@shibao

Causes

It's not exactly clear what causes exercise-induced bronchoconstriction. For a long time, the cause was thought to be cold air. However, recent studies found dry air to be a more likely culprit. Cold air contains less moisture than warm air. Breathing in cold, dry air dehydrates the air passages. This causes them to constrict, reducing air flow. Other factors, such as chlorine or other fumes, can irritate the lining of the airways and contribute to breathing difficulties as well.