From Kathleen MacNaughton, R.N.,
Your Guide to Asthma.
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About.com Health's Disease and Condition content is reviewed by Sanja Jelic, MD

A. Your environment also seems to have some impact on whether you will develop asthma. This is because repeated exposure to certain substances that act as asthma triggers over time can raise the sensitivity of your airways and result in asthma symptoms.

In people who don't have asthma, exposure to such triggers produces no real response in terms of breathing. But in people at risk for asthma, the trigger causes your body's immune system to respond to the substance as though it were a threat to your health. As a result, your immune system cells set off a series of reactions in an attempt to "fight off" this threat.

The reactions cause your airways to narrow and tighten. Extra mucus is also produced, which makes it even harder to breathe. The result is an asthma attack.