Guardian: When Justin Bieber collapsed last week at the O2 arena in London and was taken to a private clinic feeling "short of breath" and needing oxygen, the rumours started flying that he had had an asthma attack. They were denied by his management, but it would have been understandable if he had. Most of last week, London's air was heavily polluted, with many of the capital's pollution monitors recording "high" nitrogen dioxide (NO2) levels as an acute photochemical smog of fumes and microscopic particles...