As parts of the US and other nations start to open up from the COVID-19 lockdown, there is a question of how best to safely do that. The goal, obviously, is to maximize economic activity while minimizing the spread of the virus. Meanwhile the virus is still surging in some parts of the world, including some US states, and the current surges seem to be related to poor policy choices – opening up too quickly or without proper safeguards.

The US state that seems to be surging the most right now is Arizona, yet a mass Trump rally was just held yesterday in Phoenix at the Dream City Church. The mass gathering in the hottest red zone of COVID-19 in the US was defended because the church recently installed an air purification system:

“We’ve installed these units and it kills 99.9% of COVID within 10 minutes,” Zastro says. “When you come into our auditorium, 99% of COVID is gone, killed, if it was even there in the first place,” Barnett says. “Thank God for good technology.”

Do air purifiers actually work to filter out the SARS-CoV-2 virus? Yes and no, but mostly no from a practical point of view depending on how you use it. Let’s take the most basic question first, can a good HEPA filter in an air purification system filter out viral particles and remove them from the air? The answer is clearly yes.

Coronaviruses are about 0.125 microns in size. HEPA filters are rated to 0.3 microns, so it might seem at first that they would not filter out the smaller viruses. But a NASA study showed that the HEPA filters are very efficient at filtering out particles down to 0.01 microns. Further, with COVID-19 viral particles are often suspended in very small droplets, and those droplets are much bigger than 0.3 microns and can be efficiently filtered as well.

The makers of the air filtration system in this case also claims that it cleans viruses out of the air with ionization. The idea here is to create ionized air particles that are then attracted to and bind to viruses and other small particulates in the air, making them easier to filter, causing them to drop to the ground, or in the case of germs, deactivating them.

Ionizers may be helpful in some setting, such as minimizing contamination in a sterile environment, but are not generally recommended for home use. They don’t really add much to a good HEPA filter (which is the industry standard) and they generate ozone, which can trigger asthma attacks in some people.

There has not yet been time to test many air filtration systems specifically for COVID-19, but even if we assume that the core claim is true, that a HEPA filtration system will clear out 99.9% of the virus, the more important claim is still – will it prevent the spread of COVID-19? The answer to this question depends on context, but for most situations the answer is a resounding no. The reason for this has to do with volume and how the virus spreads.

When thinking about and describing technology it is often the case that consumers focus on one number or feature and miss perhaps more important features. Cameras are rated by their megapixels, and computers by their processor speed. In this case everyone is focusing on the filtration efficiency, with the rally organizers emphasizing over and over the “99.9%” efficiency of the air filtration system they installed. But that number tells us nothing without another, arguably more important, number – the CFM, or cubic feet per minute.

The CFM rates how much air the purifier will process in a minute. This number is then multiplied by the efficiency to get the CADR, or clean air delivery rate, roughly analogous to replacing the air in a room with fresh air. So a CADR of 400 will bring 400 cubic feet of clean air into a room every minute. That may sound like a lot, and it is, for a regular room in a house. A large auditorium, however, will be in the hundreds of thousands of cubic feet at least. This means that even a robust system with a CADR in the thousands would take hours to replace all the air in the auditorium once (not 10 minutes as claimed). Of note, the “10 minute” claim was based on a study in a small sealed room – which cannot be generalized to a massive open auditorium.

The limiting factor on how big an air purification system can be is mainly noise. Bigger systems are noisy, causing a background of masking noise like being in a wind tunnel. This obviously would not work for an event where you need to hear people speak. It is safe to say that, unless there was a loud background roar of massive air purifiers, whatever system they had installed in the large auditorium likely had a negligible effect on the circulating virus and small droplet particles in the air.

Further still, spread of the virus in a mass gathering likely is not significantly from circulating air that would have a chance to go through a filter. Rather, it is from direct person-to-person spread. If you are talking without a mask to someone relatively close you are spraying them with small droplets, and if you are infected those droplets may be teeming with virus. Droplets can also spread the virus through intermediaries, like door handles, walls, or whatever people may touch. The bottom line is that in a mass gathering an air purification system is likely worthless in terms of reducing the spread of COVID-19.

However, claiming that an air purification system can protect people in a mass gathering can arguably be counterproductive. It may give some people a false sense of security, so perhaps they will be less careful about social distancing, wearing a mask, or touching things. In fact, it seems to have created the justification for the mass gathering in the first place.

I should note that this specific context does not necessarily mean that air purifiers are useless in every setting. If a sick person is isolating in a small room, whether at home or even in a hospital, a good and powerful enough air purification system can reduce circulating droplets and virus and reduce the chance of spreading the infection to others in the same building. We don’t have the evidence to know how effective this is for COVID-19 but that limited claim is plausible. The recommendation is that the system should have as high a CADR as possible without creating so much noise that it disturbs sleep.

Perhaps the best air purification system, however, is to just be outdoors. For now that may be the only way to safely have mass gatherings in places where the virus is still spreading – outside, with masks, and with enough space to allow for adequate social distancing. There is no way, however, to make an indoor mass gather safe where the virus is spreading.