Global health has been significantly impacted by the COVID-19 pandemic. In 2020, over 50 million people were exposed to the virus and over 1.5 million people died from associated respiratory complications.1
Can air purifiers help protect against viruses like COVID-19? The answer here lies in knowing how how air purifiers work and how viruses, in particular COVID-19, are transmitted. Then, you can assess whether an air purifier will help protect you from viruses in the air.
How mucus-based viruses are transmitted
COVID-19 is a newly emerging disease. Through scientific and medical research, we are still refining our understanding about how it spreads.2
COVID-19 is a contagious infection caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). When the virus contacts mucous membranes, like those in the eyes, nose, and mouth, infection can result in illness and possibly death.
Much like measles and influenza, COVID-19 is a viral infection and can be spread through aerosol transmission.3,4 However, it can be transmitted by other means.
In general, mucus-based viral infections can occur through:5
- Droplet spray in short range transmission: the virus can spread through respiratory droplets produced when an infected person exhales (breathes, coughs, sneezes, sings, yells, or talks). These droplets tend to be larger (>5 microns in diameter) and fall out of the air rapidly within seconds to minutes. Air purifiers are not helpful for this form of transmission. Social distancing and masks, and engineered filtration pressure differentials are helpful for preventing this form of transmission.6
- Contact (direct or indirect): the virus can spread by touching an object or surface with virus present from an infected person, and then touching the mouth, nose, or eyes. Air purifiers are not helpful for this form of transmission. Good hygiene, a form of source control, is helpful for this form of transmission.7
- Aerosol in long-range transmission (airborne transmission): when an infected person exhales, they produce typically small respiratory droplets (≤5 microns in diameter). Smaller, aerosolized viruses can remain in the air indefinitely and travel far from their source on air currents.8 Air purifiers and engineered filtration pressure differentials can be helpful for this form of transmission.
Note that engineered filtration pressure differentials means maintaining positive or negative air pressure in a room. In a medical setting, negative pressure can be important in controlling airborne infection using an air purifier equipped with an outflow kit in order to direct airborne contaminants out of a room.9
The first line of defense in protecting against viral infection is source control: hand washing, wearing masks, and maintaining social distancing. When source control practices are in place, some air purifiers can help reduce airborne viruses.
How an air purifier filter works
Mechanical air purifiers use filters to trap particles with a mesh filter typically woven of glass or specialty synthetic fibers.
High Efficiency Particulate Air (HEPA) and HyperHEPA filters are mechanical air purification filters. Mechanical filtration is the safest and most effective method for removal of airborne particles from the air.
There are significant differences in the functionality between air filters.
- Mechanical filters trap big particles and let air pass through like a strainer catching food while water and dirt filter through
- Synthetic air filters use a charged media comprising synthetic fibers with an electrical charge to increase the “stickiness” of the filter
Due to the advantages of high airflow, most HEPA filters today utilize synthetic media. But these fibers lose their charge over time, as particles “stick” to the filter and the filter becomes too “loaded.”
In fact, the HEPA efficiency dramatically decreases as the filter is loaded with pollutants – ultimately, it becomes “overloaded” with particles, and the “stickiness” is reduced.
Mechanical media can provide better performance over time when helping filter viruses.
Since there is such a variance in long-term performance of mechanical versus synthetic filter media, mechanical media provides better performance over time when helping filter viruses in most cases.
Furthermore, HEPA filter media must only be tested to capture particles down to 0.3 microns. That’s almost three times the size of the average virus.
And there’s two problems with the HEPA testing requirement:10
- HEPA filters aren’t tested when they’re fully “loaded.” So any efficiency test no longer applies after as little as a few weeks.
- The Clean Air Delivery Rate (CADR) test used for HEPA filters is unreliable for testing the actual efficiency of HEPA filters when they’re used in air purifiers.
What is CADR in an air purifier?
Theoretically, CADR measures how many particles are removed multiplied by the rate of airflow (cubic feet per minute, or cfm) through the purifier.
But the way the test is done is problematic. Here’s how a CADR test is conducted:11
- Air purifiers being tested are placed in a test space of 1,008 cubic feet (an 11′ x 11′ x 8′ room).
- Dust, pollen, and tobacco smoke pollutants are introduced into the test space.
- The air purifiers being tested are turned on for twenty minutes (testing for pollen ends after only ten minutes).
- After running for this short time, the remaining contaminants are tested and converted into the final CADR rating.
The CADR rating system has flaws, including:
- CADR only tests performance for the first twenty minutes of operation. But the majority of HEPA air purifiers start losing efficiency after only one hour of usage, and some lose as much as 50% of their efficiency after only 8 weeks of testing.
- CADR does not test performance against particles smaller than 0.1 microns, which includes many viruses.
CADR only tests performance for the first 20 minutes of operation and doesn’t test for particles smaller than 0.1 microns, which includes many viruses.
The virion for COVID-19 and other related SARS viruses is anywhere from 0.06 to 0.14 microns.12 Way tinier than the 0.3 microns HEPA specification.
Look for air purifiers that are tested and certified to filter ultrafine particles (UFPs) and have high manufacturing standards.
The COVID-19 virus is anywhere from 0.06 to 0.14 microns – way tinier than the 0.3 microns that the HEPA standard tests for.
The bottom line on air purifiers and viruses
Air purifiers are not meant to be the first line of defense against viruses. Social distancing, wearing masks, and practicing good hygiene should be your first lines of defense. But an air purifier that can filter airborne ultrafine particles, including viruses like COVID-19, is a good part of a defense plan to help prevent virus transmission.
Viruses can be captured by filters designed for tiny particles, but this doesn’t guarantee that you’re fully protected – airborne transmission is only one way that viruses are spread. Even the most efficient air purifiers are not 100% effective at preventing the spread of viruses.
A common way viruses and diseases are transmitted is direct contact with someone who’s infected or carries the virus, spread in two primary ways:
- through the exchange of bodily fluids with an infected individual
- by breathing in infected respiratory droplets from coughing or sneezing
Any manufacturer that claims to 100% prevent virus transmission is falsely advertising. Air purifiers can help remove viruses from the air but cannot stop all modes of transmission.
Any manufacturer that claims to 100% prevent virus transmission is falsely advertising. Air purifiers can help remove pollutants, including viruses, from the air but cannot stop all modes of transmission.
 Johns Hopkins University of Medicine – Coronavirus Resource Center.(2020). Mortality analyses.
 Morens D, et al. (2020). Emerging pandemic diseases: How we got to COVID-19. DOI 10.1016/j.cell.2020.08.021
 Zhang R, et al. (2020) Identifying airborne transmission as the dominant route for the spread of COVID-19. DOI: 10.1073/pnas.2009637117
 World Health Organization. (2020). Coronavirus disease (COVID-19): How is it transmitted?
 Zanin M, et al. (2016). The interaction between respiratory pathogens and mucus. DOI: 10.1016/j.chom.2016.01.001
 Galbadage T, et al. (2020). Does COVID-19 spread through droplets alone? DOI: 10.3389/fpubh.2020.00163
 Aiello AE, et al. (2008). Effect of hand hygiene on infectious disease risk in the community setting: A meta-analysis. DOI: 10.2105/AJPH.2007.124610
 Liu Y, et al. (2020). Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. DOI: 10.1038/s41586-020-2271-3
 The Joint Commission. (2020). Utility systems.
 DOE technical standard: Specification for HEPA filters used by DOE contractors. (2015).
 Ginestet A. (2012). Development and evaluation of a new test method for portable air cleaners.
 Bar-On Y, et al. (2020). SARS-CoV-2 (COVID-19) by the number. DOI: 10.7554/eLife.57309