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 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. 
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