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.


Along with wearing masks, air purifiers with HEPA filters are also one of the best ways to stay safe from a variety of pollutants in our air including viruses and dangerous PM2.5. A recent CDC study confirmed significantly lower COVID-19 infection rates in schools that used HEPA air purifiers. HEPA filters can significantly lower the risk of a variety of deadly diseases including diabetes, heart disease, cancer, and high blood pressure.

Read More: Four Steps to Choosing the Best Air Purifier

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