If the evaporation coils get dirty, their performance is reduced. Air blowing across the coils may contain dust, pollen, and other particles. Since the coils are normally damp from the draining process, contaminants adhere very easily to them. Dirty air filters in the system (or the complete lack of filters) can increase the amount of material that comes into contact with the evaporator coil. In just a short time, enough material can accumulate on the coils to affect their performance.