About half the replacement air-bag kits that Takata Corp. shipped to automakers last month have inflators made by competitors, including some with a different chemical than the one linked to explosions that have killed six people.

That portion is expected to reach 70 percent by the end of the year, Takata said in a statement on Monday. The Tokyo-based company has been relying on competitors to help it replace defective air bags in about 34 million vehicles, the largest auto-safety recall in U.S. history.

Takata’s choice of chemical propellant, ammonium nitrate, has been found to be rendered unstable by high humidity and moisture, causing it to vaporize with so much pressure that the inflator canister bursts and hurls metal and plastic shards at drivers and passengers. Takata said it’s still safe to use a stabilized version the chemical.

“We continue to use phase-stabilized ammonium nitrate in our propellant, which is safe and effective for use in air-bag inflators when properly engineered and manufactured,” Takata said. “We are confident that our replacement air-bags are safe.”

Takata’s executive vice president of North America, Kevin Kennedy, is set to appear before Congress Tuesday. In written testimony submitted to a panel of the House Energy and Commerce Committee, he said the national recalls for driver-side inflators will proceed in stages.

“We are working with our automaker partners to transition to newer versions of driver inflators in our replacement kits or inflators made by other suppliers that do not contain ammonium nitrate propellant,” Kennedy said.


BMW, Honda

In his testimony, Kennedy seeks to assure the public that a defective air-bag inflator is an exceedingly rare event. For driver’s side units, there have been fewer than nine failures out of every 100,000 air-bag deployments, he said.

Takata isn’t transitioning away from using ammonium nitrate, according to a statement from the company.

Among the carmakers using Takata air bags are Honda Motor Co., Bayerische Motoren Werke AG, Ford Motor Co., Mazda Motor Corp. and FCA, the U.S. unit of Italy’s Fiat Chrysler Automobiles NV

Takata agreed on May 19 to expand recalls to fix faulty air-bag inflators in vehicles from 11 different carmakers. It may take weeks for the companies to sort out and notify all consumers affected.

“It is unacceptable to us and incompatible with our safety mission for even one of our products to fail to perform as intended and to put people at risk,” Kennedy said in the testimony posted to the committee’s website on Monday. “We deeply regret each instance in which a Takata air-bag inflator has ruptured.”