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CO2 and sleeping while driving

  1. Jan 10, 2015 #1

    Ivan Seeking

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    After almost falling asleep while driving the other day, I got to thinking about CO2. I started to wonder if due to normal respiration of the driver and or occupants, CO2 levels in a car can rise to levels that can inhibit responses or even lead to unintended sleep, and make driving unsafe. I was struck by how quickly I regained composure as soon as I opened the window for fresh air. Maybe cars are too air tight for our own good?
     
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  3. Jan 10, 2015 #2

    SteamKing

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    Spending time in any sealed enclosure without adequate ventilation is not recommended. Fortunately, you learned (or re-learned) this lesson without injury to yourself or others. Cars are built with multiple means to provide ventilation to the cabin. Please use at least one at all times, even if it's just leaving a window open a crack.
     
  4. Jan 10, 2015 #3

    Doug Huffman

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    My VW owner's manual is clear that the HVAC vent should be open but for exceptional circumstances. For the usual position of the function control, recirculation is defeated.
     
  5. Jan 10, 2015 #4

    russ_watters

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    Was the fresh air cold?

    In winter, it would be very difficult to have a CO2 problem before a humidity/condensation problem. Most cars have an indicator that tells you whether the outside air damper for the HVAC is open or closed. In summer it should almost always be closed and in the winter it should almost always be open.
     
  6. Jan 10, 2015 #5
    Do you turn off the air coming from the outside?
     
  7. Jan 11, 2015 #6

    NascentOxygen

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    Maybe also too warm or too comfy to maintain a high state of alertness.
     
  8. Jan 11, 2015 #7

    mfb

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    ~1kg of CO2 per day and person, or half a cubic meter. Let's assume the car has 3m3 of volume inside, then you get ~0.7% of CO2 per hour (by volume, a bit more by mass). Driving for more than two hours without any ventilation is not a good idea, and with multiple passengers you'll certainly need some sort of air exchange.
    I would expect temperature had a larger effect there.
     
  9. Jan 11, 2015 #8
    A slow build up of excessive CO2 will result in hypercapnia. The normal reflex to hypercapnia is to awaken so that you can reposition yourself during sleep.

    Unless you are missing this reflex (a potentially fatal condition), what you are describing is not consistent with a hazardous buildup of CO2.

    If there was a buildup of CO2, that may have been what awakened you.
     
  10. Jan 11, 2015 #9

    OmCheeto

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    According to our government, a difference of only 1.4% in the level of oxygen means the difference between normal, and minimum required, to have no effect.

    * 20.9 percent: Percentage of oxygen found in normal air. No effect.
    * 19.5 percent: Minimum permissible oxygen level. No effect.

    From my calculations, if you were in a 2015 Subaru Outback, with a very voluminous 100 ft3 of air, it would only take about an hour and 20 minutes before there were noticeable effects.

    Of course, a smaller vehicle, and extra occupants, would reduce the time.

    ps. It looks like mfb and I have come up with very similar numbers, so I'll not recheck my maths. (available upon request)
    pps. Please use the homework template in the future. :oldwink: :biggrin:
     
  11. Jan 11, 2015 #10

    Ivan Seeking

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    Normally I have the exchange open but noticed later that it was closed. It wasn't very cold out that night but it was raining. And it is possible that the windows not receiving forced warm air were fogged, but I don't remember. If they were I wouldn't have necessarily noticed.

    I did find this
    http://ehp.niehs.nih.gov/1104789/

    I've never heard of it being potentially dangerous to drive without an air exchange. If true it would be news to a lot of people I'm sure. It could be argued to be a dangerous design flaw.
     
    Last edited: Jan 11, 2015
  12. Jan 11, 2015 #11

    russ_watters

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    This also assumes the car is completely air tight, of course, and it isn't anywhere close.
    It isn't dangerous. Cars are nowhere close to tight enough for CO2 buildup to be a problem.

    You breathe out a lot more water than CO2 -- much faster than you'd notice high CO2 content, it would practically be raining in the car.
     
  13. Jan 11, 2015 #12

    Doug Huffman

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    As I recall, and it has been many years now since I was last at sea as responsible crew or irresponsible civilian, the minimum [O2] concentration on submarines then was 17.5%!

    When we were at quiet-ship for long durations, [O2] was minimized to keep everyone in their bunks, groggy and miserable. Watch standing was a challenge. The roving watches would fill a drum liner from the O2 discharge and huff from it, but only for the psycho-ilogical boost.
     
  14. Jan 11, 2015 #13
    rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/
    At sea level, the O2 partial pressure is 0.2095 atmospheres.
    At 12,500 feet MSL, the partial pressure is 0.5950 x 0.2095 = 0.1247 atmospheres (equivalent to 12.47% O2 at sea level).
    At 15,000 feet MSL, the partial pressure is 0.5363 x 0.2095 = 0.1124 atmospheres (equivalent to 11.24% O2 at sea level).

    http://en.wikipedia.org/wiki/Barometric_formula
     
  15. Jan 11, 2015 #14

    Ivan Seeking

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    With inhibited function at possibly as low as 0.1% CO2, I don't see how you can support that statement.
     
  16. Jan 11, 2015 #15

    russ_watters

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    Cars are not very air-tight....but perhaps more to the point, do you really think that if this were a real issue it wouldn't already be well known? Ivan, you're blind-guessing here. You have an idea in your head and you are chasing it for no reason. Don't let your imagination get the better of you here.

    It was warm and humid in your car, which does cause people to feel drowsy. Then you opened the window and the cold air hitting your face woke you up. That's it. This isn't rocket science.
     
  17. Jan 11, 2015 #16

    russ_watters

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  18. Jan 12, 2015 #17

    Ivan Seeking

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    I just don't take arm waiving arguments as convincing; especially given the calculations already done and the paper cited. Frankly, I expect more from PF. And I did cite impaired function and not just a loss of consciousness as a point of interest. I'm not arguing that I fell asleep due to CO2. It may have been a blood sugar issue. But as soon as I did the math in my head I couldn't help but wonder.

    Also, anyone who has ridden in a closed car with a smoker may question just how much air exchange takes place during normal driving.
     
  19. Jan 12, 2015 #18

    Doug Huffman

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    @.Scott, yeah, I know I have climbed Mount Whitney's 14,500 feet twice, once the easy way and once the hard way. Anyone seriously mountaineering in the Sierra Nevada knows to acclimate for a week before pushing.
     
  20. Jan 12, 2015 #19

    RonL

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    A car in motion can have some serious air flow conditions, that produce a vacuum on parts of the body. Depending on seals and insulation, sucking air out of the cabin is likely if vents are closed or windows not opened a little.
     
  21. Jan 12, 2015 #20

    mfb

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    Sure.
    That is a good comparison. ~30g of water per cubic meter at 100% relative humidity, so 100g H2O would make it rain. Give or take a factor of 2 depending on temperature and car size.
     
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