Air pollution means in some cities around the world exercise does more harm than good
In cities such as Allahabad, India or Zabol, Iran long term damage from inhaling fine particles could outweigh the health gains of cycling in the city after just 30 mins. In Riyadh Saudi Arabia the time is 45 mins and in Delhi, India and Xingtai China the time is 1 hour.
A study published in the journal Preventive Medicine modelled the health benefits of active travel and of air pollution. Air quality was measured through average levels of PM2.5s (tiny pollutant particles that can embed themselves deep in the lungs). This type of air pollution can occur naturally from dust storms or forest fires but is mainly created by motor vehicles and manufacturing.
15 cities have annual mean PM2.5 levels of 115ug/m3 or above, so the break even point of exercise benefit v harmful effects of pollution is reached after 1 hour of active travel. Fine particulate levels above 80ug/m3 were found in 62 cities making cycling more harmful than beneficial after 2 hours.
The study found western cities such as London, Paris and New York do not reach the point where PM2.5 air pollution's negatives outweigh the positives of exercise in the long term. London's annual average PM2.5 pollution was estimated at 15ug/m3 by the World Health Organization (WHO) above the ideal WHO target of 10 but still at a level which the study estimates active travel is always beneficial. Paris had PM2.5 levels of 18ug/m3 and New York had levels of 9ug/m3. However the study did not consider the health impacts of short term spikes in PM2.5 pollution levels.
NOTE: The World Health Organization has published maps detailing ambient air pollution data in cities around the world. See WHO Global Urban Ambient Air Pollution Database (update 2016).