Breathers, also known as emission control filters, filter fresh air that's drawn into the engine's crankcase. Because the engine must breathe to purge itself of fuel and water vapors, breathers are used in conjunction with the PCV valve. Although small in size, Purolator breathers are big in importance. Like any other filter, the breather is used to trap harmful contaminants and prevent them from damaging the engine. Failure to change the breather will allow unfiltered air (with all of its contaminants and dirt) to be drawn into the engine crankcase which can cause engine damage.