Watching mastermind tonight, one of the questions is what gas causes airbags to inflate. The contestant guessed at Carbon Dioxide but the answer was given as Nitrogen.

That got me racking my brain, what explosion produces Nitrogen, most ones I know of give off oxides and heat.

Looking this up, the explosive is Sodium Azide (a compound of Sodium and Nitrogen and nothing else), when heated it separates into Sodium and Nitrogen, the Nitrogen going to a gas from a solid compound is in copious volume.

Problem .... Both the explosive Sodium Azide and the residue Sodium are extremely toxic, even in contact with the skin and especially to the surfaces of the lung. Very small amounts of Sodium Azide can be fatal.

Why on earth didn't they just use a compressed gas (or liquid CO2) which would be much safer.

Before anyone quotes numerous internet sites that state nobody has died from air bag chemical, they should check out the coroners report on Ronald Smith who walked away from the crash a healthy man but very rapidly went downhill and died of bronchial pneumonia three weeks later in 2012. The coroner stated this was from the airbag chemicals.

I have seen peoples skin after being attacked by an airbag and believed it was just from "bag slap", however, it could also be from skin burns caused by the sodium.

I should mention that the white powders associated with airbags are just lubricants such as talc which are relatively harmless.

I think they don't use other gases because they are not quick enough, ie they cannot inflate the bag fast enough to save you
from the impact. On the sodium problem...
According to Wiki...

"Older airbag formulations contained mixtures of oxidizers and sodium azide and other agents including ignitors and accelerants. An electronic controller detonates this mixture during an automobile crash:-
2 NaN3 -> 2Na + 3 N2
The sodium that is formed is a potential hazard alone and, in automobile airbags, it is converted by reaction with other ingredients, such as potassium nitrate and silica. In the latter case, innocuous sodium silicates are generated. Newer generation air bags contain nitroguanidine or similar less sensitive explosives."

I would think that all the sodium azide would be consumed in
the explosion, however as Wiki says they are using different
charges in newer airbags.

Liquid CO2 is certainly fast enough, its used to power air rifles. At 0 centigrade its pressure is about 35Bar at 20 centigrade its pressure is about 58 bar.

Thanks for the heads up on nitroguanidine, in the back of my mind I thought there had been a fuss and subsequent change a few years ago. It means that the Mastermind answer was incorrect - but of course the contestant was still wrong.

Surely, whatever the combustion products are, they are contained safely in the bag. They are supposed to be strong enough not to burst.

Liquid CO2 in my experience rapidly turns to 'dry ice' when the pressure is released. I think that might result in the slowing down of the expansion, and you wouldn't really want dry ice shot into your face anyway even in a bag.

No, the bag has a big hole in it. If it was sealed their is a high risk it would explode. It's also there to ensure it deflates easily afterwards.

It's not dry ice that happens, it's ordinary (water) ice. The water in the air condenses and freezes. It wouldn't be a problem with a very short blast. It is definitely fast enough if released properly.

I think the reason CO2 is not used is because nobody has invented a fast enough release mechanism.