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Health Team

Study: Mask type really matters, ventilation may matter more

Posted August 31, 2021 4:00 p.m. EDT
Updated August 31, 2021 5:42 p.m. EDT

— A recent study out of Canada shows wide disparities in the effectiveness of different types of masks and suggests ventilation improvements can do better than common cloth and surgical masks in reducing transmission of COVID-19.

University of Waterloo researchers used a sealed room, a masked mannequin and atomized olive oil to simulate breathing in their study. They used lasers to measure the aerosols, tiny particles that can float in the air, getting past masks and circulating in the room.

Then they added ventilation, courtesy of an air purifier equipped with the sort of high-powered HEPA filters used in labs and on airplanes.

Though the virus behind COVID-19 is present in larger droplets, which all masks help block, it's also thought to transmit through these much smaller aerosols. In fact a separate study released last week suggests airborne/aerosol transmission may be the dominant form of transmission.

The University of Waterloo 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."

"Indeed, our research shows that fresh air exchange or air purification can be very effective in controlling aerosol buildup indoors," Professor Serhiy Yarusevych, one of the researchers, told WRAL News via email. "The most beneficial strategy would be to employ both masks and ventilation to minimize the risk of virus transmission. However, given the relatively low efficiency of cloth and surgical masks in aerosol control, it is essential to complement their use by adequate ventilation for prolonged indoor occupancy."

"Other studies have shown before"

WRAL News shared this study with four experts here in North Carolina: A virologist, an epidemiologist, a chemist who previously tested masks in his own experiments and a mechanical engineer who studies fluid dynamics.

"This paper confirms with a more controlled set-up (and more math) what other studies have shown before," said Dirk Dittmer, director of virology and global oncology programs at the UNC Lineberger Comprehensive Cancer Center. "N95 is much better than anything else."

Dittmer said that, in Germany, the government routinely sends his parents and others N95 masks. He also said ventilation is important, along with mask fit. Because he works in a lab with coronaviruses, Dittmer said he gets annual fitting lessons.

Pia MacDonald, a UNC professor of epidemiology and senior director of applied health research for RTI International, said ventilation should be considered crucial.

"While masking is one strategy for reducing risk of transmission indoors, the combination with ventilation (air exchange) is a critical strategy to reducing risk of SARS-CoV-2 transmission," she said.

Tarek Echekki, a professor of mechanical and aerospace engineering at N.C. State, noted that the Waterloo study had an average droplet size of one micron.

"A human exhalation may involve larger droplets as well," he said. "It is not clear if the use of smaller droplets is placing the more common face masks (surgical and cloth) at a disadvantage and thus reducing their efficacy."

Echekki noted that the study doesn't account for any protection you get for your self in wearing a mask, it only measures aerosols exhaled. He said the experiment deviates from normal life in a number of ways, but he called it "a nice addition to the available literature."

Martin Fischer, a Duke University chemistry professor who compared masks in his own experiment last year, said the Waterloo study shows yet again the complexity of evaluating masks.

"I would take this study with a grain of salt," Fischer said. "How many of the super-spreader events involved a bunch of folks sitting still doing nothing but steady breathing in a room with absolutely no air movement? Nevertheless, the message as far as masks and ventilation should be: Wear a mask, try to make it fit to avoid gaps, and exchange your air (filter or exchange the air in a room, or go outside). And go get vaccinated!"

Ventilation efforts limited

Ventilation has gotten less attention during this pandemic in part because the U.S. Centers for Disease Control and World Health Organization went back-and-forth last year on whether the virus was likely it was to transmit as an aerosol, in spite of growing scientific consensus.

A study published last week in Science argues that "unequivocal evidence indicates that airborne transmission is a major pathway for the spread of SARS-CoV-2." It makes a number of recommendations, including a focus on mask fit and increased focus on ventilation.

Figure 1 from "Airborne transmission of respiratory viruses" study, Science Magazine, Aug. 27, 2021.

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

Asked about the Waterloo study, North Carolina's Department of Health and Human Services called ventilation "part of a layered strategy to reduce exposure to the virus that causes COVID-19."

"Increasing the amount of outdoor air into a building, increasing filtration, and enhanced air cleaning, such as using HEPA filtration systems, are important factors to improving indoor air quality and reducing exposure to viruses," DHHS spokeswoman Catie Armstrong said in a statement. "Improving indoor air quality and using a properly fitted mask are additive – they work better together to help control the spread of COVID-19."

It's difficult to pin down how much, out of the billions in federal coronavirus dollars that flowed to North Carolina in the last year, has been spent on ventilation. The state Department of Public Instruction, for example, said Monday it couldn't provide a figure for schools. It's online COVID money tracker lists only a single HVAC contract totaling $1.78 million, but it's likely more has been spent, just identified differently or bundled with other projects.

Wake County schools spokeswoman Lisa Luten said the system upgraded its HVAC filters to MERV-13's and plans to spend about $940,000 this year on filters alone. That sort of filter captures at least 75 percent of particles below 1 micron in size and at least 90 percent of particles above that, according to Yarusevych, one of the researchers in the Waterloo University study.

HEPA filters used in the study catch at least 99 percent of particles across this entire range, he said in an email.

A few days into the school year last week, both Wake and Durham public schools moved toward getting students outside more often for lunch. Before that, parents in Durham raised their own money to buy tents for lunch and air purifiers for classrooms.

Luten said the system couldn't point to any spending to move students outside, or to improve ventilation, beyond the filter improvements. But she also said school HVAC systems are tested every year and meet higher standards than, say, a home. She said system leadership has reviewed a number of studies on ventilation and COVID.

"It’s hard to read those studies and compare them to our schools," she said.

Whatever the ventilation needs in schools around the state, they're not limited to COVID. In the state's latest facility needs survey, schools listed nearly $700 million in desired HVAC work.

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