If you exercise outdoors, you’re more exposed to more toxins in the air because you're breathing more rapidly and heavily. Learn how to gauge your city’s "pollution mood" and keep your exposure to a minimum:
Avoid exercising near areas of heavy traffic – choose quieter roads, parks or sports fields.
Early morning and evening (before and after peak traffic, i.e. before 7 a.m. and after 7 p.m.) are generally the best times for outdoor exercise. If safety’s a concern then get a group together. Still winter mornings, however, are often conducive to higher pollution levels – on such days, try exercising a little later in the day.
Wind and rain can help disperse and wash pollution out of the air, so after a rainstorm or a blustery spell is a good time for outdoor activities.
If you must exercise when pollution levels are high, decrease your workout’s duration or intensity, and rather make up for it on a less polluted day.
(Olivia Rose-Innes, Health24, February 2012)