Today's smog in London is over three times as bad as New York and two and a half times the annual average for LA, according to figures from the Department of Environment, Farming and Rural Affairs (Defra).
Globally, how does London compare? You can use this info-graphic to explore how London usual measures and Thursday’s high, compare to annual averages across the country and beyond.
How pollution is measured
The two most common measures are of PM10 particles - pieces of organic and non-organic matter measuring 1/100 of a millimetre or less - in a cubic metre of air, and PM 2.5 particles which are smaller allowing them to enter the body and be absorbed into the blood.
The annual average level of PM10 particles for London is 29, according to the World Health Organisation (WHO), but today some parts are measuring as high as 64, which is higher than cities such as Bangkok (54) and Rio (63.7).
For the UK, London (29) is consistently the most polluted city - no other metropolitan area gets within 5 PM10 particles a cubic metre of it. The city with the second highest annual count is Glasgow with 24. Birmingham lags behind on 16.
Ambient air pollution is a big problem globally: it is responsible for 3,732,500 deaths a year, according to more data from the WHO.