You're continuously producing CO2 whenever you're alive. If you fall down and land in a normal atmosphere then your normal breathing reflex will save you. Low oxygen atmospheres don't stop you from breathing; they're dangerous because you continue breathing as normal. The CO2 is carried away from your lungs with each breath, so there's no feeling of suffocation.

> If you inhale a lot of helium and pass out, your body doesn't start breathing again because there is no CO2, so you just suffocate instead of inhaling oxygen.

Other posters have pointed out that this is false(you will continue producing CO2 while unconscious just fine, chemoreceptors will notice). I would like to add that I have personally witnessed people falling unconscious from helium. They recovered just fine.

Really, the greatest danger is the fall.

you don't breathe because there is CO2 in the air, you breathe because there is CO2 in your blood

If that were true then divers would have to bring canisters of CO2 down with them.

Well helium rises. So falling down is a good thing, as most vents are above you. If you go to a lab with heavy gases you'll notice vents on the floor for exactly this reason.