Opening your windows often, even in the winter improves indoor air quality and it is especially important to do during cold and flu season. Air purifiers require a lot of energy to run, and typically because of their design do not remove some indoor pollutants. It is not necessary to leave the window open for a long time, so bundle up for a little bit and let the breeze come in. Adding plants to a room also help indoor air quality by absorbing carbon dioxide and replacing oxygen.