Edit: do your chemistry, kids. What's the molecular weight of carbon monoxide versus carbon dioxide versus regular 21% O2 breathable air? Think about how much CO would have to be in solution in a space to achieve the effect of stripping O2 molecules from hemoglobin in the amount of time it would take a similar volume of CO2 to displace enough air from your lungs to asphyxiate you. Read up on enclosed space entry. Most people who die from CO poisoning don't die immediately, as it's a generally a leaky furnace, and the exposure is cumulative over time, depending on volume and concentration in a regular atmosphere. Walking into an atmosphere of mostly CO2 will kill you in three minutes.