Tower Bridge and Canary Wharf, are just visible through the haze and smog in London, Friday, April 10, 2015. (AP/Alastair Grant)

Fact-check: Does Air Pollution Kill 40,000 Brits Every Year?

Reports state that air pollution “causes” or is “linked to” 40,000 premature deaths in the UK every year — or 29,000, or 50,000, depending on what you are reading. But can these figures be trusted — and what do they actually mean?