When evaporator coils get dirty, their performance in these two functions is significantly reduced. Air blowing across the coils can contain dust, pollen, and other particulates. Since the coils are usually damp from the dehumidification process, it is very easy for contaminants to stick to them. Dirty air filters in the system (or the lack of filters entirely) can increase the amount of material that makes contact with the evaporator coil. In just a short amount of time, enough material can accumulate on the coils to affect their performance.