Favourable weather and good planning avoided major air pollution problems during the Olympics, prompting the University of Manchester to suggest that the London Games may have been the least polluted Olympics in recent times. Good news, but the Games were not pollution free.
In the final four days of the Games, air began circulating over England and the near continent causing increases in ground level ozone. Increased air pollution presents a health risk to us all but it can be a much bigger problem for marathon runners due to sheer volumes of air that they breathe during the race. By scheduling the race in the morning the organisers avoided the afternoon peak in ground level ozone, and pollution was below the World Health Organisation’s guidelines.
via Pollutionwatch: London Games least polluted Olympics in recent times | Environment | The Guardian.