It's not exact science. Each individual will remain conscious at different levels of hypoxia (lack of O2 in the blood) The range is narrow. The variables are these. Cubic volume of air (room size hieght X Length X width) air Oxygen concentration (21% is normal) O2 exchange rate for individuals involved - here is where it get difficult. activity level, size, muscle mass% stress level, general health all affect the rate at which an individual will convert O2 to CO2. In a normal healthy individual without breathing problems the 21% O2 concentration in becomes somewhere near 14% O2 concentration out. This 14% concentration is still sufficient to revive someone through Mouth to Mouth resucitation. I would expect that O2 concentrations below 8% might begin to be a serious problem. Don't know the exact level. As O2 concentrations become lower the other gasses become important. If there is any CO (carbon Monoxide) in the room the blood will bind it preferentially over O2 so you could suffocate in a rather high O2 concentration if there is also a high concentration of CO. There is also a reflex that kicks in for some during hypoxia and hypothermia called the dive responce. When this happens the individuals use of O2 is drastically reduced and they may survive for a much longer time. It has been reported that some Yogi's (practitioners of Yoga) can reduce their breathing rate to very low levels thus reducing thier usage of O2 drastically. Though the Yogi's may be exchanging O2 more efficiently and exhaling a lower concentration of O2 than a person breathing normally.<br><br>BeachDoc, is there any lower threshold of O2 concentration in otherwise normal air mix that would be considered deadly? (O2 replaced with CO2, no other change in the normal gas mix in air.)