Key takeaways:

• Health experts recommend using air purifiers to help prevent illness, especially during the cold months and flu season, as viruses like influenza and the COVID-19-causing SARS-CoV-2 are carried in respiratory aerosols. 
• Health experts recommend operating your air purifier continuously at a low speed to minimize the risk of illness. If someone is actively sick, prioritize a high fan speed.
• Our exclusive lab data reveals that the Shark HP232 performed best at particle removal on a low fan setting, while the Coway Airmega 400S performed best on a high fan setting.
• Other important factors for illness prevention include an air purifier with a true HEPA filter and strategically placing the unit in your home.

I've become dependent on my air purifier to capture allergens such as dust and pollen during the spring allergy season, as well as smoke from wildfires in Southern California. However, I've always wondered whether air purifiers can prevent illness. If they can capture allergens, smoke, mold spores and pet dander, can they also remove viruses from the air?

According to CNET's exclusive lab data, the answer is yes. Health experts agree.

Pediatric allergist and clinical immunologist Dr. Zachary Rubin tells CNET that viruses such as influenza and SARS-CoV-2, a member of the coronavirus family, are carried in respiratory aerosols, which are particles that get exhaled while you're breathing, coughing, talking or sneezing. These are usually under 5 μm in size, but many can be smaller than 1 μm, and your air purifier should be able to remove them.

Primary care and urgent care physician Dr. Steven Goldberg, the chief medical officer at molecular testing laboratory HealthTrackRX, notes that an air purifier capable of removing 0.3 μm particles is a particularly vital benchmark for viral aerosol removal, especially since manufacturers often rate purifier filters by their efficiency in removing particles of this size.

To put this to the test, CNET Labs placed 12 popular air purifiers in a smoke chamber to measure their ability to filter five different particle sizes, both fine and coarse, from the air. Two models emerged as top performers for their speedy results, making them must-have devices for cold and flu season.

The best air purifier for preventing illness, according to our tests

We tested all 12 air purifiers at both low and high fan speeds to evaluate their effectiveness in removing fine particles. These are the models that emerged as the top performers.

At low fan speed

On a low fan setting, the Shark HP232 performed the best at removing fine particles, averaging 1 minute and 15 seconds. It also performed best at this speed for all particles, with an average time of 1 minute and 20 seconds. This makes it our "Best at Particle Removal on Low Setting" winner.

Figure 1: The y-axis displays the 12 air purifiers we tested, and the x-axis displays T90, or the time it took for 90% of particles to be removed, in the following format: [minutes].[seconds]. This is for low fan speed. The black bars represent fine particles measuring 2.5 μm and under, while the red bars represent coarse particles measuring 10.0 μm and under.

Designed for spaces up to 1,200 square feet, the HP232 uses Shark's Anti-Allergen Nanoseal filter with true HEPA, which the company says captures 99.98% of large, small and micro-sized particles ranging between 0.1 and 0.2 microns. HEPA stands for "high-efficiency particulate air" and is a type of air filter used in air purifiers.

The Shark HP232 is $330 at full price, making it the seventh cheapest model we tested. The most affordable option tested is the Morento Indoor air purifier, priced at $79, but it did not fare as well in our particle tests. 

Second place for removing fine particles at low speed goes to the Coway Airmega 400S, which is the fourth most expensive unit at $565. The BlueAir 311i came in third place, while the Oransi Mod Plus came in fourth place. The BlueAir model is the fourth least expensive unit at $160, while the Oransi is the third most expensive model at $640. 

At high fan speed

At high fan speed, the Coway Airmega 400S was the best at removing 90% of fine particles in an average of 34 seconds. It also reigned supreme in removing all particles on average at a high speed in 35 seconds. It has won our award for "Best at Particle Removal on High Setting."

Figure 2: The y-axis displays the 12 air purifiers we tested, and the x-axis displays T90, or the time it took for 90% of particles to be removed, in the following format: [minutes].[seconds]. This is for high fan speed. The black bars represent fine particles measuring 2.5 μm and under, while the red bars represent coarse particles measuring 10.0 μm and under.

Interestingly, the Shark HP232, our low-fan particle removal winner, ranked seventh for fine particles at high fan speed with an average time of 1 minute and 15 seconds. Ranked higher in second place is the Oransi Mod Plus with 47 seconds. In third place, the BlueAir 311i and Levoit Core 400 S tied at an average of 49 seconds for fine particles and 48 seconds for all particles. The $181 Levoit model, however, is $21 more expensive than the BlueAir 311i.

Which air purifier speed is best for preventing sickness?

Based on our data, you may be wondering: For cold and flu season, should I opt for an air purifier that works best at removing finer particles at low or high speed? According to Rubin, continuous low-speed operation is better for preventing illness because "it keeps background aerosol concentrations low at all times."

Goldberg echoes this, adding that many manufacturers' operation guides recommend a continuous, low-speed mode for optimal performance.

"High-speed bursts are only useful in addition to continuous use -- for example, immediately after someone coughs, sneezes or leaves the room -- but they should not replace baseline filtration," says Rubin. 

If someone is sick, you'll want to prioritize cleaning speed at a medium or high setting. If these fan speeds are too loud for sleep, you can switch to a low fan speed at night, allowing it to still filter particles in the background. However, if you can tolerate it, Goldberg emphasizes that a higher fan speed will provide a higher airflow rate, which is more effective for rapidly removing aerosols when someone in your home is sick.

"In bedrooms, quieter settings are acceptable as long as the unit can still clear fine particles within about 5 minutes," says Dr. Raj Dasgupta, chief medical advisor for Sleepopolis, who is quadruple-board certified in pulmonary, sleep, internal and critical care medicine.

In other words, an air purifier that performs effectively at a low speed is best for the cold and flu season. Our tests reveal that this would be the Shark HP232. However, if someone is actively ill, you'll want your device to purify effectively at high speed, which the Coway Airmega 400S was best at in our tests. 

"If someone is sick, combining a purifier on medium or high and opening a window for even 5 to 10 minutes dramatically improves air quality," Rubin says. 

According to our tests, air purifiers that performed best at removing fine particles on both low and high fan speeds are the Coway Airmega 400S, priced on the high end at $565, and the Oransi Mod Plus, which is more expensive at $640. A more affordable model that still ranks in the top four for both fan speeds is the BlueAir 311i, priced at $330. 

How long should it take an air purifier to remove fine particles?

While there is no universally accepted removal time for fine particles, Goldberg states that rapid removal within a few minutes is most effective in preventing illness.

"A purifier should ideally remove 90% of fine particles within one to 3 minutes," says Dasgupta. "Up to 5 minutes is still helpful for reducing exposure."

For an air purifier to be considered effective at removing fine particles when someone is ill, Rubin provided the following benchmarks:

• Under 3 minutes on high: excellent
• 3 to 5 minutes: acceptable for most situations
• Over 8–10 minutes: too slow to meaningfully reduce risk during active illness or gatherings

"If a unit takes about 10 minutes or more to remove 90% of fine particles, it likely won't keep up with the continuous production of aerosols when someone in the home is sick," Rubin says.

Our testing revealed two air purifiers that took 10 minutes or longer to remove 90% of 0.3 µm particles at a low fan speed: the $519 Airdog Green Technology Air Purifier X5 and the $750 Dyson Formaldehyde TP09, the second most expensive unit tested at just $10 less than the $760 RabbitAir A3.

On low speed, the Dyson also struggled with all other particle sizes, lagging behind the 11 other models tested. On average, it took the Dyson 8 minutes and 26 seconds on low speed to remove 90% of fine particles. For all particles, the average time was 7 minutes. 

The Dyson Formaldehyde TP09 is specifically designed to detect and destroy formaldehyde with a catalytic filter. The device uses a HEPA H13 filter with a 99.95% removal efficiency for particles down to 0.1 microns per liter of air, as well as activated carbon filters that capture 99.97% of particles down to 0.3 microns in size, including allergens, bacteria, viruses, pollen and mold spores. Dyson purifiers also feature Air Multiplier technology, which helps purify an entire room.

At high speed, the Airdog also took at least 10 minutes to remove 90% of 0.3 µm particles, but had no issue with other particle sizes. This air purifier uses TPA (Two-Pole Action) Technology, which claims to "capture and destroy pollutants 20 times smaller than conventional air purifiers -- all without using non-recyclable, environmentally-damaging HEPA filters." It states that it can remove pollutants that are at least 0.0146 microns.

For why Dyson specifically wasn't as effective as the other purifiers, CNET Senior Lab Engineer Gianmarco Chumbe has an explanation: "Our smoke chamber is designed as a torture test that pushes purifiers to remove a very dense concentration of particles in a confined space as quickly as possible. Dyson's purifiers use an Air Multiplier and projection-style airflow system that's optimized for circulating air across a larger volume."

Due to Dyson's specific system, it won't work as intended in a smaller chamber like the one used for our testing. Chumbe emphasizes that this doesn't indicate a malfunction. "It simply reflects how this specific fan technology behaves under our high-intensity, small-volume test conditions."

Added 'germ' features are something to sneeze at

Air purifier brands advertise various additional features, such as special "germ" filters or germicidal UV-C light bulbs, claiming they can reduce germs in the air. However, our testing reveals that what's most important for preventing illness is performance. 

"The most important factors are airflow and a high-quality HEPA filter," says Rubin. "'Germ' filters rarely provide measurable benefit. Built-in UV-C often uses weak bulbs with too-short exposure time to reliably inactivate viruses, so it shouldn't drive a purchasing decision."

For HEPA filters, you'll want a true HEPA, which captures 99.97% of 0.3 μm particles. Activated carbon can help reduce odors, but it doesn't eliminate viruses. You'll want to avoid filters labeled HEPA-type, HEPA-like or those claiming ionic- or ozone-producing technologies.

As Airdog points out in its product description, HEPA filters are typically not reusable. 

"Standard HEPA filters (H13/H14) are defined by their ability to remove at least 99.97% of particles greater than or equal to 0.3 µm and are the most established and widely recommended technology for removing airborne viruses and fine particles," says Goldberg.

Why your air purifier should appreciate the finer particles in life

In our CNET Labs testing, five different particle sizes were accounted for: 0.3 µm, 0.5 µm, 1.0 µm, 2.5 µm, 5.0 µm and 10.0 µm. The time it took for 12 different air purifier models to remove 90% of these particles (represented as T90) released by a smoke bomb made of 50% potassium nitrate, 40% sugar and 10% baking soda in a 96-cubic-foot chamber was measured with a particle counter at both high and low fan speeds for each particle size. 

Explaining why T90 was the benchmark, Chumbe says, "T90 is more useful than T100 [100% of particles] because the last few percent of particle removal is slow, noisy and often dominated by measurement limits, making T90 a more reliable and practical metric for comparing air purifier particle filtration performance."

An average time was also calculated for all particles, fine particles measuring 2.5 μm and under and coarse particles measuring 10 μm and under.

When asked which measurement is most beneficial for cold and flu season, Rubin points to the T90 for fine particles (2.5 μm and below). He adds that the 0.3 μm and 0.5 μm performance is especially relevant because these two sizes represent the most penetrating particle sizes (MPPS) for HEPA filtration.

Other air purifier factors that don't deserve a cold shoulder

In addition to performance, there are additional air purifier features to consider.

Size

On both high and low fan settings, the top performers were large- and medium-sized air purifiers. According to Rubin, this is supported because larger models typically have larger HEPA filters with a greater surface area for capturing particles. They also have stronger fans and higher clean air delivery rates (also known as CADR).

"Even on low, [larger units] can move enough air to keep particle levels low, which is what your testing reflects," he says.

Placement 

Once you have your air purifier, ensure it's in the right location for optimal performance. This should be in a room where your household spends the most time, like the bedroom or living room. Or, if someone is sick, place it where that person spends most of their time. 

"In a bedroom, place it near the bed but not blowing directly on your face," Rubin recommends.

For optimal airflow, position the air purifier 3 to 6 feet away from walls, avoiding corners and areas behind furniture. Goldberg advises against placing the air purifier near open windows or doors, as this can cause it to short-circuit and reduce its effectiveness.

Air quality indicators

Air quality indicator lights or automatic modes can be beneficial for people who don't want to manually adjust their air purifier's settings, but Rubin explains they're not effective for preventing illness. 

"These sensors are very good at detecting larger particles like dust, cooking smoke or candle soot, so the purifier can automatically ramp up when the indoor air becomes visibly or measurably dirtier," Rubin explains. "However, these sensors generally cannot detect the smallest respiratory aerosols that carry viruses, so the purifier may stay on a low setting even when someone in the room is breathing out infectious particles."

When someone in the home is sick, it's more important to run the air purifier at medium or high speed, regardless of what the device's indicator may show. Dasgupta adds that during cold and flu season, continuous operation provides more reliable protection than relying on air quality sensors.

Our final recommendations

Based on our testing and health experts' analysis, we recommend the Shark HP232 for effectively removing fine particles continuously at low speed to prevent illness during the cold and flu season. For achieving similar results at high speed, particularly when you're sick, we recommend the Coway Airmega 400S. 

"Don't underestimate continuous use," concludes Rubin. "Even an average purifier can be very effective if it runs 24/7." 

The next time you're trying to stave off sickness, don't just stock your medicine cabinet -- make sure your air purifier is working to decontaminate the air you breathe. That way, we can all take a deep, cleansing breath.