Before you buy a true-HEPA air purifier, try some simple, common-sense steps to reduce indoor air pollution.

1. Don’t smoke inside your living space.
2. If your vacuum doesn’t include a HEPA filter, upgrade your vacuum. If you have outside housecleaning help, it is recommended they use your HEPA vacuum and not a public vacuum.
3. Seal windows and exterior doors. This will help keep City air pollution outside.
4. Inspect the kitchen, bathroom, and laundry exhaust systems to confirm they are clean and all exit outside the building. This can be tricky in San Francisco. Do the best you can – we can’t rebuild the City.
5. Move all furniture a minimum of 6″-12″ away from perimeter walls, unless you are sure those walls are insulated.
6. Remove from your living area any contents that show discoloration, water damage, or growth. Some things can be remediated (cleaned for mold); others can’t.
7. Minimize candles and wood fires.
8. Don’t store toxic cleaning (or other) chemicals inside.

Before you invest in portable air purifiers, make sure carbon monoxide detectors are working and properly placed throughout your building. Things like wood stoves, fireplaces, chimneys, and vents are all places where carbon monoxide leaks can hide. And don’t idle your car, run fuel-burning power equipment, or light a barbecue grill in your garage, basement, or in confined spaces near your home.

Better True HEPA air purifiers do especially well at filtering pollutant particles such as dust, tobacco smoke, and pollen. Add charcoal filters to a true-HEPA to remove some of the volatile organic compounds (VOCs) and other types of gaseous pollutants.

Some portable models with carbon pre-filters are claimed to filter VOCs, known respiratory irritants that arise from adhesives, paints, and cleaning products. But the Environmental Protection Agency warns that such filters are specific to certain gaseous pollutants, not for others and that no air purifiers are expected to remove all gaseous pollutants found in the typical home. Carbon filters also must often be replaced, typically every 3-6 months, or they stop working–and can even, when full, release trapped pollutants back into the air. The safer course: Heed strict product label warnings such as “use only in well-ventilated spaces.”

Air-purifier models with an electrostatic precipitator remove pollutant particles by charging them as they pass through and collecting them on an oppositely charged metal plate or filter. In the process, they produce some ozone as a byproduct. You’ll also find dedicated ozone generators, which produce relatively large amounts of this gas by design. While ozone in the upper atmosphere protects us from the sun’s ultraviolet rays, ground-level ozone is an irritant that can worsen asthma and compromise your ability to fight respiratory infections. We believe that air purifiers that emit even small amounts of ozone are a poor choice if someone in your household has pulmonary problems or allergy symptoms. We also suggest that you avoid dedicated ozone generators entirely, given their high ozone emissions.

The very best portable models we tested were effective at cleaning the air of dust, smoke, and pollen at high or low speed. For whole-house purifiers, our recommended models did best at filtering dust and pollen without impeding airflow of forced-air heating and cooling systems. The worst models weren’t terribly efficient at any speed.

How to choose

If you want a purifier and don’t have a forced-air system, consider a large portable. In addition to removing more particles at high speeds, the better large models still did well at lower, quieter speeds.

Weigh features carefully. Most air purifiers have an indicator that tells you when first to clean or replace the filter to maintain efficiency. But some indicators turn on based on length of time the unit has been running, not how dirty the filter is. Skip odor-removal features. In past tests, it took up to an hour for them to make a difference–when they did anything at all.

And the certifications on the box? All tell how well a model filters particles at its highest speed. The certifications all also allow up to 50 parts per billion of ozone, a respiratory irritant. We advise against using models that produce any ozone, even if they are effective cleaners.

Check an air purifier’s efficiency rating

If you still want one, use this air purifier guide to picking the appropriate model for your circumstances. The Association of Home Appliance Manufacturers certifies most room models as part of a voluntary program that includes appropriate room size and maximum clean-air delivery rate (CADR), a measure of cleaning speed. We judge a CADR above 350 to be excellent and below 100 to be poor. Choose a model designed for an area larger than yours for better cleaning at a lower, quieter speed. Many whole-house filters list a minimum efficiency reporting value (MERV), developed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. The top performers in our tests typically had a MERV higher than 10.