Study Shows That HEPA Filters Combined With The Right Masks Lower COVID Transmission

Since the beginning of the pandemic, there has been a great deal of debate about the effectiveness of masks in lowering COVID transmission. But, while the right masks are an important tool in the fight against COVID, a new study suggests that ventilation may matter just as much.

The Study

University of Waterloo researchers used a sealed room, a masked mannequin, and atomized olive oil to simulate breathing in their study. Lasers were used to determine the concentration of tiny particles (known as aerosols) that were circulating in the room. Then, an air purifier equipped with high-powered HEPA filter was added to remove the remaining particles. 

The Results

The study reached several conclusions:

  • N95 masks and similar masks filter exhaled air much better than cloth and surgical masks. Cloth and surgical masks caught 10 to 12 percent of aerosols breathed out in the experiment. Various N95, KN95 and R95 masks stopped 46 to 60 percent.
  • Even N95 masks leak, primarily around the nose, allowing aerosols to circulate. This led to “notably higher” concentrations of aerosols more than six feet away from the mannequin.
  • N95 masks with valves lost half their ability to stop aerosols. N95’s without valves remain the recommended choice, “if worn correctly,” but a loose-fitting N95 “provides a negligible” filtration efficiency.
  • The study found “increased ventilation/air-cleaning capacity significantly reduces the transmission risk in an indoor environment, surpassing the apparent mask filtration efficacy.” The university put that another way in a press release on the study: “Even modest ventilation rates were found to be as effective as the best masks in reducing the risk of transmission.”

The Implications

Two things are clear from the study, one is that fitted N95 masks without valves are the preferred choice and the other is that ventilation and filtration is just as important as mask-wearing. 

Why is HEPA filtration so important?

HEPA filters are unique in that they are able to eliminate almost any size particle from the air. They do this in a number of different ways.

Particles larger than 1 microns: As air flows through the filter, the larger particles are heavy enough that the airflow from the filter propels them forward into the fibers of the filter where they get stuck.

Particles that are .3  to 1 microns: Particles this size can fit between the gaps in the filter. But, they are too heavy and slow to follow the air flow around the HEPA filter and end up getting stuck in the fibers.

Particles smaller than .3 microns: While HEPA filters only need to filter particles of .3 microns, the truth is that they are also very effective at filtering out smaller ones. Because of a phenomena known as Brownian Motion, tiny particles bounce wildly off of other larger particles in the air in random patterns that send them careening off into different directions. It’s these zigzag patterns that cause them to hit the fibers of the HEPA filter and get stuck.

Have other studies confirmed the importance of ventilation and air filtration?

Yes! HEPA filters have long been used in hospitals, and a good track record of peer-reviewed clinical evidence shows that they can reduce viral infections. “The consensus is that HEPA filtration is the best technique to remove viruses from the air,” says John Holecek, a senior scientist at nanoComposix, a nanomaterials consultancy and contract research company based in San Diego.

A number of other studies have concluded ventilation deserves more attention, including several that found plastic barriers may increase transmission by impeding airflow.

If ventilation is so important, why aren’t we talking about it more?

One possible reason that ventilation is not getting more attention in the fight against COVID is that the U.S. Centers for Disease Control and World Health Organization went back-and-forth last year on whether the virus was likely to transmit as an aerosol, in spite of growing scientific consensus. 

Now that it is been confirmed that aerosols play a significant role in transmission, masks seem to have dominated public discourse.  But, while masks are helpful, the best way to lower transmission is to use a layered approach including vaccination, N95 masks, contact tracing, and a high quality air purifier with a True HEPA filter.

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