How can automobile airbags inflate in a few milliseconds? With some clever chemistry! When sodium azide, NaN3, is ignited by a spark, it releases nitrogen gas which can instantly inflate an airbag. The problem, however, is that the reaction also forms sodium metal which reacts with moisture to generate sodium hydroxide, a highly corrosive substance. A burst airbag can therefore wreak havoc. Chemical ingenuity, however, comes to the fore. If potassium nitrate and silicon dioxide are also included with the sodium azide, the only products that form in addition to nitrogen are potassium silicate and sodium silicate. Both of these are inert, harmless substances.
An airbag is designed to release some of the gas just after it deploys to help cushion the impact against the body. Hitting a fully inflated, unyielding airbag, though, could be catastrophic. So before widely promoting this protective device, the safety of the contents had to be ascertained. In the 1970s Mercedes settled this issue by putting a cage full of canaries in a car and deploying an airbag. Canaries are extremely sensitive to minute traces of toxic gases, but the birds survived the experiment with no casualties. By the late 1980s airbags had become a common feature in automobiles and have since saved thousands of lives.
But, like with any other scientific advance, there is a “but.” Airbags are not problem-free. While the chemistry involved in curbing sodium hydroxide production is clever, it is not foolproof. Small amounts of the caustic material have been released from deployed airbags, in rare cases causing severe eye injuries, including blindness. The most serious concern, however, is damage that can be done by an airbag as it rockets out at an astounding speed of up to 330 km per hour. A blow to the head by the rapidly inflating bag can be lethal. Unfortunately, over a hundred people, mostly children and small adults, have been killed in this fashion, ironically sometimes in low speed collisions when lives were not at risk. There is a great deal of research underway to find ways to ensure that airbags are only deployed when necessary and that they are deployed in the safest way. This includes various sensors that can gauge the weight of the passengers to allow a computer to calculate how and if an airbag should be inflated. In any case, children must not be allowed to sit in the front seat of an airbag equipped car! Some researchers even argue that better protection can be had by stronger promotion of seat belts than by emphasizing airbags.
There is a further problem that needs to be addressed. Sodium azide is more toxic than cyanide. What will eventually happen to all the azide in cars headed for the junk heap? What if the canisters are not removed? If sodium azide is released, it can react with water to form hydrazoic acid which is not only toxic, but is highly explosive. Sodium azide can also react with metals such as copper or lead to form explosive copper or lead azides. Just ask the plumbers who were called to a lab where sodium azide solutions had been used. When they took out a piece of copper pipe and tossed it into the garbage it exploded. An unfortunate and shocking way to learn about the chemistry of azides!