What are indicators of a chemical reaction would you want in a reaction used in airbag?

The development of airbags began with the idea for a system that would restrain automobile drivers and passengers in an accident, whether or not they were wearing their seat belts. The road from that idea to the airbags we have today has been long, and it has involved many turnabouts in the vision for what airbags would be expected to do. Today, airbags are mandatory in new cars and are designed to act as a supplemental safety device in addition to a seat belt. Airbags have been commonly available since the late 1980's; however, they were first invented (and a version was patented) in 1953. The automobile industry started in the late 1950's to research airbags and soon discovered that there were many more difficulties in the development of an airbag than anyone had expected. Crash tests showed that for an airbag to be useful as a protective device, the bag must deploy and inflate within 40 milliseconds. The system must also be able to detect the difference between a severe crash and a minor fender-bender. These technological difficulties helped lead to the 30-year span between the first patent and the common availability of airbags.

Reaction must be FAST and volume change must be BIG; in order to inflate the airbag quickly :)