How an Ingredient in Airbags Might Turn Explosive

An airbag can save your life, but if improperly manufactured, it could mean your death. At least five people have died after airbags made by Japanese company Takata exploded during deployment in crashes, bombarding passengers with sharp metal fragments. Now more than 14 million cars using the airbags are up for recall worldwide. The recall highlights a delicate balance of electrical, mechanical, and chemical processes inside an airbag—all of which are vulnerable to contamination and failure.

First, some background on how airbags work: Before an airbag deploys, the control unit has to detect a crash through various sensors on the car. Crash sensors are rigged to detect the sudden deceleration of a crash, but not be affected by the normal stopping and starting of driving. One form of sensor is the “ball and tube” setup, where a small metal ball is held in place by a magnet. In the event of a collision, the ball detaches from the magnet, rolls down and completes an electrical circuit that triggers the inflation. Similar sensors use weights connected to a coiled-up spring that unrolls with a sudden stop. Another type of sensor can be located inside the front doors, where it monitors air pressure; a collision from the side that pushes the door inwards will change the air pressure, and trip the sensor.

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If the sensor detects a crash, it tells the airbag’s inflator system to kick into gear. Most airbags are inflated when the inflator unit ignites a pellet of a compound called sodium azide (NAN3), kickstarting a swift chemical reaction that fills up the airbag with nitrogen gas (N2), sending it bursting out to cushion a car’s occupants. All of this happens within less than half the time it takes you to blink once.

But in the 1990s, Takata started looking for alternatives to sodium azide, due to the fact that the compound could release toxic fumes when the airbags deployed. First, the company’s engineers replaced the sodium azide with a compound called tetrazole. But tetrazole, while less toxic than sodium azide, proved more expensive. Eventually, over the objections of some employees, Takata developed a propellant using the ammonium nitrate (NH4NO3), more commonly used as a fertilizer.

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Ammonium nitrate “shouldn’t be used in airbags,” Missouri University of Science and Technology explosives expert Paul Worsey told the New York Times, saying the compound is really better for large-scale demolitions. “But it’s cheap, unbelievably cheap.”

Part of the danger with ammonium nitrate lies in the compound’s ability to transition through various solid states due to changes in temperature, pressure and moisture. The transition point between state IV, called beta-rhombic, and state III, called alpha-rhombic, occurs at 32.3 degrees Celsius (89.6 degrees Fahrenheit). The temperature cycles that a car experiences through days and nights, especially in hotter and more humid areas, may be enough to cause the compound to switch between these crystalline states, making it less stable.

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Takata senior vice president Hiroshi Shimizu told U.S. lawmakers in early December that the true cause of the airbag ruptures is still unknown, and the company has advocated for recalls to be limited to humid regions. Meanwhile, while still not characterizing its airbag propellant as defective, Takata has quietly modified the recipe for the propellant used in the replacement bags for recalled cars—though ammonium nitrate still remains a key component.

This article originally appeared on World Science Festival.