HEPA air filters are excellent for keeping your air free of particulates, unwanted bacteria, and mold spores. If you're using a HEPA air purifier, you should change the filter at least once a year. If you live in a city or other polluted area, you should replace it more often. If you notice a musty smell when you turn on your air purifier, you must replace it to avoid mold growth. Replacement filters are a must if you are storing prized items that need to be preserved or if you care about the air you breathe.