If too little outdoor air enters a home, pollutants can accumulate to levels
that can pose health and comfort problems. Unless they are built with special
mechanical means of ventilation, homes that are designed and constructed to
minimize the amount of outdoor air that can "leak" into and out of the home may
have higher pollutant levels than other homes. However, because some weather
conditions can drastically reduce the amount of outdoor air that enters a home,
pollutants can build up even in homes that are normally considered "leaky".