Indoor air can be two to five times as dirty as outside air, so it makes sense that interest in air purifiers spikes in winter, when cold weather keeps us indoors with the windows shut. But we’re just as likely to be shuttered inside in hot weather with the air conditioner cranking. To clear the air, more and more consumers are turning to air purifiers, according to Gfk, a market research firm. But unless you have allergies, you probably don’t need one, and the Environmental Protection Agency does not endorse manufacturer claims of healthier indoor air resulting from the use of air purifiers.

Norman Edelman, M.D., a professor of preventive and internal medicine at New York’s Stony Brook University, agrees that keeping indoor air clean is the best strategy but adds that it can be difficult to keep all allergens and particulates out of the house. “The data on air purifiers are not clear,” he says. “When used properly, they have been shown to modestly reduce allergy symptoms.”

Air purifiers aren’t cheap and can be expensive to operate because they typically run around-the-clock (see chart below). And they have limitations in terms of their primary job. For example, an air purifier can remove only airborne particles, not the dust and pet dander that has already settled into the carpet. “For that you’ll need a good vacuum,” says Dave Trezza, who has tested both appliances for Consumer Reports.

Finally, our tests found that air purifiers can be noisy, especially at higher speeds, and may keep you awake if you use one in your bedroom.