Scientists have actually randomized people to wear N95 or regular surgical masks, then tracked how many people got infected with colds and flus similar to the coronavirus. This data can tell us whether N95 or surgical masks are more effective at preventing transmission of viruses. The results surprised me.
Coronavirus & Mask Livestream
Wondering whether masks work to protect you against the coronavirus? Check out our livestream recap covering all the info here!
People’s Intuitions About N95 Masks and The Coronavirus
The results are surprising because I’ve seen several people claim that we need N95 masks to stop the coronavirus. They say surgical masks just won’t cut it when it comes to the coronavirus.
Similarly, a doctor interviewed on CNN said that masks don’t work.
And to be honest, that’s my intuition too. N95 masks fit a lot tighter than surgical masks, and they capture a higher percentage of particles. They’re what I wear in Delhi and Beijing.
The Surgical Mask vs. N95 Mask Test
Scientists tested this question by randomly assigning over 2,000 nurses to wear N95 or surgical masks. Then they tracked how many of them caught the flu.
I polled people on Twitter to see what people guessed. A majority (68%) guessed N95, so I’m not alone in my intuition.
But when the data came back, it turned out that rates of infection were the same!
Differences weren’t significant, although surgical mask users actually had slightly lower infection rates.
OK, Maybe That Study Was a Fluke
Data can be fluke-y. That’s just one study. But it’s not the only one.
Researchers in Canada randomly assigned 446 nurses to wear N95 or surgical masks during a few months of cold and flu seasons (September to December). Then they tracked how many got the flu or a cold.
Again, no significant difference! Both masks performed just as well at preventing the transmission of the viruses. Twenty percent of nurses wearing surgical masks got sick versus 22% wearing N95 masks.
Entirely Reasonable Skepticism
At this point, there must be at least a few savvy readers thinking, “aha!”
I know what’s going on! These studies didn’t have a control group, so we don’t know whether the masks actually worked. If they both don’t work, then of course there’s no significant difference!
That’s a great question. It’s just hard to test because, what are we going to do? Assign nurses to NOT wear masks? That seems irresponsible.
What we need is some situation where people face infection but don’t normally wear masks. It turns out, scientists found just such a place:
The No-Mask Test on Viruses
Researchers in Australia studied parents taking care of their children, who were sick with the flu.
Because people often don’t wear masks at home (even around sick people), researchers could defensibly randomly assign people to wear masks or not. They randomly assigned parents to wear no mask, a surgical mask, or an N95 masks like the one I’m wearing here.
Then they tracked how many parents got the flu.
Compared to parents not wearing masks, parents wearing surgical masks had 27% odds of getting the flu versus 24% wearing N95 masks (called “P2” masks here).
Thus, masks seemed to work! But again, surgical masks were just as effective as N95 masks. And the effect size was fairly large—75% reduction in infection risk.
The table above reports hazard ratios, not raw percentages. In raw percentages, 17% of all participants got sick versus 5% among surgical masks and 4% wearing N95 masks.
Perhaps not surprisingly, the masks only worked for people who actually wore them. There was no benefit among people who often forgot or just gave up on wearing the mask.
How on Earth Could Surgical Masks Work as Well as N95 Masks at Capturing Viruses
OK, at this point, there must still be some skeptical readers out there. There’s no way that surgical masks can capture particles that small! I saw a doctor on Quora say that “most masks” can’t capture particles the size of viruses.
But when we ignore our intuition and look at actual test data, reality is far more interesting. Test data shows surgical masks are surprisingly effective, even for tiny particles. For example, in one study, researchers tested particles down to .007 microns (even smaller than viruses) and found that a simple surgical mask blocked 80%.
So it’s downright surprising that surgical masks are just as effective! Maybe virus particles are actually easy to capture because they fly on water droplets. Or maybe mask usage prevents people from touching their mouth and nose. For now, I can only speculate.
Bottom line: There’s scientific evidence finding that (1) masks prevent flu infection and (2) surgical masks prevent infection of viruses like the coronavirus (Covid-19), as well as more sophisticated N95 masks.
P.S. Do we even know whether masks capture tiny virus particles? Aren’t they too small for masks?
This is a solid intuition I’ve seen several times, including here on Quora, repeated by a doctor.
The only problem it doesn’t fit the data. I summarize that data here: Can wearing masks stop the spread of the coronavirus?
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.