Air purifiers are best for people who are sensitive to indoor allergens, like dust mites, animal dander, or molds, explains Miguel Wolbert, MD, a board-certified allergist in Midland, Texas. They won't necessarily fix all of your allergy issues all the time, but they can certainly help. For example, if you come and go during the day (to go to work or run errands), then you're constantly changing your environment from "outside air" to a "filtered environment," so it's not going to make a huge difference, he says. However, when you're home for prolonged periods of time, like while you're sleeping, then air purifiers can effectively clean the air, and possibly keep your symptoms under control.
Source : http://www.refinery29.com/2017/09/169210/best-air-purifiers-reviews-modern-style