Cost of replacement filters: As a general rule, you should replace filters (or clean those that can be vacuumed) every six to 12 months if they are pleated and every three months for activated carbon filters. Most of the units we test have an indicator light that lets you know when to change (or clean) the filter. The costs of filters vary widely: In our tests of large air purifiers, they range from $20 to more than $200. Filters with odor-removing carbon can cost as much as $50. 

Clear Air Delivery Rate (CADR): This value is provided by air purifiers tested by the Association of Home Appliance Manufacturers. Many air purifiers have undergone AHAM’s voluntary certification program, which provides room size guidelines along with the CADR on its AHAM Verifide seal. The CADR reflects, in cubic feet per minute, the volume of clean air that an air purifier produces on its highest speed setting. For example, a purifier with a CADR of 250 for dust particles reduces particle levels of dust to the same concentration that would be achieved by adding 250 cubic feet of clean air each minute. The higher the CADR, the faster and more efficient the air purifier is. Room air purifiers with HEPA filters often achieve the highest CADR. There are different CADR ratings for removing tobacco smoke, dust, and pollen. Focus on the CADR for your main pollutant of concern. For instance, if you live with a smoker, choose an air purifier that has a high CADR for tobacco smoke. 

Energy Star certification: Air purifiers must run around the clock to be effective, so you should factor in the energy cost when you shop. Energy Star certified purifiers are 40 percent more energy-efficient than standard models. 

Room size: If an air purifier has an AHAM Verifide seal, you can trust that the unit can handle the suggested room size listed on the seal. Be wary about manufacturers’ claims, though. We have tested many air purifiers that are not suitable for their claimed room sizes. You can check our ratings to see what room-size range we suggest for each model based on our test results. Also, consider sizing up: Most models suitable for large rooms (350 square feet and larger) can also work well for smaller rooms at lower speeds. Lower speeds tend to be quieter—which is nice for when you’re watching TV or sleeping.

Noise: Judge an air purifier not just by how well it performs but also by how well you’ll be able to live with it. Because these machines should always be running, ideally they should also be quiet. (For reference, a noise rating around 50 decibels is roughly equal to the hum of a refrigerator.) You may be able to find a model’s decibel levels on its packaging or website listing before you buy it. Or check our air purifier ratings; we rate models on noise levels at both high- and low-speed settings.

Clean or replace filters regularly. An air purifier can’t run efficiently if it has a dirty filter. Typically, you should replace filters (or clean those that can be vacuumed) every six to 12 months for pleated filters and every three months for activated carbon filters.

Place it wisely. If you have just one unit, put it in the room where you spend the most time. For most people, that’s the bedroom. (Some units can be heavy and clunky to move around, so if you want an air purifier in multiple rooms, you may want to buy a unit for each room.) Make sure to place the air purifier in a spot where nothing can obstruct airflow—away from curtains, for instance.

Adjust the speed. To avoid noise disruptions, we suggest running the unit on its high-speed setting when you’re not in the room and turning it down to low when you’re nearby. Or buy an air purifier certified for a larger area so that you can run it on a low speed and still have it work effectively. 

Vacuum regularly. Air purifiers can’t remove the larger allergens—dust mites and pet hair, for example—that settle on furniture and carpets unless they get disturbed and redistributed into the air. Use a vacuum cleaner with HEPA-certified filtration once or twice a week to clean floors and furniture.

Use an exhaust fan in the kitchen. Do the same in bathrooms and laundry rooms. 

Stop smoking indoors. That also goes for burning candles and wood fires. 

Ventilate. Open your windows on nice days to let in clean, dry outdoor air. If pollen or related allergies keep you from opening windows, run your air conditioner or forced-air cooling system with a clean air filter.

Reduce the use of chemicals. Limit the use of chemical-heavy cleaning products, and stow house paint, glues, and insecticides in a basement or another area that’s away from where you spend most of your time at home.