Scientists have actually made people wear N95 masks and measured their blood oxygen. So we have a solid answer to this question, and it’s not what this question assumes.
This question itself doesn’t ask IF wearing masks decreases oxygen, but HOW MUCH. It assumes there’s an effect.
Elsewhere on Quora, someone asked if wearing a mask will give them permanent brain damage.
The news media is on this question lately too. For example, a recent news story in the US said masks lower our oxygen.
The Oxygen Test
Fortunately, we don’t need to act based on our intuition. Scientists have already tested this by randomly assigning people to wear tight-fitting N95 masks or not.
While participants wore masks, the researchers tracked their oxygen levels through their skin.
Participants weren’t just sitting there wearing masks. The researchers made them exercise with a mask.
OK, OK, we’re not talking about running an Ironman. It was a moderate exercise: 1 hour of walking 2.5 mph on a treadmill. That’s about 4 kilometers in an hour.
I don’t know about you, but I’ve never measured people’s oxygen levels, so I don’t know what numbers are normal. According to the Mayo Clinic, normal oxygen levels are 95–100%. In the mask study, when people were just sitting without a mask, they were right in the middle of that range:
And what would we expect if masks starve us of oxygen? Maybe just lower oxygen levels throughout the test:
Or no, maybe they’d start out the same, but then they’d slowly start to suffocate as they breathe more of their own air in, eventually reaching hypoxemia at 90%.
Here’s what actually happened:
A whole lot of nothing. Their oxygen levels were similar to baseline. They even ended ever-so-slightly higher than when they started.
Hang on. You might have noticed that mask had an exhaust valve on it.
Well duh! Exhaust valves make it a lot easier to breathe. But when the concern is an infection, people shouldn’t be wearing masks with valves anyway. So without the exhaust valve, I bet they would’ve been much worse.
Skeptical readers are in luck! The researchers ran that condition too. They had people do the same thing with a valveless mask. And here’s what happened to their oxygen:
Even without a valve, there was no difference in oxygen levels. There wasn’t even a downward trend.
All right, so it isn’t about the valve. Maybe it’s about time. Maybe 1 hour is too short?
Researchers have got that covered too! In another study, researchers tracked nurses working 12-hour shifts while wearing N95 masks. Even over 12 hours (!), they found no decrease in oxygen.
OK, OK, But There Are Sensitive Groups
All right, maybe that’s true for these healthy adults. But what about people who are more sensitive?
OK, for sensitive groups, how’s about pregnant women?
So they seem like a good candidate for a sensitive group. And—you guessed it—those same researchers did another study, this time with pregnant women.
They asked 22 women in their third trimester to pedal an exercise bike for 20 minutes while wearing an N95 mask or not. And what happened to their oxygen levels? A whole lot of nothing.
Bottom line: Even during moderate exercise, randomized studies have found no evidence that N95 masks decrease oxygen levels in people’s bodies.
So Is There NO Physiological Effect of Wearing a Mask?
At this point, it seems like we’re reaching an extreme conclusion. Masks have absolutely zero physiological effect on people?
It turns out masks DO have an effect, it’s just not what this questioner is imagining. It’s not about oxygen, it’s about carbon dioxide.
Masks Increase CO2 Levels
Humans (and dogs) breathe out CO2, and that CO2 builds up inside the mask.
In those same studies, when researchers measured the amount of CO2 in people’s bodies, it did increase when participants were wearing the N95 mask.
Yet even without an exhaust valve, the rise in CO2 was minimal—about 3%. And it was below the CO2 limit of 45 mm Hg (above that is what’s called “hypercapnia”). What’s more, their CO2 stabilized after 15 minutes. In other words, CO2 did NOT seem to accumulate in their bodies more and more over the course of the hour.
Bottom Line: Wearing N95 masks during moderate exercise does not starve us of oxygen, but it does modestly increase blood CO2.
P.S. We make the same incorrect assumption about room air
I’ve seen people ask about oxygen levels in closed rooms with air purifiers. So I put that to the test in the modest-sized Smart Air office.
The data showed people’s intuitions are wrong in the same way! Oxygen was virtually unchanged throughout the day, but CO2 rose significantly.
P.P.S. Caveat: Other Sensitive Groups
Pregnant women are surely a sensitive group, but there are many other potential sensitive groups out there that could have breathing problems with N95 masks. The fact that one study found N95 masks are safe for pregnant women does not mean they’re safe for everyone. For example, I’d worry about people with COPD or black lung disease.
P.P.P.S. Caveat: Other Physiological Markers
Oxygen and CO2 aren’t the only physiological markers out there. The researchers in these studies looked at other markers like blood pressure and heart rate, and they generally did not differ between the mask and the no-mask group. But the human body is complex, so I’d be hesitant to say it has no other effect.
Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people across the world breathe clean air without shelling out thousands of dollars for expensive purifiers.