Outdoor triggers are substances normally found outside that can set off asthma symptoms in sensitive people. Typical outdoor triggers include: dust, dirt, diesel emissions, smoke, air-borne particulates, garbage storage, garbage disposal, pollen, smog, ozone, heat, cold, factory smelter emissions, volatile organic compounds, many gases including sulfur dioxide and the nitrogen oxides.
Outdoor triggers are extrinsic (outside the body).
Pollen is an airborne allergen that may set off numerous asthma symptoms in sensitive people at certain times of the year. Pollen consists of tiny, egg-shaped male cells found in flowering plants. You may know pollen better as the tiny, powdery granules that plants use during the fertilization process.