Sodium azide in car airbags poses a growing environmental hazard
provided by University of Arizona
utomobile airbags use a chemical compound that is so toxic that even small amounts can kill. Yet trucks loaded with hundreds of pounds of sodium azide routinely travel the nation's highways, and discarded airbags sit like environmental time bombs in the nation's auto junkyards, a University of Arizona scientist says.
Scientists really don't know where or how all this sodium azide will wreak greatest environmental havoc, UA atmospheric scientist Eric A. Betterton said March 26 at a national meeting of the American Chemical Society in San Francisco.
For the past few years, he and his undergraduate studentshave been doing laboratory experiments to find out.
Although sodium azide has long been used in many industrial products, such as broad-spectrum biocides, explosives detonators, anticorrosion solutions, and airline safety chutes, a much larger threat emerged with the advent of the automobile airbag, Betterton said.
"As the demand for airbags increases, and as vehicle fleets age over the next few decades, the amount of sodium azide that could potentially be released to the environment will greatly exceed the approximately 5 million kilograms (11 million pounds) that has already been incorporated into inflators in the United States alone," Betterton said. "Given the huge surge in production, there exists a greatly increased potential for significant accidental spills and subsequent human exposure to this material."
Sodium azide (NaN3) looks like common table salt. But it kills everything from bacteria and fungi to mammals - including humans. It is as powerful a poison as sodium cyanide.
As a graduate student, Betterton learned firsthand that even a whiff of hydrazoic acid (HN3) sodium azide's conjugate acid can be dangerous. While conducting a laboratory experiment with the dangerous compound, he suddenly felt dizzy, his blood pressure dropped, his heart raced and his eyes flushed bloodshot red.
Eating as little as 50 milligrams (less than two-thousandths of an ounce) of sodium azide can lead to collapse and a coma-like state within five minutes as blood pressure plummets and heart rate skyrockets. Ingest a few grams, and death occurs within 40 minutes.
Studies done in the 1970s show that, at 10 parts per million in the soil, sodium azide kills or degrades the seeds of many plants, Betterton noted. At 200 ppm, sodium azide not only sterilizes the soil - killing soil bacteria and fungi but also changes soil chemistry.
Just how sodium azide is metabolized is unclear. "Practically nothing is known about the environmental chemistry or biology of azide," Betterton said.
What is known is that sodium azide is water-soluble. "Spills therefore could potentially migrate into sewers, streams, lakes, and groundwater systems," Betterton said. The compound easily pronates (adds a proton) when wet, becoming volatile hydrazoic acid, a potential threat to sanitation workers, for example, he added.
Azide spills are not just "possible." They already have happened. In December 1996, a tanker truck hauling 80 fifty-five-gallon drums of sodium azide overturned and burst into flame 65 miles south of Salt Lake City. Rain intensified the giant toxic vapor plume released by the burning chemical. When the plume blew toward Mona, Utah, the town's nearly 2,000 residents were evacuated.
In Arizona, millions of pounds of sodium azide are shipped on Interstate 10 for airbag manufacture in Mesa, Betterton noted. A Utah-sized spill could be disastrous in population-dense Phoenix, he said.
Sodium azide tablets are stacked like small hockey pucks in two-inch-diameter metal canisters inside airbags. The driver-side airbag can is about 1 and 1/2 inches long and holds about 50 grams of sodium azide. The passenger-side airbag can is about six inches long and holds about 200 grams to inflate a bag big enough to fill the front-seat passenger area.
On impact, an electromechanical trigger heats sodium azide to explosively decompose, forming nitrogen gas - the main constituent of the air we breathe and metallic sodium. Additives like silica or iron oxide sometimes are used to scavenge the metallic sodium, which could cause burns.
There are no regulations requiring the detonation of airbags when cars are scrapped - "a smart way, I think, to get rid of this stuff," Betterton said. Scrap yard operators can remove car airbags and set them aside to accumulate in junk yards. Or, they are left in cars as they rot on the lot. Even worse, they are sent along with cars through crushers, and worst of all, wet crushers. The airbag canisters could be smashed, spilling sodium azide over the ground and generating sodium azide dust.
In laboratory experiments at the University of Arizona, Betterton and his students tested how readily sodium azide oxidizes (combines with oxygen) when exposed to some environmental oxidants that may be found in water, such as hydrogen peroxide, an ingredient in natural rainwater, and ozone, a very powerful oxidant in the atmosphere.
Oxidation is one way sodium azide degrades in the environment, just as the burning (oxidizing) truckload of sodium azide spewed up the spectacular toxic plume in Utah.
Betterton and his students found that only ozone is a potential oxidant for sodium azide.
However, sodium azide combines with water to form the highly volatile hydrazoic acid. Betterton and his students determined the "Henry's Law constant" for hydrazoic acid, or the ratio of how much hydrazoic acid in water will remain in solution and how much will be released as gas into the atmosphere. The Henry's Law constant number is very low. That is, much more of the acid is released as gas into the atmosphere than remains in water.
"I don't know - no one knows what the lifetime of azide is in the atmosphere," Betterton said.
Currently, Betterton and a student are running experiments to determine how sodium azide might migrate through wet soil where there has been an azide spill.
|Students who work on this project are funded through the Arizona/NASA Space Grant Program. Contact: Eric A. Betterton, University of Arizona, 520-621-2050; betteratmo.arizona.edu.|