Originally Posted by csuhickey

Second, how well ventilated is the room you train in? here's some food for thought.

Let's say that your average VO2 is 4L/min and your RER is .85. That would mean you're producing 3.4L of CO2 per minute. Times 90 minutes that's 306L of CO2 produced. In a sealed room that is 2mx3mx3m there is 18000L of air. 306/18000 is 1.7% CO2... by comparison air outside averages 0.038% CO2 and 5% CO2 is fatal. Breathing air that is 1.7% CO2 will probably give you a headache.