An asthma trigger is any object, act or event that makes asthma worse. These can be exercise, viral infections, air pollutants such as cigarette smoke or perfume, cold air, grass, animals, mold and pollen, coughing, yelling and some medicines and chemicals. 

Asthma triggers can be avoided by not allowing smoking in your house or car and removing dust catchers from the bedroom (stuffed animals or toys, carpets, drapes, etc). Heating vents can be covered with an air filter. The humidity in the house can be kept between 25 and 50 percent. (Humidity over 50 percent encourages the growth of mold and mites.)

Although exercise is the most common trigger of asthma it should not be avoided. Exercise increases stamina and lung capacity and should be encouraged unless otherwise directed by your doctor. Almost every person with asthma should be physically active and able to play any sport.

This information is taken from One Minute Asthma by Dr. Thomas F. Plaut