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Millions of people have breathed in air contaminated with record-breaking levels of a harmful pollutant for which there is no known safe exposure.

Every air quality monitoring site in London hit “high” or “very high” during the smog episode from Thursday March 10 to Sunday March 13.

The levels are the worst since records began.

More than 10 million people live or work in the capital and millions more Brits visit every year.

Read more:Air pollution 'is a direct cause of asthma and heart attacks'

The tiny particles, known as PM2.5, are particularly dangerous because they can penetrate deep into lung tissue and are too small to be filtered out by the body’s normal defences.

(Image: GEOFF ROBINSON PHOTOGRAPHY)

The spike in smog was caused by dirty air from industrial areas of Germany, Holland and Poland.

The peak day was the Saturday when 11 out of the 18 measuring sites in London hit “very high” with the rest on “high”.

These were the worst readings since PM2.5 started to be measured in 2012 because of growing concern about its impact on health.

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Prof Sir Malcolm Green, founder of the British Lung Foundation, has described breathing in high levels of PM2.5 as “like inhaling little particles of tar. They go right down into the lungs and can pass through the membrane into the bloodstream.”

Simon Birkett, founder of the Clean Air in London campaign, said: “It’s a national disgrace.”