Whether it is summer or winter, checking your air filter is very important. When the filter gets a build-up of dirt (on the outside coils) and those coils get wet the conditions are just right for mold spores.  These spores can get blown all over your home.  Individuals with allergies will suffer and not know why. Also, if left unchecked, a build up of dirt on the air filter can cause the entire system to break down.  Make sure the air filters are clean and free of dirt and change the filters on the inside of your air exchanger according to the filter instructions.  The hepa filters usually indicate the need to change every three months. Regular "blue" filters, however, should be changed monthly.  Then we can all breathe easier!