And if you are? Not everyone has an open Jeep and you can't predict when a fire happens. It's just something to be aware of. 15% isn't much.
here's what i think and i'm far from being an authority in the matter so take it with a grain of salt:
your body has a built in alarm system for excess of CO2, if you try to hold your breath that's just what happens - you'll eventually start gasping for air and that is due to increase co2 content in your lungs, an automated failsafe trigger that will force you to open your mouth and breathe in fresh air - it's not the lack of oxygen that sends the alarm, it's the increase content of co2, hypoxia is one of the known issues, where for example, at high altitude the lack of oxygen causes the brain to malfunction with no warning or discomfort other than getting tired quickly and that is since there is not enought co2 to "tell" you something's wrong, you'll be losing consciousness without realizing what is happening to you. An inert odourless gas could have the same result in high concentrations which would result in low oxygen percentage, you'd be out without warning and without enough oxygen you might sustain brain damage.
At the other extreme is for example deep diving, i can't remember all details but in this case the air you breathe in has added nitrogen to prevent too much oxygen from going into your lungs and bloodstream (would be due to increased air pressure going in your lungs so you can breathe and that would result in more air mass and therefore a higher oxygen count per each breath, without the increased pressure at those depths to compensate your lungs would collapse).
On the other hand i don't see anyone stopping the vehicle with smoke coming out and not opening the door, stepping out and deploying the f/e from the outside (side of the car) somehow, if you do run into trouble (too much co2) your body will tell you and even if you continue to the extreme (unlikely, try doing that by holding your breath) you'll pass out and release the trigger, should dissipate quickly and you won't lose your life (but then again that's with a door open, doesn't have to be with the top off.
Enclosed spaces with automated gas based fire suppression are dangerous as you might not be able to make it out of the room and colapse in which case your life is in danger - that's why all of them have pre-deployment alarm and have override emergency switches in that area for cancelling the process.
but then again nothing is 100% failsafe, regardless of what type of f/e you are using since it is under pressure if you mishandle it you can get hurt and that includes what you are saying (releasing the gas and not take precautions of breathing in excessive amounts - in that case yes there is a risk and that also includes the dry-chem ones which i doubt are safe in an enclosed space either).
but that's just my opinion - i'm no expert in the matter so if anyone knows more i'd like to hear it (i guess i'm too lazy to google it)
