Air bags provide protection against injuries which are due to collision with the car interiors during an accident. The mechanism behind air bags does not involve inflation of the compressed gas rather it is due to a chemical reaction involving sodium azide or NaN3. Here’s how it works.
When an accident occurs, the crash trigger sensors that detect collision in the car and send an electric signal to the sodium azide container. The electric signal sparks a small amount of an igniter compound in the container and heat is generated. The heat causes sodium azide to decompose into sodium metal and nitrogen gas which inflates the air bag.
Sodium azide produces about 67 liters of nitrogen gas which is enough to inflate a normal air bag. Amazingly, the time taken from the sensor detection to the air bag inflation is only 30 milliseconds or 0.03 second.
Therefore, in about 50 milliseconds after an accident, the driver hits the air bag and the deflation absorbs the energy produced due to forward motion of the occupant.
What happens to the sodium metal formed in the process? Sodium is very reactive and if mixed with water can form sodium hydroxide which is harmful for eyes, nose or mouth. Therefore, in order to minimize the harmful effect, other chemicals are mixed with sodium azide which react with sodium metal and turn it into less poisonous compounds.