If you have an air purifier at home, you get a gold star. “HEPA filtered air cleaning devices—the most efficient kind—are important because according to the EPA, indoor air quality is 25 to 100 times worse than outdoors,” says Philip Tierno, PhD, a microbiologist with New York University. In fact, the average 1,500 square foot house can accumulate 40 pounds of dust (40,000 dust mites per ounce). “One in five Americans suffer from allergies or asthma, which can be exacerbated by dust, mold, and bacteria in the air so a good filter system goes a long way,” he adds. Just don’t forget to replace the filter every so often or you could actually be growing mold and bacteria, and blowing contaminants back into your air. How often depends on what kind you have, so check with your manufacturer and use common sense. One telltale sign it needs to be tossed is a musty smell.