I notice this the most in cars, when I set the air con to recycling air (instead of pulling in fresh air). After a while, I start to feel sleepy, and the sleepiness passes almost as soon as I open a window or change the air con setting to bring in fresh air.

I notice it also in shopping centers, or long bus rides, where there is inadequate fresh air. I know a lot of people who get sleepy from this, but also some people who just notice the air smelling 'stuffy' - but there's no particular smell, just 'stuffy'.

What causes recycled 'stuffy' air to smell stuffy? Is the sleepiness to do with increased Co2 / decreased O2 due to people breathing? Then is the 'stuffy' smell just the smell of CO2?

Any ideas how big of a change in concentrations we're talking about? Wikipedia says concentration of O2 in air is 20.95%. So are we talking about a drop by less than 1%, or a drop by a few %, etc.?

Is this 'stuffy air' bad for you? e.g. hypothetically if I have trouble sleeping, could I seal my bedroom (as sealed as an ordinary passenger car is), install an air con, and have it start recycling air when it's bedtime. Would this be bad for me long term?

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    $\begingroup$ do you have any reference besides your own experience to confirm that other people are also feeling sleepy with recycled air? $\endgroup$ Commented Mar 6, 2016 at 3:44
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    $\begingroup$ Yeah, just google "recycled air drowsiness". It doesn't seem to be unusual, and there are some articles / forum threads where people talk about CO2 levels. Although nothing about O2 levels (Does higher levels of CO2 necessarily mean lower levels of O2?). I'm more interested about whether it's bad for you, i.e. the sealing my bedroom and using an aircon hypothetical. $\endgroup$ Commented Mar 6, 2016 at 5:39
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    $\begingroup$ Happens with me too!! :D But wouldn't recycled air also have lesser oxygen? $\endgroup$
    – Polisetty
    Commented Mar 6, 2016 at 9:52
  • $\begingroup$ @Polisetty yup that's kind of what I'm wondering about... $\endgroup$ Commented Mar 6, 2016 at 23:26
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    $\begingroup$ Lot of questions in your post. You should try to emphasize one of them $\endgroup$ Commented Mar 23, 2016 at 4:51

2 Answers 2


Carbon dioxide is not the only waste product which is exhaled. NASA scientist Wolverton goes into detail in his book, "How to Grow Fresh Air". We exhale 70% of the body's waste products in gaseous forms. Breathing these in repeatedly and at ever-higher concentrations causes a whole host of health problems, starting with, you guessed it, an inability to stay alert and sustain full brain function.


Good question. CO2 doesn't smell though, it's one of the many odourless gases. That stuffy feeling is a very mild CO2 toxicity. But there would most likely need to be a change of more than 1% for a significant change in comfortability as we exhale 15.74% of that of that 20.95% unchanged. The sleepiness is caused both by the mild CO2 toxicity and the lack of O2 to your brain and muscles. Installing an air con largely depends on the size of your room (bigger is better), your age (younger is better), and your overall health, specifically how much physical activity your sustain on a day to day basis. Assuming you sleep for 8 hours you would only need 0.2 kg of O2 (0.6kg/24hr is needed for the avg. person at rest). If your room was the size of a twin bed (hypothetically) there would be around 0.15kg of O2. Taking the exact measurements of your room, you can calculate the amount of oxygen available using the molecular masses available on a periodic table. Personally, I feel you should be fine even without an air con but consider the factors listed above.

This is a very useful equation: kg = number of moles over molarity. But, you can always use a site like this: http://www.unit-conversion.info/molar-mass.html.

This is a copyrighted material so I can only give a reference: Organic and Biochemistry for Today, 7th Ed. ISBN 0538734310 Seager, S.L.; Slabaugh, M.R. Thomson/Brooks/Cole

  • $\begingroup$ Isn't it 75.13% of the 20.95%? $\endgroup$
    – Nathan
    Commented Mar 23, 2016 at 2:50
  • $\begingroup$ No 15.74% (approximately, most text books will give a range orbiting around 15.7%) $\endgroup$
    – 360ueck
    Commented Mar 23, 2016 at 4:33
  • $\begingroup$ Which part of your answer is the reference relevant to? $\endgroup$ Commented Mar 23, 2016 at 4:52
  • $\begingroup$ It's semantics. You've stated it "15.74% of that 20.95%", which sounds like 20.95% times 0.1574 giving you a return of 3.30% from the 20.95%. 75.13% of that 20.95% is 15.74%. $\endgroup$
    – Nathan
    Commented Mar 23, 2016 at 13:37

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