34 million cars that use Takata airbags have been recalled for fatal defects in them

For more than a decade, the Japanese company Takata, one of the largest suppliers of airbags, denied that its products were defective even as motorists were killed by exploding airbags and automakers around the world recalled millions of cars equipped with its products.

But on Tuesday Takata admitted that its airbags were defective and agreed to double the number of vehicles recalled in the United States, to nearly 34 million — or about one in seven of the more than 250 million vehicles on American roads — making it the largest automotive recall in American history. The airbags can explode violently when they deploy, sending shrapnel flying into a car’s passenger compartment. Six deaths and more than 100 injuries have been linked to the flaw.

Danger in the Steering Wheel

Airbags made by the Takata Corporation have a potential defect when they deploy. Some have ruptured, sending metal fragments into the car.

But since the appointment of a new administrator, Mark R. Rosekind, the agency has shown greater assertiveness toward companies like Takata.

Takata, in a statement, said that the announcement was the culmination of a year of work with automakers and the safety agency.

“We are committed to continuing to work closely with N.H.T.S.A. and our automaker customers to do everything we can to advance the safety of drivers,” said Shigehisa Takada, Takata’s chairman and chief executive.

Despite the sweeping nature of the announcement, the agency said it would not know exactly which models of cars would be recalled until it coordinated with automakers, which could be several days. The final number may change as more tests are performed, Mr. Rosekind said. Ten automakers, including Honda, Chrysler and Nissan, have recalled cars in the United States because of the defect.

Honda, the automaker that has been most affected by the Takata airbag recalls, said that it was reviewing the announcement to determine what fresh recall measures might be required. Honda already said that it was looking to other suppliers to provide replacement airbags.

Nissan, Chrysler, Toyota and BMW also said they needed to review the announcement before taking any further action. None of the automakers would say whether they expected to have access to enough replacement parts to repair all the cars potentially carrying defective airbags.

Officials at other affected automakers, including Ford, did not immediately respond to requests for comment. Even now, Takata and automakers continue to search for the root cause of the defect. But in new filings with the safety agency, Takata went beyond its previous statements that there had been some errors in manufacturing and admitted to flaws in the airbags’ design and components.

For example, Takata said that the propellant in the inflaters — the explosive material that generates the gases to inflate the airbag — could degrade over time if exposed to high humidity and changes in temperature, making it prone to “overaggressive combustion.”

Former Takata engineers told The New York Times last year that they had raised concerns over a decade ago that ammonium nitrate, the explosive material Takata uses, was sensitive to moisture and temperature swings. But those concerns went unheeded, they said.

And for the first time, Takata also acknowledged that its testing had uncovered leaks in some of its inflators that could allow moisture to seep into them over time. When that happens, the propellant breaks down, making it more susceptible to exploding violently.

Last week, a former Takata consultant said that tests he carried out on prototype Takata airbags in the early 2000s revealed leaks, and that he had urged the company to use a different leak testing method, one that he had devised and was offering to sell to the company. His advice also went unheeded, he said.

But in a statement on Tuesday, Takata argued that under industry-mandated testing, it could not have been expected to spot such complex problems.

“The potential for this long-term phenomenon to occur was not within the scope of the testing specifications prescribed by the vehicle manufacturers,” the supplier said in a statement.