Your home was built to help keep you warm during chilly weather by holding in heat. Unfortunately, sealing off your home to protect against the cold traps moisture and raises humidity levels, which can lead to problems with mold and mildew and allow dust mites to thrive. Beyond controlling sources of pollutants and ventilating your home with clean outdoor air, an air purifier will help improve the air quality in your home. For dealing with particulate matter, such as mold, mildew, dust mites and animal dander, look for a quality HEPA (high-efficiency particulate air) filter. And as it warms up, open your windows to help circulate fresh air.