With winter approaching and the end of patio season in sight, the dread of several isolating months under COVID-19's shadow is palpable in Minnesota.
Is there anything you can do to safely have friends or family visit your home, when the yard is filled with snow and the temperatures fall below zero?
One possibility emerging in internet ads and science blogs is the portable air purifier. It sounds simple: Clean the coronavirus particles out of your indoor air to make it safe for breathing.
But will it work? We went to Professor Chris Hogan at the University of Minnesota's College of Science and Engineering to find out. He studies the transport of small particles in the air and is the editor-in-chief of the Journal of Aerosol Science.
The short answer: An air purifier can help mitigate risk, but it is no substitute for social distancing and wearing a mask.
That being said, there are indeed supplemental things you can do to make the air in your home cleaner - and safer - for your family and loved ones.
So you want to buy an air purifier
OK, so you know that your best options are to limit the number of people in your home, social distance and wear masks. But you still want that added protection of an air purifier for the necessary meetings - or those times you choose to take the risk and have someone over.
There are three main types of purifiers you can look at.
HEPA filters
The most common air purifiers you'll see advertised these days are those that incorporate a HEPA filter. HEPA stands for High Efficiency Particulate Air. This type of filter is actually theoretically fine enough to catch coronavirus particles - which are smaller than most lower-grade air filters will capture.
"HEPA filters are extremely efficient," Hogan said. "99.97% or better. They're really good."
HEPA filters are designed to catch particles as efficiently as possible. The air purifiers that incorporate those filters basically just bring air in and recirculate it.
"So they really clean the air in the room," Hogan said. "You can think of it as dirty air in, particle-free air out for all particle sizes."
Of all the types of air purifiers available right now, Hogan said the one that's the safest bet is probably one that incorporates a HEPA filter, because to have the HEPA rating the company has to have done collection efficiency tests.
"The challenge there is to make sure that it is cleaning enough air per minute or per hour to actually help reduce concentrations in that room," Hogan said.
To verify this, check the Clean Air Delivery Rate (CADR). This is usually in cubic feet per minute or per hour. You want to estimate the dimensions of the room you're putting it in, and then make sure it's cleaning the air in that room more than five times an hour. Hogan said above seven times an hour is even better, although that can be harder to find.
"Make sure that that's actually been tested and not, you know, estimated by somebody," Hogan said.
That's part of the problem - companies aren't actually required to test the CADR specifically. Hogan said, though, that companies that have done the test are pretty quick to say it, and to put the testing report on their website.
"You should feel free to reach out to companies and ask them for that," he said. "I think there's pressure between companies to do that."
Hogan said most companies are likely doing CADR testing now - and if they're doing it, they're bragging about it.
"It would be really weird for a company to have the test done and then not mention it," Hogan said. "And if they don't mention it, they probably don't have it done or they weren't happy with the result."
Finally, don't expect the device to clean your whole house - focus on one room. Hogan says if you have someone over, the most effective place for your air purifier is right in between the two parties.
One more warning: The higher the CADR, the louder the device will be - kind of like a furnace.
UV light
Other air purifiers may utilize different technology, such as UVC light. That's a tactic that's been used for 100 years, Hogan said, and was even used in school ducts to reduce measles infections before there was a vaccine.
"The one concern is, there's so many companies out there selling these and there are not a lot of standards, in the sense that there are not a lot of standard tests for things," he said.
So even though UVC light may be able to kill coronaviruses in theory - in fact, Hogan has shown this in his U of M lab - you may not know if the particular device you're buying has been tested sufficiently.
Some HEPA filter devices also incorporate UVC, Hogan said. The filter captures the particles, but it's the ultraviolet light that kills them - or attempts to. The most important thing to check with UVC light devices is that they're designed safely, since exposure to that light can be harmful.
Electrostatic precipitator
A third type of air purifier uses an electrostatic precipitator, which ionizes particles, charges them up and gets them to stick to a wall inside the device.
"They can be very efficient, too, if designed properly," Hogan said. "But again, the company will need to have done the proper testing in order for it to work."
Improving air circulation
It may seem simple, but improving the air circulation in your home is another way to decrease the risk of COVID-19 particles hanging around.
Hogan said that to clean the air in your home, your number one option should be to improve the air change rate in the home. This could mean getting stronger fans or blowers to replace the ones that are already circulating your home's air.
There's also already a filter in your home, near your furnace, that cleans the air. It may be tempting to replace this with a HEPA filter, but Hogan said don't do it. If your fan isn't strong enough, the super-fine HEPA filter will actually slow down your air change rate because the system isn't strong enough to push air through.
Instead, you'll want a higher MERV rating - that's the typical home air filter rating, a step below HEPA. MERV ratings go from zero to 16. Eight is common in home systems, and 16 is the best. A higher MERV rating could be a good option for your home.
The tough truth: Distance, distance, distance
"First and foremost, the number one way to protect your home from COVID is to still maintain social distancing," Hogan said.
The classic public health recommendations of social distancing and mask-wearing are still the best ways to keep yourself and your loved ones safe, Hogan said.
"I would say if you are really concerned about it, the number one thing you can do is limit who comes into your home," he said.
The other somewhat simple tactic is this: If you do have people over, limit the number of people in the space and the time that they're there. Hogan said there is a big difference in risk between a 10-minute visit and a 30-minute visit.
"Number of people in the room times time, that's your risk," he said. "Those two multiplied together."
Hogan said in the cold months (OK, maybe not February), bonfire settings are some of the safest places to hang out with other people.
"Campfires actually trade a lot of air too, and it's pretty hard for viruses and bacteria to survive the temperatures that fires have," he said. "And you're outside."
What do the experts do?
Professor Chris Hogan can't endorse a particular brand of HEPA filter. In fact, he's not allowed to - he studies this stuff for work. But he will admit to having one in his home. He said he actually bought it to improve the air quality in his basement, not to prevent COVID transmission.
"I do think they can be effective, but they can also be ineffective, depending on, you know, the one that you buy," Hogan said.
When he bought his purifier, he looked at the CADR and the size of the room to make his calculation.
For others, he said, he doesn't necessarily recommend going out and buying one.
"If you're going to have large family gatherings, I would encourage you to think about: One, how long is that going to be? Two, how many people are going to be there? Three, how can I keep the space well ventilated, with ventilating meaning, how am I clearing the air out of here?" Hogan said. "But I would never say you have to buy this technology to do that. It's an option. It's definitely an option."
Lastly, if you do buy one, Hogan has a warning: "If there's something that feels kind of gimmicky about the way that it works, it probably hasn't been tested yet."