Takata, the Japanese/German auto-parts maker, that supplied airbags used in millions of recalled vehicles employed an unusual explosive chemical to inflate the safety devices, which may have contributed to the spraying of metal shrapnel at vehicle passengers.

Bloomberg Businessweek reports that the chemical will likely become the focus of National Highway Traffic Safety Administration investigators as they continue a probe into the defective airbags that have so far been linked to four deaths and 30 injuries in the United States.

Chemicals have long been the powerful mechanism behind airbags. That’s why after some crashes, the driver or front-seat passenger in a vehicle may have chemical burns on their skin.

Typically the inside of an airbag contains an igniter that heats an aspirin-sized tablet of compressed chemical. The ensuing reaction fills the airbag with gas, inflating it at speeds reaching a few hundred miles per hour.

Takata began using ammonium nitrate in its airbags in the late 1990s, because of the chemical’s ability to make airbags inflate in a matter of milliseconds.

Jochen Siebert, a Shanghai-based managing director of JSC Automotive Consulting, says Takata – the only auto-parts supplier to use ammonium nitrate – favored the chemical because it allowed for the creation of smaller and slighter airbags.

“It was all about technology; it wasn’t even about price,” Siebert tells Businessweek. “But it all went wrong.”

Regulators believe that issues with the airbags have been caused by the presence of moisture, which led automakers to initiate recalls in areas of high humidity such as the southern United States.

Scott Upham, president of Valient Market Research, tells Businessweek that the presence of moisture can render the ammonium nitrate unstable.

When the safety device becomes activated the combination of the unstable chemical and igniter inside the airbag can create an environment where too much force is present.

The high-pressure force created by the chemical reaction has been found to send small pieces of metal flying at drivers and passengers in the affected vehicles.

Officials with Takata declined Businessweek’s request to comment on the use of ammonium nitrate in their airbags.

So far this year, 10 automakers have recalled nearly 8 million vehicles to replace the Takata airbags.

While NHTSA issued an unusual warning last week urging owners of affected vehicles to get them fixed, a shortage of parts has left millions of potentially dangerous vehicles on the roadways.

Takata’s biggest customer Honda, which has been linked to at least three of the deaths related to the airbag defect, said last week that it doesn’t have enough parts to fix the 2.8 million vehicles the company has recalled.

Instead, the manufacturer is sending out recall notifications only as parts become available, with priority being reserved for areas of high humidity.

Toyota, which expanded its recall of vehicles with Takata airbags last week, says because of the lack of parts, the company would in some cases disable the airbags, leaving a note urging customers not to ride in the front passenger seat.

For their part, officials with Takata say they are “working night and day” to enhance the safety of parts.

According to Businessweek, this isn’t Takata’s first issue with the chemicals used in its airbags.

The company was found to have previously improperly stored chemicals and mishandled explosive propellants used in its airbags at a plant in Mexico. In March 2006, a series of explosions at the factory led authorities to evacuate nearby residents.

Air-Bag Maker in Global Crisis Used Unusual Explosive [Bloomberg Businessweek]

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