By Janet Raloff
From San Francisco, at the spring national meeting of the American Chemical Society
While in graduate school, Eric A. Betterton was working with sodium azide when some grains of the white chemical got wet. Immediately, the crystals transformed into the highly volatile hydrazoic acid, and the chemist inhaled just the barest whiff.
At once, his skin reddened and he fell dizzy with a wildly racing heart. As his head throbbed, Betterton’s breathing grew difficult. “I thought I was toast,” he recalls. Sodium azide is an explosive, broad-spectrum biocide with toxicity comparable to sodium cyanide’s. “Lethal to practically all forms of life,” just a few grams can kill a person, notes Betterton, an atmospheric chemist now at the University of Arizona in Tucson. So, imagine his surprise a few years ago when he read that most airbags pack 50 to 200 grams of sodium azide (NaN3) each. It’s the source of the nitrogen gas that inflates the devices.
U.S. vehicles are a reservoir for some 5 million kilograms of sodium azide, he learned, with lots more due to hit the streets as the latest-model cars increasingly include passenger-side airbags, side-impact bags, and ceiling bags. Inside airbags, the azide is safe, Betterton notes.
However, his team’s new data indicate that if the compound gets released—either during airbag manufacturing or when a car is junked—it could persist stably until moistened. Then, transformed to hydrazoic acid (HN3), it could move with air or water through the environment.
Auto recyclers who might shred or smash canisters of sodium azide face the biggest safety threat, says Scott Schmidt of the Alliance of Automobile Manufacturers in Washington, D.C. When airbags were rare, he says, many recyclers didn’t know to remove or trigger the devices before a car was shredded. But today, he says, the azide “is something recyclers are definitely aware of.”
He concedes the possibility of accidents, however, especially by thieves or recyclers “who may take out the driver- and passenger-side airbags and [miss the newer] side or roof airbags.” Betterton’s studies have confirmed a widespread suspicion that once released from its cannister, hydrazoic acid breaks down in sunlight. His team’s studies reveal, however, that it doesn’t break down quickly. “We’ve shown that it can stay in the atmosphere for time scales that could range from minutes to hours, maybe longer,” he told Science News. That’s long enough, he worries, for a plume of the gas to waft into residential neighborhoods or collect in sewers and tunnels.
Because hydrazoic acid can oxidize into harmless nitrogen—as the azide does when airbags inflate—the Arizona group began scouting for naturally occurring agents that might neutralize the poison in water or moist soil. Though dissolved oxygen and hydrogen peroxide seemed good candidates, Betterton reports that at concentrations typical in the environment, neither neutralized the acid in less than weeks or months.
He and his colleagues then explored ozone, which finds use in some water-treatment plants. Though it oxidized the poison in seconds to minutes, Betterton reports, the azide could trigger ozone’s destruction. “So, you need about 30 molecules of ozone for every molecule of azide you want to destroy,” he says.
