Sodium azide is a white powder that explodes when it hits metal. When it hits water, it turns into an acid that can eat through your skin. When it's inhaled, it shreds your lungs. But for a long time, it was in your car.
A simple compound that's distributed as a white salt, sodium azide consists of a sodium atom and three nitrogen atoms. The instructions on how to deal with it are long, and most recommend having at least two people in the room, including one person specially trained to deal with sodium azide, at all times. Before you get the sodium azide out of its container, you want to clear the area of metals, including the metal spatulas typically used to measure out the chemical and any metal powders or residue on the counter. You definitely don't want to flush it down the sink when you're done, because it will hit copper piping, and then.. boom.
You also don't want to flush the sodium azide down the sink because water turns it into hydrogen azide, a powerful acid that causes a great deal of damage to human skin. Particles of sodium azide dispersed in the air will rip apart the lungs and fill them with fluid, suffocating you. And just to top things off, sodium azide can be absorbed through the skin. It will stop cells from using oxygen, suffocating you at a cellular level.
For those of you who are unfamiliar with them, cars are objects that contain a lot of metal, are often exposed to water, and have been known to be out in the kind of wind that can disperse salt through the air, so it seems counterproductive to put sodium azide inside it. However, it has one useful property. When sodium azide is heated to around 300°C, it quickly decomposes into two harmless components, sodium residue and nitrogen gas. It does so very, very quickly. So when a car takes a hit, the emergency system can heat or shock the small package of sodium azide in the steering wheel and the entire thing will explode into a nitrogen-filled pillow.
However, sodium azide isn't found in many air bags these days — its nasty properties caused it to be phased out in the 1990s — but in its time, it saved a lot of people. If you want to see a demonstration of sodium azide in action (saving a stuffed panda), watch this video from the Royal Society of Chemistry.
Top Image: Daniel Grohmann.