CO2 Build up in your pain cave

Guys, I haven’t proven this, but I want to relay two completely different experiences that happened roughly a month apart and I suspect this may be an important thing to consider.

I recently did the Uber Pretzel route badge and after about 3.5 hours I was losing focus, bonking hardcore, starting cramping and could barely finish. It was miserable. And that was the beginning. After the ride, I felt sick and was generally completely in a walking coma the rest of the day.

Saturday I did a century on zwift and felt like $1 million bucks the entire time. Finished strong barely even registered the rest of the day that I had spent 4 hours on the trainer at an average power much higher than the one I used on the uber pretzel attempt.

The thing I attribute this to was opening the window. After the uber pretzel i was goofing around on youtube and one of @JustinDoesTriathlon vids popped up and one of the tips he mentioned was around C02 build up. I had never thought of it. I’m sitting in a 10x10 room for 5 hours, with doors closed, breathing hard. I am almost certain my symptoms from the previous ride were based on either too low oxygen or too high CO2. It may sound silly and many of you may be saying you knew this, but I had never seen anyone mention this until Justin did. I was thinking of buying a sensor, and still may, but thought I would try it and it was ride-changing. Thanks Justin!

Anyone else experience this?


Does the room has any ventilation?
Like, is it connected to the main ac of the house?

10x10 by presumably 10 feet is a very big room… It would take a long time to fill the the room with co2…

That said, it might not be just co2, but a combination or heat build up and co2 build up…

In any case… My pain room is pretty big, and it’s connected to the ac system. And I either open a window if temperature is bellow 70 or turn on the ac to 70 degrees while I’m on the bike…

yes it has an AC vent, but not a return.


You probably need to open the door or something…

My house is newish construction (5 years?) And every room has a return to avoid vacuum issues

That’s BS. As long as you have a fan blowing on you, the zone of humidity and gases around you move. The air molecule content of the room is almost always gonna be the same as the atmosphere. The cracks of the doors and vents are big enough for the molecules to move around. If you’re in a closed room, heat is more of a factor than co2 content.

Furthermore, your body will breathe in and out proper amount of stuff based on your body metabolism and it’s needs. Also what you’re implying is carbon monoxide not dioxide overload.

You will have good and bad days in your workouts. That’s just how it is.


I would say it was far more likely to be heat than CO2.

You don’t start to get negative symptoms associated with high CO2 until ~4000ppm. The air in a house should be around 400ppm. Yes, you are exhaling CO2, which is quite concentrated, but the total volume of the room is large enough that you shouldn’t make a huge dent in that, especially if you have a fan going or something to mix the air a bit. I have done multiple 3+ hour trainer sessions in a room smaller than this (5x10ish) without these symptoms. You sweat a lot on these indoor rides and even fans are poor cooling. Drink a lot, stay cool.


Not sure on the co2 part but I do notice a huge difference in how I feel with how much I open the window, there’s been times where the wife has come and opened it another 6” and it’s made a huge difference for the rest of my workout

Yeah, there’s some interesting stuff on it for sure!

Here is a study showing the concentrations of CO2 at three large gyms:


We also know that with low/moderate CO2 increases, ability to concentrate and decision-making performance decreases:,in%20CO2%20concentration%20increases.

And lastly, we know that mental fatigue directly affects RPE and ability to work hard over long efforts in endurance sports:

So as pointed out, no we’re not in perfectly sealed rooms, but especially if you’re in a small room (I’m in something like 11x11x9) and you’re working out many hours, it seems very reasonable that you’re going to be experiencing some of the effects of high CO2. Airflow and heat over the body is a huge consideration, of course; I’ve sat on a trainer in ~35f with no airflow and I was burning up. So yeah, airflow is key, but some exhaust / intake is likely worth it if you’ve got the option #MarginalGains


I have a Netatmo that measures Co2 in my pain cave. If I don’t open a window the levels change from 400ppm to over 1000ppm in 45-60 minutes. Looking back I can find peaks up to 1500ppm.


For context…

How big is your pain room, and whats your ventilation situation?

I ask because the OP talks about his room size of 10x10…
and that he does 3 hours rides…

to clarify, my ride that day was over 5 hours. I started feeling a little squishy at about 3.5. Trashed at 5.

Of course it was 5 hours and the finish was on the Alpe, and it may have gotten hotter in the room.

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My room is roughly 18 sq m / 190 sq feet, no ac only vents in the walls.

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Glad there is some hard data posted here - my gut reaction was ‘no way is CO2 build up a problem’, so my gut is wrong in this case :slight_smile:


Has anyone seen any data/studies on how (if at all) rising CO2 levels in a closed training room impact athletic performance other than as a possible result of effects on cognition? I don’t know whether any such affect, if it exists, might be directly a result of CO2 levels or perhaps a result of decreasing O2 concentration in inhaled air because of an increase in CO2 concentration. I think it may be right to say that under “normal” circumstances inhaled oxygen levels are not generally a limiter, but not sure what happens in a poorly ventilated space.

Have become interested in this question since moving house - my old trainer room was ground level with opening windows, my new trainer room is in a basement reliant on mechanical ventilation. I know that the ventilation unit records increases in CO2 levels in extracted air from around 400ppm to around 900ppm + after an hour of riding. This measurement is however an average of the levels from the pain can itself and other basement rooms serviced by the same ventilation unit - so local levels in the pain cave are presumably somewhat higher than recorded at the unit. I intend to get myself a Netamo to take local readings.