When air flows through a respirator, it passes through a dense mesh of fibers. Those tiny particles collide with the fibers and get stuck, thanks to electrostatic forces—the same force that makes hair stick to a balloon.
There is “a huge reduction in the number of particles that get through,” Marr said. (Indeed, the "95" in the N95 rating indicates that a mask, used properly and under the right conditions, is designed to capture roughly 95 percent of airborne particles.)
In the laboratory, researchers can actually test out respirator performance. For one paper, published in 2020, scientists placed two mannequin heads in a translucent box. Using a nebulizer and actual SARS-CoV-2 virus, they piped “a mist of virus suspension” through the mouth of one mannequin, mimicking an exhaling person. They used a ventilator to draw air into the other mannequin’s mouth. Finally, they fitted the mannequins with various combinations of masks, respirators, or nothing at all, and tested how much of the virus evaded capture as it journeyed between the mannequins. Cloth and surgical masks did have an effect — but were substantially outperformed by the N95s, which captured most of the viral particles.
Just because an N95 captures particles in the lab, however, doesn’t necessarily mean it will stop an actual person from getting infected out in the world. Part of the issue is that people don’t always wear respirators properly. And, even if the respirator performs well, the viral particles that slip through could be enough to make a person sick anyway. In the mannequin study, even an N95 taped snugly to a mannequin’s face failed to capture all the particles.
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Over the past 15 years, a handful of research teams have tried to test out mask and respirator performance in the real world, through randomized controlled trials. Such studies are often considered the highest standard of evidence, because they can minimize sources of bias. In one such study, conducted in the winter of 2009 and 2010, the Australian epidemiologist Raina MacIntyre and several colleagues divided nearly 1,700 health care workers in Beijing into three groups. People in one group were told to wear surgical masks at work. Another group was instructed to wear an N95 at all times. And a third group was asked to wear an N95 only during certain high-risk procedures. Then, for four weeks, the team tracked how often the participants got sick.
MacIntyre and her colleagues reported that the people who wore N95s all day were significantly less likely to develop a respiratory illness than everyone else.
Other studies have produced mixed results. Some found that the masks or respirators had a small effect on someone’s odds of getting sick, but not always enough to be considered statistically significant. Others didn’t find any benefit at all when comparing N95s to surgical masks, or even surgical masks to non-masking.
Do those findings apply, though, when millions of people are masking together, in the middle of a pandemic? At this scale, the question of whether or not masks work can be treated as a policy question: Did mask requirements actually reduce the spread of COVID-19? But doing a randomized controlled trial to answer this question is probably impossible, said Jing Huang, a biostatistician at the University of Pennsylvania’s Perelman School of Medicine. It’s not easy to just ask a few dozen randomly selected cities to implement mandates, and a few dozen to avoid mandates, and then track what happens.
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And yet, this scenario did happen naturally during the COVID-19 pandemic: Some places put in mask mandates, and others did not. This sort of natural experiment opened up an opportunity for researchers to sift through health data in these different locations and try to suss out patterns—and Huang and her colleagues recently did just that. They matched 351 counties in the United States that had implemented mask mandates with counties that did not have a mandate, but that were otherwise similar in several other respects. This means that, when possible, the COVID rates in a Republican-leaning, suburban county in the South that implemented a mask mandate during moderate COVID-19 spread would be measured against infection rates in another right-leaning, suburban Southern county that did not put a mandate into place at the same time.
Huang's analysis found that mask mandates were associated with substantially dampened COVID-19 spikes, although the benefit waned over time in some counties. The reason behind that waning was unclear, but could perhaps be could be due to fatigue with the mandates, the researchers suggested. Similar studies have often—but not always—found a positive effect.
Whether the masks were responsible for those benefits, though, was hard to pin down, Huang said. It’s possible that other factors—such as other policies implemented alongside mask mandates, or greater social distancing—actually kept COVID-19 rates lower, rather than the masks themselves. “I think it’s very difficult,” Huang said, “to make a causation conclusion.”
The CDC has cited other observational studies to justify its masking recommendation. One 2022 study found that people in California who chose to wear N95s were less likely to catch COVID-19 than people using other kinds of respiratory protection, who were themselves less likely to fall ill than people did not wear a mask at all. But the study was criticized for doing little to control for all the other ways people who wear N95s may behave differently than people who never wear masks. Was it the masks that made the difference? Or was it those other cautionary behaviors that people who tend to wear N95s also engage in that reduced their risk?