an air filter for a well is installed within a well casing of the well . the air filter forming an airtight seal around the well casing of the well , such that all the air that enters and exits the well must pass through the air filter . the air filter preferably includes a filter housing , at least one removable gasket and a removable filter cartridge installed within an opening extending through the filter housing . the air filter prevents dirt , dust , bacteria , gaseous chemicals , volatile organic compounds , , insects and other air born contaminants from entering and possibly contaminating the well . an air filter may also be installed within a vent pipe extending through a well seal attached to the top of a well casing of a well .