Air pollution: How polluted is central London?
3 April 2014 Last updated at 10:06 BST
Andrew Grieve, who is the senior air quality analyst at King's College, London, talks to Carol Kirkwood about pollution levels in the city.
High levels of air pollution will continue to affect parts of the UK on Thursday, experts have warned.
Environment department Defra said parts of eastern and north-west England, the Midlands, north Wales and south-west Scotland would be the worst-hit.
It said London and the South East had reached the maximum level 10 - meaning very high - on Thursday morning.