Different kids with asthma have different triggers - things that set off asthma flares. There are many different kinds of triggers. A lot of kids have asthma flares when they are near furry animals. Cats and dogs both have what's called animal dander in their fur. This is sort of like dandruff, and it's a trigger that can cause a powerful reaction in the airways.
Smoking is always a bad idea, and especially around someone who has asthma.
Sometimes an infection can be a trigger and set off an asthma flare - if a kid comes down with a cold or the flu, her airways may become more sensitive than usual. In some kids, cold air itself can cause an asthma flare, and so can exercise. In fact, some kids have what's called exercise-induced asthma. This means that their airways are sensitive to exercise only - they could pet a dog and smell cut grass without a problem. But if they try to run a race without preparing, they could have an asthma flare.