Soon after the coronavirus pandemic hit, people started snatching up portable air purifiers for their homes and offices. Their thinking? The devices will scrub the air of the virus (it can float for up to three hours indoors), reducing their risk of falling sick.

But do air purifiers actually work? The answer is yes—to a point, according to the U.S. Environmental Protection Agency.

The effectiveness of air purifiers depends on how well they catch air particles and their size in relation to the space they must filter. And it helps to use the devices in concert with other techniques such as increasing ventilation indoors, using high-end filters in central heating and air conditioning systems, and adhering to social distancing.

Here's what you need to know before buying an air purifier.

Who needs a portable air purifier?

Air purifiers are expensive relative to some other solutions—improved ventilation, for instance, can be as simple as opening a few windows. You should think of them as an extra precaution, or as a last resort when other options are unavailable.

For instance, windows in high-rise buildings often can’t be opened. Or maybe you’re at home with a coronavirus-infected family member, or with people who are regularly in risky situations outside of the home. For example, teachers worried about exposure from their students may want the extra protection for their families. Experts have even recommended air purifiers as part of schools’ broader reopening plans.

What types of air-purifying technologies are available?

Air purifiers aren't created equal. The first thing to look at is how a purifier filters the air.

The most widely used air purifier technology is what’s known as mechanical filtration. This simply means a purifier pushes air through a physical filter that catches airborne particles, in essentially the same way a sieve removes particles when you pour water through it.

A second, increasingly widespread method involves ionization. This technology adds an electric charge to particles, which makes them stick to surfaces instead of floating airborne. However, some research has found ionizing purifiers are less effective than other options. And the EPA has warned that some ionizing devices generate ozone at levels that may be damaging to human lungs. Thanks to better technology and pressure from regulators, ionizing filters that don’t generate ozone are now available, and the California Air Resources Board has created a list of those that emit only safe levels of ozone.

Finally, there are a number of unconventional air purifying technologies available. Some of these, such as the proprietary PECO filtration technology used by Molekule, are widely vetted. But since air purifier claims are subject to limited regulatory oversight, approach experimental technology with caution.

Which filters catch the coronavirus?

The easiest way to ensure your air purifier can filter coronavirus is to buy a mechanical purifier that uses a HEPA filter, which capture 99.97% of particles that are 0.3 microns or larger. This is the filter grade commonly used by hospitals, and is widely considered effective for removing coronavirus from the air.

The size of the coronavirus is only around 0.1 microns, or smaller than what a HEPA filter is rated to reliably catch. But to get airborne, the virus requires a carrier, usually water droplets, which are much larger and therefore can be captured.

Adding to the confusion, some purifiers may be advertised as having "HEPA type" filters, which could be misleading, since these may not be as effective as a true HEPA filters.

Other devices, including ionizing and electrostatic purifiers, clean the air without using filters. That can save hassle, but the effectiveness of filterless purifiers varies widely, and research on their effect on the coronavirus specifically is still limited. So in general, a mechanical HEPA filter is the safest bet.

There is one notable exception: Laboratory tests show Molekule's proprietary PECO technology catches particulates more effectively than HEPA, and destroys them instead of just trapping them. But Molekule's devices are among the most expensive on the market (see below).

Is your purifier big enough?

Air filters are rated on a metric known as Clean Air Delivery Rate, or CADR. This rating reflects how well a purifier removes contaminants, though understanding the number is difficult.

Most purifiers have three CADR ratings, for pollen, dust, and tobacco smoke, which are filtered at slightly different rates. The Association of Home Appliance Manufacturers (AHAM) recommends buying a filter with a tobacco smoke CADR of at least two-thirds of the volume of the room in which you'll use the device.

For example, if you’re buying a purifier for a room that’s 12 feet by 10 feet, or 120 cubic feet, you’d want a purifier with a tobacco smoke CADR of at least 80.

If you want to be especially cautious, you can buy a purifier rated for a larger room, which will simply mean your air is being filtered even more often than necessary. Also, for rooms with ceilings higher than eight feet, AHAM says you’ll want a purifier rated for a larger room (though the group doesn’t specify how to adjust your measurement).

This doesn’t apply to all purifiers, though: Neither Dyson nor Molekule have submitted their devices for measurement under the CADR standard. But they offer their own recommendations for the room size each model can protect.

Freestanding air purifiers are largely intended to work for single rooms, so if you want protection in more than one part of your house, you may need multiple purifiers.

Which air purifiers are best?

A few air purifier models are consistently highly ranked by reputable reviewers, though you’ll have to make your own judgment when balancing factors like price, room size, and design.

The Coway AP-1512HH Mighty purifier was the top pick from both Tom’s Guide and the New York Times’ Wirecutter, and was also highly rated by Cnet. The Blueair Blue Pure 411 and the Honeywell HPA 300 also placed consistently high in all three rankings.

How much does an effective air purifier cost?

Air purifier prices can vary a great deal depending on their filtering capacity and the technology they rely on.

Among the most expensive air purifiers is the $799 Molekule. On the other end of the spectrum, the highly-rated Blueair 411 purifier costs just $119.

Also worth considering: An air purifier's total price includes not only the sticker price, but also the cost and frequency of replacement filters, and the electrical cost of running the device. That’s one clear advantage for ionizing purifiers, which don’t have a filter that needs replacing. But they do generally cost more up front than mechanical filters, and you have to clean the metal plates that collect particulates, which some users say is unpleasant.

Where to find more information about air purifiers？

One very useful tool for evaluating air purifiers is this directory of room air cleaners maintained by AHAM. The directory includes room size (CADR) and performance data for a wide variety of purifiers, verified by independent lab tests.

You can also look for AHAM’s “Verifide” mark on a purifier's packaging or marketing materials. This indicates that the manufacturer’s claims about filtration power and other details have been independently verified.