billion and stop a million hospital admissions over the next 30 years.

Mr Khan has also introduced 12 low emission bus zones during his term, stopped licensing diesel taxis, and recently announced a lorry scrappage fund to help companies replace heavily polluting trucks.

But he warned that the country could “sleep walk” from the coronavirus pandemic into another major public health crisis without action on toxic air.

“Air pollution remains a major public health challenge and it’s time for Government to step up, set ambitious national targets and provide the powers and funding we need to consign air pollution to the history books,” he said.