[krool1969 (OP)]

When I was a welder a very big deal was to weld in an enclosed space. All kinds of safety measures need to be taken. Even a completely brand new, virgin tank (that has never contained anything) can be really dangerous because the iron absorbs ALL the oxygen inside leaving just the nitrogen. (this can happen really quickly)
Even deadlier is an environment with pure carbon dioxide. In a pure nitrogen environment you take a breath you pass out but if someone can get you out in time, you should be okay. Pure CO2, one breath and you're stone dead. Why?

[RD]

Nitrogen is an asphyixiant gas : it's not toxic , it just does not contain oxygen, so causes suffocation.

Whereas carbon monoxide is toxic (poisonous) ...

[carbon monoxide] combines with hemoglobin to produce carboxyhemoglobin, which is ineffective for delivering oxygen to bodily tissues. Concentrations as low as 667 ppm may cause up to 50% of the body's hemoglobin to convert to carboxyhemoglobin. A level of 50% carboxyhemoglobin may result in seizure, coma, and fatality.

https://en.wikipedia.org/wiki/Carbon_monoxide#Toxicity

[Bored chemist]

Carbon monoxide is not the same as carbon dioxide.
Carbon dioxide and nitrogen both kill you by not being oxygen.
One breath of CO2 wouldn't do much harm, though it might make you cough a bit.

[CliffordK]

Your body, of course, has to get rid of CO₂.  The higher the concentration of atmospheric CO₂, the harder it is to do so.   CO₂ does bind to hemoglobin, and while it is a different site than oxygen binds to the hemoglobin, it does reduce the oxygen carrying capacity of the blood.

The red blood cells also catalyze the dissolving of carbon dioxide in the blood, and the formation of carbonic acid.  The higher the CO₂ concentration, the more acidic the blood.

[Bored chemist]

There's another factor. The body produces CO2 and it has a system for monitoring how much CO2 there is  in the blood. If that level is high, it increases the breathing rate to get rid of the excess.
In an atmosphere of pure CO2 that would tend to drive residual oxygen out of the body faster than an atmosphere of, for example,  nitrogen.
However there's another mechanism that's more likely to make a difference.
If the brain is deprived of oxygen you faint.
Normally, that's a good thing since it makes you lie down and that stops gravity draining blood + oxygen away from the brain.
However, if you are in a room with no oxygen and you faint then you are not in a position to save yourself.

The practical outcome of all this is that you should never enter a confined space unless you have tested the atmosphere and know for certain that there is enough oxygen.

In the UK, it's considered important enough to have its own law.
http://www.legislation.gov.uk/uksi/1997/1713/contents/made

[CliffordK]

How quickly does CO₂ mix with air?  It is about 150% of the weight of O₂ and N₂, and thus if you introduce pure CO₂ into a room, it could be more concentrated near the floor.  Thus, fainting might in fact be worse than if you could remain standing.

[Bored chemist]

Good point, it does "pool" and that's sometimes been a problem.