In addition to HEPA, you’ll also see activated charcoal or carbon filters on our list. Carbon is effective in removing smoke particles, gasses, and odors from the air—just note that, while these purifiers may pack an extra punch, they’ll likely come with a higher price tag.

Cost of filter replacements

Because purifiers require a filter change every 3 to 12 months (the time frame varies by brand and how often you run your device), you’ll want to consider the cost of new filters on top of the machine’s cost. Each brand has its own type of HEPA filter, and they all come at different prices—some brands even offer a subscription discount for scheduled refills.

Room size

It’s important to consider the size of the room when choosing a purifier,” says Mehrotra.The filtration process can vary from product to product, but the goal of air purification is about five or six air-changes per hour in the room, a rate that is sufficient to decrease particle levels in the room by about 70%.”

One way to gauge the effectiveness of a purifier is the product’s CADR, or Clean Air Delivery Rate—a measurement developed by the Association of Home Appliance Manufacturers (AHAM) in the 1980s. “CADR measures the amount of particle-free air being delivered into the room,” explains Mehrotra. “It reflects both how effective the filter is and the amount of air going through the filter. The higher the clean air delivery rate, the larger the size room that can be properly cleaned.”

Mehrotra adds that a good CADR rating is equal to at least two-thirds of the room’s area in square feet. For example, a room that measures 10 feet by 15 feet has an area of 150 square feet. For this room, a good CADR rating would be at least 100 cubic feet per minute (cfm).

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