Air purifiers or cleaners can help reduce the airborne SARS-CoV-2 virus particles in closed spaces that aren’t well ventilated.
The target for these devices is the spray spewed forth by talking, coughing, sneezing, laughing, singing, etc., especially in crowded places.
The larger-sized virus-carrying particles are called respiratory droplets. They can go out about 6 feet from their source and drop down to surfaces like floors or tables (is that why they are called droplets?).
Particles that lose a lot of moisture, become lighter weight, and float around over wider areas for longer time periods are labeled aerosols.
Aerosols are felt now to be a substantial source of viral transmission. If you have a well-ventilated (drafty?), spacious home, or you work in a building with a central ventilation system that cycles air rapidly with good quality filters, you might not need air purifiers. Most air cleaners fall into two basic categories: filters or sanitizers. Some combine both features in the same unit.
Filters are designed to improve indoor quality by physically removing tiny particles of matter floating around, such as dust, pollen, pet dander and viral particles.
The most common types of home filters right now are HEPA filters. HEPA stands for “high energy particulate air.” HEPA filters are dense mesh that traps 99.97% of particles of 0.3 microns. Particles between 0.7-10 microns are trapped even more efficiently. The more times the air passes through the filter, the more particles are removed. The aerosolized droplets that the viruses snag a ride on are 0.1-0.3 microns, and are trapped with their tinier hitchhikers.
Many manufacturers use the Clean Air Delivery Rate system to rate air cleaner performance. This measures airflow through the filters, which are designed to trap particles as small as 0.01 microns in diameter, just a tiny fraction of the width of a human hair.
Air “sanitizers” contain short wavelength ultraviolet light, UVC, which can kill viruses, bacteria and fungi. These have been used for a century in various capacities, and are a mainstay in hospital ventilation systems. They don’t move air. The air has to be vented past them in the ducts for exposure. Ionizers give air particles an electrical charge, which attracts them towards something with an opposite electric charge (do they get a charge out of it?). Once trapped, they tend to stay there, similar to static electricity.
If your closed space has good ventilation, even with the UVC lights in the ducts, you may not need an air purifier. Most places don’t. There are a bewildering bunch of brands to choose from. One rating to check is the air changes per hour or ACH. For medical buildings such as hospitals, 20 to 40 changes per hour is desirable. For the home portable models, six is a decent number. You need to consider the square footage involved. The price range is from a hundred to several hundred dollars. One with a good HEPA filter for $200 to $300 will mostly fill the bill (that you have to pay).
There have been a few studies of the freestanding portable filters that show the aerosol load reduced by 90% in less than 30 minutes. It’s indirect evidence that the viruses would be filtered out with them. They are not silver bullets, but they are tools in the viral war chest. One researcher used the analogy to airbags in cars. They save lives, but you still need your seat belt. That means masks, physical distancing, and hand washing are STILL our main weapons in this war. If you have a household with at-risk, elderly or immunocompromised folks, it could be a valuable investment.
In the random musings in my Bohemian mind, I have pondered whether air purifiers could clear the air of the stereotypic political promises pollution particles that are promulgated during the bombastic campaign speeches we hear every two years.
Dr. Bures, a semi-retired dermatologist, since 1978 has worked Winona, La Crosse, Viroqua, and Red Wing. He also plays clarinet in the Winona Municipal Band and a couple dixieland groups. And he does enjoys a good pun.