Does Closing the Windows Starve Us of Oxygen?

Air purifiers work best in a closed room environment and this worries some who are concerned that there wouldn’t be enough oxygen coming in when all doors and windows are closed. Yet recent tests in a real apartment — with as many as seven people in a small room — show less than 1% change in oxygen levels in a closed room.


Fears of Low Oxygen

Fears of low oxygen levels are easy to find. Quora user Arghya Roy, says a closed room running air condition leads to low levels of oxygen.



Playing on this belief, companies rushed in with products that fill this supposed void. For example, the popular Three Dad’s purifier claims to solve the oxygen problem by using “submarine technology” to create oxygen.



The logic is simple: The room is closed, humans breathe in oxygen, so oxygen levels should decrease over time. Or should it not?


The Experiment

Smart Air tested this question by tracking oxygen levels with an iBrid MX6. We used the Smart Air office as an extreme testing site because the humble Smart Air office has seven people packed into just 34.5m² room.



On top of that, the office windows and doors are sealed with insulation tape to prevent polluted air from getting in (the office has no central air system too). We tracked oxygen levels throughout normal workdays, keeping the door shut except for when people left and came back from lunch at midday.



Even in these extreme conditions, oxygen levels showed almost no change. By the end of the day, oxygen levels had dropped by 0.3%. Simply put, humans don’t take in as much oxygen as we think we do. Based on oxygen alone, estimates are that an average person could survive in a completely sealed room for 12 full days!


oxygen level in a closed room


The Change Is in Carbon Dioxide

While oxygen levels are pretty much constant over the day, what does change is the amount of carbon dioxide in the air. Tracked by the Air Visual Node, carbon dioxide levels went from 0.08% to 0.35% — more than tripling over the course of a day!


carbon dioxide level in a closed room


How Can Carbon Dioxide Triple when Oxygen Remains Unchanged?

If humans breathe in oxygen and breathe out carbon dioxide, how can one remain almost unchanged while the other skyrockets? The key is that regular air has a lot more oxygen than carbon dioxide. Oxygen constitutes about 20%, while carbon dioxide is only 0.038% of the air. Thus, carbon dioxide is capable of tripling from such a small number, while oxygen is mostly unchanged.



Carbon Dioxide Can be Harmful

Although we don’t have to worry about running out of oxygen, too much carbon dioxide can be harmful. At levels in the Smart Air office during the test (1,000-3,500PPM), people felt drowsy and may even perform worse on cognitive tests.


The Simplest Ways to Lower CO2 Levels:

#1  Open your windows from time to time

Yes, periodically opening the windows will let in polluted outdoor air, but our tests have found that purifiers, take the Cannon and Blasts for example, can cut PM2.5 levels by 50% in just 10 minutes after closing the windows.



#2  Leave the windows opened, just a tiny gap

If you have a strong purifier, open the windows by just a bit and leave it there. Our tests have found that purifiers can still be effective even when the window is left open. What’s more, if you get a super-powerful purifier like our Blast air purifiers, opening your window shouldn’t affect the clean air you breathe indoors!

#3  Get a place with a central air system

Choose an office or apartment with a central air system that brings in outdoor air (hopefully purified ones too). If your apartment or office, like many of the ones in China, doesn’t have a central air system, you can get an air exchange system installed. But it is expensive.


Bottom Line

In indoor spaces, oxygen levels are not of much concern even though CO2 can be. So the next time you come across a marketing hype about purifiers that claim to create oxygen, you know you don’t quite need that.

Breathe safe (and smart) folks!

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Araz Reof

I believe that inhaling a fresh air will make you to have better quality of personality! Thanks for sharing this information.


Love this article! (And many others). In general people are unaware of how little oxygen we need. We breathe in air with 21% oxygen, we breathe out air with 13-16% oxygen. More than a quarter can’t fit in our blood! This doesn’t mean that a significantly lower percentage in the air would be okay though, because the air pressure would get lower too and we would be able to absorb even less (this is what happens at altitude, there is more than enough oxygen, but the pressure is too low to get the maximum amount in our blood). Also, because… Read more »

Liz - Smart Air

Hi Kelly, Liz here from Smart Air. Wow, thank you for the informative comment! I’m learning more from you too! Yes, I totally agree, when we feel dizzy or a headache in the afternoon, the first thing I do is to check the CO2 level in the office. If it’s too high, opening the window can resolve the symptom shortly.


Great article. So, CO2 levels reach dangerous levels in as short as 8 hours in a closed room ?! Vow. My elderly parents with respiratory and cardiac problems sleep in a small room with air conditioner and Dyson Air purifier on, in Chennai, India – with doors and windows closed – for about 8 hours. Chennai pollution levels are not as bad as North India but I would prefer to keep windows/doors shut. With windows open, the purifier doesnt do a great job of bringing down PM2.5, 10. I see values like 40. With windows closed, it comes down to… Read more »

Exactly right Balaji, you’ll need to trade-off between CO2 levels and PM2.5/PM10 levels. That’s why I’d suggest you open your window for short periods of time to let out CO2, then close them to stop any more PM2.5/10 coming in. If you have a very powerful air purifier, you might even be able to run your purifier with your windows open ever so slightly. You can check out the research we did running air purifiers with windows open here.


How about with plants in the room does it harm us to sleep with plants in the room?

Hey Stephan, great question! Many people think plants can improve air quality, so we decided to test it. Check out this blog post on whether plants can improve indoor air quality.


I was also having the same doubt about purchasing new purifier for my room, but your blog helped me to select a purifier more specifically, so i purchased the same kind of purifier