Elevated CO2 increases ventilation volume (rate & depth) (Respiratory Minute Volume = RMV), thereby increasing the rate of CO uptake by the body. Lowered partial pressure of (depleted) oxygen shifts the Haldane Equation in favor of increasing COHb saturation of the blood, as also occurs at high altitude. Thus, the other gas composition changes that occur in both complete or incomplete combustion exacerbate the toxic/lethal effects of CO on the body.