Do N95 Masks Deprive Us of Oxygen?

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.

When wearing N95 mask, oxygen intake reduce or not, lots of people ask on Quora

Can wearing N95 mask bring any brain damage due to the low oxygen level, lots of people asked in Quora

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.

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.

Wearing 3m n95 mask with valve reducing the oxygen that people breathe in.

Physiological effects on of wearing n95 masks for healthcenter workers

While participants wore masks, the researchers tracked their oxygen levels through their skin.

 researchers tracked their oxygen levels when wearing N95 face mask through their skin

Participants weren’t just sitting there wearing masks. The researchers made them exercise with a mask.

Effect of running and exercising on oxygen levels when wearing a face 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.

How does exercising with face mask affect oxygen we breathe and in blood

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:

Wearing N95 face mask for 60 minutes does not reduce oxygen in blood

And what would we expect if masks starve us of oxygen? Maybe just lower oxygen levels throughout the test:

First hypothesis of while wearing N95 face mask, blood oxygen decreased and stayed same

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%.

Second hypothesis of while wearing N95 face mask, blood oxygen decreases by time

Here’s what actually happened:

blood oxygen while wearing N95 mask and doing exercises has no differences compare with no mask

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 mask had an exhaust valve on it.

N95 mask with valve and no valve affects the blood oxygen level.

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:

No differences in oxygen while wearing N95 masks during moderate exercise

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.

Oxygen (O2) levels and blood pressure did not change when nurses wore face masks for 12 hours

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?

Wearing N95 face mask affect pregnant women blood oxygen or not

They’re breathing for two, and pregnancy makes it harder to breathe. That’s a double whammy.

How pregnant woman breathe more oxygen while wearing N95 face mask since they have more demand on oxygen level

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.

Data shows oxygen levels in pregnant women were not affected when wearing N95 respirator masks

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.

Wearing N95 face masks have no effect on blood oxygen levels in pregnant women

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.

Humans (and dogs!) breathe out CO2, wearing face masks affect carbon dioxide levels in blood

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.

Blood CO2 do increased while wearing N95 face 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.

Blood CO2 slightly increased while wearing N95 face mask for pregnant women

Bottom Line: Wearing N95 masks during moderate exercise does not starve us of oxygen, but it does modestly increase blood CO2.

Breathe safe!

Wearing N95 face mask outside does not reducing the oxygen breathe in

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.

Oxygen Levels in a Closed Room stayed same

The data showed people’s intuitions are wrong in the same way! Oxygen was virtually unchanged throughout the day, but CO2 rose significantly.

CO2 Levels in Closed Room has increased

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.

Caveat Other Physiological Markers

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.


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10 thoughts on “Do N95 Masks Deprive Us of Oxygen?”

  1. 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 who ever said he wasn’t feeling well, and only a week earlier he had gone to the ER because he was having difficulty breathing; par for course with his health issues over the years. The doctors did a test for Covid and told him he didn’t have it. My father was convinced he did, but aside from the trouble breathing he didn’t exhibit any other symptoms. Then at the end of May of this year, 2020, after returning home with my mother from a trip out, he told her wasn’t feeling well and went to sit on the front porch. My mother said she looked for him and found him, limp and not breathing, in the chair on the front porch. The rest is a little blurry because my mother told me one thing; my sister told me another. All I know is that someone called 9-1-1. The EMTs restarted my father’s heart on the front lawn and then again in the ambulance. He went to one hospital and was transferred to another one a long way out near Boston, and while his body was revived his mind was gone. I heard he had a heart attack; I heard he had a stroke; maybe both. He and I had spoken only a week before his death and he seemed fine, and while I personally don’t think he had the virus, I do believe that wearing the masks compromised the oxygen in his system and that the lack of oxygen caused a heart attack and/or stroke which ended his life on May 31st. So I truly believe that while it is necessary to wear these damn masks, doing so also compromises the oxygen levels in our systems.

    • Don’t mistake correlation for causation.
      Your father had significant heart disease and, from your description of him possibly having a stroke as well as a heart attack, an arrhythmia as well.
      An arrhythmia is so dangerous because it can generate blood clots which lodge for a while in the body, typically the lower extremities (legs). At some point, often after exertion, one or more clots, called a thrombus, dis!dodge from the wall of the vein or artery and travel towards the heart, brain, lungs, or a combination of these.
      Mask wearing does not decrease blood oxygen; it slightly increases blood CO2 levels. This would not cause a blood clot to break loose and cause problems elsewhere.
      Masks are unpleasant for some, but they do save lives and preserve health.
      My sympathies to you and your family for your loss.

  2. 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 room and it is a one-to-one ratio (or very close to it).

    • There is also the consideration that people wearing masks are doing so for hours and hours straight, continuously. The testing is showing 1 hour of exercise for healthy people. What about the people with medical conditions – heart attacks, asthma, etc – who are sitting and working, 8 to 10 hours, while wearing the masks. I reach 6 hours of wearing a mask and I have very direct chest pain. I am on day 4 of the week and I developed chest pain by 8 AM. Give me a few days off and no pain. Then I go back to work on Monday and CO2 starts building up again in my blood.

      • Laura, sounds like you may have health issues.
        Get a thorough check up, preferably by a cardiologist.
        But don’t stop wearing your mask. A surgical one is ok, as long as it doesn’t gap at the sides. If it does, try using plastic medical tape, ie. Transpore tape, to hold down the sides to your face.
        Happy masking!😷😏

  3. 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.

    • There is also the consideration that people wearing masks are doing so for hours and hours straight, continuously. The testing is showing 1 hour of exercise for healthy people. What about the people with medical conditions – heart attacks, asthma, etc – who are sitting and working, 8 to 10 hours, while wearing the masks. The build up may go down when the mask comes off (person got home after work) and then go back up again the next day, sooner, as the blood didn’t fully clear out all the toxins overnight.

  4. 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!

  5. 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.


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