Most modern home AC systems have, as a rule of thumb, ten percent fresh air intake. This varies, depending on the home size and state and local codes. On my home the fresh air vent is high up in my attic, which should protect me from over spraying of chemicals. However, if you have a window unit or an on ground system this will affect your exposure.

In an effort to save money, some will close their fresh air vent, this is not recommended because of the lack of air trade out in the system.