As evidence mounts that the novel coronavirus can be spread through infection-laden airborne particles as well as respiratory droplets, consumers have been wondering whether portable air cleaners offer protection. So, although I explored the do’s and don’ts for buying air purifiers in February — focusing on the benefits for those with allergies — it’s time for another look.

I checked in with two of the top experts on indoor air quality: Joseph Allen, director of the Healthy Buildings program at the Harvard T.H. Chan School of Public Health, and Richard Corsi, dean of the Maseeh College of Engineering and Computer Science at Portland State University. In the fight against the coronavirus, both have championed the cause of placing portable air cleaners equipped with HEPA filters in school classrooms.

Here’s why: When infected people talk, sing, cough or even breathe, they release the virus into the air in a range of particle sizes. Although the large respiratory droplets fall quickly, the smaller aerosols can remain in the air for 30 minutes or longer until removed through ventilation or captured by an air-purification system.

“The science is pretty clear. Portables with a high-efficiency HEPA filter and sized for the appropriate room can capture 99.97 percent of airborne particles,” Allen says.

In a typical home, it may take two to three hours for the air to be exchanged. Using an air purifier can boost that exchange rate up to six exchanges per hour — cleaning the air about every 10 minutes. 

That sounds impressive, but before you rush out to buy a purifier, consider your personal circumstances. If you have been staying at home with the same family members and don’t intend to change that, you probably don’t need one. But if someone in your household is an essential worker with a greater chance for exposure or your school-age children are back to in-person learning, you might consider investing in a purifier unit.

The two experts are not advising that you rely on purifiers to entertain company. “Having people into your home for a meal or perhaps staying several days is a big concern as the holidays approach,” Corsi says. “Some may have been exposed; others don’t even know that they are infected.” They emphasize that people should not cease wearing a mask or washing their hands just because they have a purifier. “Don’t let your guard down. A purifier may lower the level of particles in the air, but it doesn’t eliminate the risk,” Corsi says.

Should you decide to buy a purifier, you’ll need to determine the best size for your space. For those mathematically challenged, like myself, try the Harvard-CU Boulder Portable Air Cleaner Calculator. Though designed for schools, it works for any measurable space. (You have to download the tool to your computer, so you can input your own data into the yellow boxes.)

The calculator will ask you for your room size, ceiling height and ventilation situation. The other important number is the Clean Air Delivery Rate (CADR). This number tells you how much air passes through your chosen purifier and its efficiency. The higher the CADR, the more air changes per hour. Your goal is three to five total air changes per hour.

You can find the CADR in the purifier’s specifications or on its packaging. If you can’t, you may want to bypass that brand. Look for a CADR of 250 to 300 or higher, Corsi says. If you see multiple numbers, use the dust rating.

A quality HEPA air purifier will cost between $200 and $300. Stick to the basics. You don’t need add-ons such as UV or ionization. There’s no benefit, and they will probably make the purifier more expensive or may introduce a secondary harm such as ozone, Allen says. Some models are quieter or come with timers and remote controls, but that’s just personal preference.

For maximum benefit, situate your purifier in the center of the room, at least three feet away from walls and corners and elevated on a stool or table. You also need to set it on the highest setting, and, yes, it will probably be somewhat noisy.

One quick reminder: Purifier filters capture airborne particles until they become inactive; they don’t kill the particles. And it’s not clear how long trapped particles stay active, so be cautious when replacing the filters. Wear a mask and goggles, place the used filter in a garbage bag and then immediately in your trash can, and wash your hands, Corsi says.

Don’t be surprised to see air purifiers claiming they have been independently studied by a prestigious school or laboratory. Most of these studies are legitimate but are still mostly hype by manufacturers trying to stand out in the marketplace. “For cleaning the air, all you need is a good fan and a good-quality filter,” Allen says.