Wearing masks for hours on end has led some people to worry about whether N95 masks deprive us of oxygen. 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 would 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.
But Wait!
Hang on. You might have noticed that the 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.
Wearing N95 mask for 12 hours
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 about pregnant women?
They’re breathing for two, and pregnancy makes it harder to breathe. That’s a double whammy.
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.
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.
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 potentially 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.
Breathe safe!
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OMG, I am so happy I’m not the only one wondering about oxygen intake from wearing masks. My father lived with emphysema and heart issues for much of his life and while occasionally he had to make a trip to the hospital because of his breathing difficulties or concerns of a heart issue, he was still going strong at 86. That is, until this year. After Covid-19 invaded our lives, my father would still go out with a mask on, to go grocery shopping or pick up his and my mother’s medicines. My father was never the kind of man… Read more »
Observation: Regarding the measure of oxygen and carbon dioxide in a sealed room: Your graphs for oxygen and carbon dioxide levels are a little misleading. The data shows that oxygen levels in the air relative to total atmosphere decreased by about 0.3% which seems very small. It also shows that carbon dioxide levels increased by 0.27% over the same time period. There is a direct correlation between oxygen level decrease and carbon dioxide increase. They are changing at the same rate relative to total atmospheric concentration. ie, oxygen is being converted to carbon dioxide by the seven people in the… Read more »
The conclusion that N95 masks cause CO2 build-up can not be made with the data shown. The only comparison we get is a baseline resting CO2 levels and a mask-wearing result whilst exercising. There is no result comparing CO2 levels whilst exercising with a mask and without a mask. Without this, your conclusion is just an assumption. Further testing with actual control variables would be required.
What about respiration rate or other measures of the difficulty of breathing? Is there a change in the effort needed to breathe while wearing a mask even though the outcome for O2 levels in the blood remains nominal?
Thanks for all your efforts here – it’s all refreshingly useful!
Just an observation. You note a modest rise in paCO2 but above that comment in the article about nurses during a 12 hour shift, in the non-highlighted portion of the excerpt you display, that study showed a more significant rise in paCO2 of about 10 mm.