There’s been recent controversy about the use of face masks for protection against coronavirus. Mainstream sources, including the CDC and most of the media say masks are likely useless and not recommended. They’ve recently been challenged, for example by Professor Zeynep Tufekci in the New York Times and by Jim and Elizabeth on Less Wrong. There was also some debate in the comment section here last week, so I promised I’d look into it in more depth.
As far as I can tell, both sides agree on some points.
They agree that N95 respirators, when properly used by trained professionals, help prevent the wearer from getting infected.
They agree that surgical masks help prevent sick people from infecting others. Since many sick people don’t know they are sick, in an ideal world with unlimited mask supplies everyone would wear surgical masks just to prevent themselves from spreading disease.
They also agree that there’s currently a shortage of both surgical masks and respirators, so for altruistic reasons people should avoid hoarding them and give healthcare workers first dibs.
But they disagree on whether surgical masks alone help prevent the wearer from becoming infected, which will be the focus of the rest of this piece.
1. What are the theoretical reasons why surgical masks might or might not work?
Epidemiologists used to sort disease transmission into three categories: contact, droplet, and airborne. Contact means you only get a disease by touching a victim. This could be literally touching them, or a euphemism for very explicit contact like kissing or sex. Droplet means you get a disease when a victim expels disease-laden particles into your face, usually through coughing, sneezing, or talking. Airborne means you get a disease because it floats in the air and you breathe it in. Transmission via “fomites”, objects like doorknobs and tables that a victim has touched and left their germs on, is a bonus transmission route that can accompany any of these other methods.
More recently, scientists have realized that droplet and airborne transmission exist along more of a spectrum. Droplets can stay in the air for more or less time, and spread through more or less volume of space before settling on the ground. The term for this new droplet-airborne spectrum idea is “aerosol transmission”. Diseases with aerosol transmission may be spread primarily through droplets, but can get inhaled along with the air too. This concept is controversial, with different authorities having different opinions over which viruses can be aerosolized. It looks like most people now believe aerosol transmission is real and applicable to conditions like influenza, SARS, and coronavirus.
Surgical masks are loose pieces of fabric placed in front of the mouth and nose. They offer very good protection against outgoing droplets (eg if you sneeze, you won’t infect other people), and offer some protection against incoming droplets (eg if someone else sneezes, it doesn’t go straight into your nose). They’re not airtight, so they offer no protection against airborne disease or the airborne component of aerosol diseases.
Respirators are tight pieces of fabric that form a seal around your mouth and nose. They have various “ratings”; N95 is the most common, and I’ll be using “N95 respirator” and “respirator” interchangably through most of this post even though that’s not quite correct. When used correctly, they theoretically offer protection against incoming and outgoing droplet and airborne diseases; since aerosol diseases are a combination of these, they offer generalized protection against those too. Hospitals hate the new “aerosol transmission” idea, because it means they probably have to switch from easy/cheap/comfortable surgical masks to hard/expensive/uncomfortable respirators for a lot more diseases.
Theory alone tells us surgical masks should not provide complete protection. Coronavirus has aerosol transmission, so it is partly airborne. Since surgical masks cannot prevent inhalation of airborne particles, they shouldn’t offer 100% safety against coronavirus. But theory doesn’t tell us whether they might not offer 99% safety against coronavirus, and that would still be pretty good.
2. Are people who wear surgical masks less likely to get infected during epidemics?
It’s unethical to randomize people to wear vs. not-wear masks during a pandemic, so nobody has done this. Instead we have case-control studies. After the pandemic is over, scientists look at the health care workers who did vs. didn’t get infected, and see whether the infected people were less likely to wear masks. If so, that suggests maybe the masks helped.
This is an especially bad study design, for two reasons. First, it usually suffers recall bias – if someone wore a mask inconsistently, then they’re more likely to summarize this as “didn’t wear masks” if they got infected, and more likely to summarize it as “did wear masks” if they stayed safe. Second, probably some nurses are responsible and do everything right, and other nurses are irresponsible and do everything wrong, and that means that if anything at all helps (eg washing your hands), then it will look like masks working, since the nurses who washed their hands are more likely to have worn masks. Still, these studies are the best we can do.
3M makes about two dozen versions of the N95, for different industrial and medical purposes. Generally they’re constructed from nonwoven materials—infinitesimal plastic strands blown together to form a random thicket that, under a microscope, “is going to look like pickup sticks,” says Nikki McCullough, 3M’s global leader for occupational health and safety. “If you’re a submicron particle, it’s quite the journey through there.” The filters can block invaders as small as 0.3 microns, or about 1/100th the thickness of a human hair. The virus is smaller than that, at about 0.125 microns, but it often travels within larger particles when an infected person coughs or sneezes.
As far as I can tell, both sides agree on some points.
They agree that N95 respirators, when properly used by trained professionals, help prevent the wearer from getting infected.
They agree that surgical masks help prevent sick people from infecting others. Since many sick people don’t know they are sick, in an ideal world with unlimited mask supplies everyone would wear surgical masks just to prevent themselves from spreading disease.They also agree that there’s currently a shortage of both surgical masks and respirators, so for altruistic reasons people should avoid hoarding them and give healthcare workers first dibs.
But they disagree on whether surgical masks alone help prevent the wearer from becoming infected, which will be the focus of the rest of this piece.
1. What are the theoretical reasons why surgical masks might or might not work?
Epidemiologists used to sort disease transmission into three categories: contact, droplet, and airborne. Contact means you only get a disease by touching a victim. This could be literally touching them, or a euphemism for very explicit contact like kissing or sex. Droplet means you get a disease when a victim expels disease-laden particles into your face, usually through coughing, sneezing, or talking. Airborne means you get a disease because it floats in the air and you breathe it in. Transmission via “fomites”, objects like doorknobs and tables that a victim has touched and left their germs on, is a bonus transmission route that can accompany any of these other methods.
More recently, scientists have realized that droplet and airborne transmission exist along more of a spectrum. Droplets can stay in the air for more or less time, and spread through more or less volume of space before settling on the ground. The term for this new droplet-airborne spectrum idea is “aerosol transmission”. Diseases with aerosol transmission may be spread primarily through droplets, but can get inhaled along with the air too. This concept is controversial, with different authorities having different opinions over which viruses can be aerosolized. It looks like most people now believe aerosol transmission is real and applicable to conditions like influenza, SARS, and coronavirus.
Surgical masks are loose pieces of fabric placed in front of the mouth and nose. They offer very good protection against outgoing droplets (eg if you sneeze, you won’t infect other people), and offer some protection against incoming droplets (eg if someone else sneezes, it doesn’t go straight into your nose). They’re not airtight, so they offer no protection against airborne disease or the airborne component of aerosol diseases.
Respirators are tight pieces of fabric that form a seal around your mouth and nose. They have various “ratings”; N95 is the most common, and I’ll be using “N95 respirator” and “respirator” interchangably through most of this post even though that’s not quite correct. When used correctly, they theoretically offer protection against incoming and outgoing droplet and airborne diseases; since aerosol diseases are a combination of these, they offer generalized protection against those too. Hospitals hate the new “aerosol transmission” idea, because it means they probably have to switch from easy/cheap/comfortable surgical masks to hard/expensive/uncomfortable respirators for a lot more diseases.
Theory alone tells us surgical masks should not provide complete protection. Coronavirus has aerosol transmission, so it is partly airborne. Since surgical masks cannot prevent inhalation of airborne particles, they shouldn’t offer 100% safety against coronavirus. But theory doesn’t tell us whether they might not offer 99% safety against coronavirus, and that would still be pretty good.
2. Are people who wear surgical masks less likely to get infected during epidemics?
It’s unethical to randomize people to wear vs. not-wear masks during a pandemic, so nobody has done this. Instead we have case-control studies. After the pandemic is over, scientists look at the health care workers who did vs. didn’t get infected, and see whether the infected people were less likely to wear masks. If so, that suggests maybe the masks helped.
This is an especially bad study design, for two reasons. First, it usually suffers recall bias – if someone wore a mask inconsistently, then they’re more likely to summarize this as “didn’t wear masks” if they got infected, and more likely to summarize it as “did wear masks” if they stayed safe. Second, probably some nurses are responsible and do everything right, and other nurses are irresponsible and do everything wrong, and that means that if anything at all helps (eg washing your hands), then it will look like masks working, since the nurses who washed their hands are more likely to have worn masks. Still, these studies are the best we can do.
by Scott Alexander, Slate Star Codex | Read more:
Image: Jamie Chung, Bloomberg
[ed. See also: How 3M Plans to Make More Than a Billion Masks By End of Year (Bloomberg). Excerpt:
[ed. See also: How 3M Plans to Make More Than a Billion Masks By End of Year (Bloomberg). Excerpt:
The N95 respirator is so named because, worn properly, it blocks at least 95% of airborne particles from entering a wearer’s mouth and nose, while still allowing respiration through the microscopically porous shell. This design protects a person from medical and other hazards; flimsier, looser-fitting surgical masks are intended to prevent the wearer from infecting others with expelled mucus, blood, or spit.
3M makes about two dozen versions of the N95, for different industrial and medical purposes. Generally they’re constructed from nonwoven materials—infinitesimal plastic strands blown together to form a random thicket that, under a microscope, “is going to look like pickup sticks,” says Nikki McCullough, 3M’s global leader for occupational health and safety. “If you’re a submicron particle, it’s quite the journey through there.” The filters can block invaders as small as 0.3 microns, or about 1/100th the thickness of a human hair. The virus is smaller than that, at about 0.125 microns, but it often travels within larger particles when an infected person coughs or sneezes.
