On Friday 5 December 1952, a thick yellow smog brought the capital to a standstill for four days and is estimated to have killed more than 4,000 people. London’s air may appear much cleaner today, but is still dangerously polluted. The coal pollution that caused the infamous ‘pea soupers’ has been replaced by invisible pollution – mainly from traffic fumes – resulting in 13,000 early deaths each year in the UK and 4,300 in London