Since 2008, 28 million cars have been recalled in the United States because they contained air bags that could explode and hurl shrapnel into drivers during even minor fender benders. Ten deaths and more than 100 injuries have been linked to the faulty air bags.
Ultimately, the problem was traced to the solid propellant, ammonium nitrate, used to inflate the air bag. When the propellant was installed in the inflator, it was typically in the shape of a wafer or bat wing. However, it changed shape when exposed to humidity over time. When the air bag was triggered, the change in the propellant’s shape led to an unexpected increase in pressure that detonated the inflator instead of filling the air bag. The force of the explosion would hurl pieces of the inflator into the cabin like shrapnel, injuring those in the vehicle.