How do I match an air purifier to my room size?

Look at the CADR (Clean Air Delivery Rate) and the air changes per hour, not just the marketing "square feet" number. Manufacturer coverage claims often assume only one or two air changes per hour, which is too low for allergies or smoke. For allergy and pet households, aim for 4–5 air changes per hour, which usually means buying a unit rated for a room two to three times larger than the one you're using.