The best air purifiers use high-efficiency particulate air (HEPA) filters.

Clinical studies suggest that a well-maintained air purifier using HEPA filtration can alleviate the symptoms of asthma and allergies. However, these same studies caution that: "Dirty filters themselves can become a source for air contamination." In other words, air purifiers are only effective if they are properly maintained.

Air Purifiers and Wildfire Smoke

There is mixed data on whether an air purifier is effective at filtering smoke. Some studies say that the fine particulates in smoke (linked with cancer) can be filtered. Other studies say that filtering doesn't work. Those who are exposed to wildfire smoke (or secondhand tobacco smoke) may benefit from an air purifier, but there is no substitute for a smoke free home.

Air Filters and Coronavirus Protection

According to the U.S. Environmental Protection Agency, “When used properly, air cleaners and HVAC filters can help reduce airborne contaminants including viruses in a building or small space. By itself, air cleaning or filtration is not enough to protect people from exposure to the virus that causes COVID-19. When used along with other best practices recommended by CDC and others, filtration can be part of a plan to protect people indoors.” Avoid electrostatic or ionic air purifiers. These ionic devices produce ozone, a respiratory irritant.

What Is a True HEPA Filter?

Beware products that are marketed as “HEPA grade”, “HEPA like” or “HEPA style.”

A true HEPA filter must have "a minimum particle removal efficiency of no less than 99.97 percent for 0.3-μm particles," according to the U.S. Department of Energy. This standard is important. HEPA filters were designed in the 1940s to protect those who were working on the Manhattan Project. Today, nuclear facilities still use HEPA filters. So do hospitals, laboratories and other clean facilities.

How Much Does a Good Air Purifier Cost?

A good air purifier (with verified, above-average reviews) costs approximately $127, according to a 12-month analysis of price trends. Those same HEPA units cost around $74 while on sale: an average discount of 42%.

Don't buy an air purifier unless you're certain that you can buy replacement filters for a reasonable price. Some Amazon customers who bought off-brand air purifiers were horrified to learn that the replacement filters they needed cost as much as the unit itself!

Lastly, don't expect a portable air purifier to clean an entire home or apartment. A review published in Current Allergy and Asthma Reports concludes that portable filters are only effective in a single room.

A Note About Plastic Smells and Burning Smells

During initial usage, some air purifiers smell like plastic (for example, models made by Philips). However, an air purifier that emits a burning smell is a potential fire hazard: Unplug it and contact the manufacturer.

Here are the best affordable True-HEPA air purifiers for the money: