WASHINGTON —
Two weeks ago, Takata, under pressure from the National Highway Traffic Safety Administration, admitted for the first time that its air bags were defective and agreed to double the number of cars to be recalled in the United States to almost 34 million, the largest automotive recall in US history. The bags can rupture when they inflate, spraying metal fragments into the cabin.
But automakers are still checking which cars need to be included in the expanded recall, leaving many owners and lawmakers confused over which vehicles need servicing.
"The messaging around these air bag recalls has been tortured at best," Representative Fred Upton, a Michigan Republican, told Mark R. Rosekind, the safety agency's administrator. Upton chairs the House Energy and Commerce Committee, whose subpanel held the hearing. "We need more information, and clearer information for consumers."
Upton added: "I am concerned that NHTSA and Takata decided to release head-turning, headline-grabbing recall numbers at a time when the information is not yet actionable for consumers."
Rosekind responded that Takata's announcement was "a necessary first step" in expanding the recall. Automakers, he said, had worked quickly to sort out which cars fell under the expanded recall and would have updated lists within two weeks.
He added that the agency now had access to testing data from Takata, the automakers, and expert labs to help identify the root cause of the ruptures, which remains a mystery.
The safety agency's consent order with Takata also gives the agency authority to approve future fixes, he said.
"Now, NHTSA is in the driver's seat," said Rosekind, who added, "This may be the largest and most complicated recall in the nation's history."
Later testimony from an executive vice president for Takata, Kevin Kennedy, and from auto groups was expected to delve into the root cause.
Experts, as well as former Takata engineers and consultants, have pointed to Takata's use of a cheap, widely available compound called ammonium nitrate in its propellant, which helps generate the gases that inflate the air bag.
Ammonium nitrate is highly sensitive to moisture and to changes in temperature. With prolonged exposure to temperature swings and humid conditions, the propellant can break down, making it prone to combusting violently, experts say. Takata's new design would alter the shape of the propellant wafers but not the composition.
Former consultants have also reported that Takata's inflaters, which hold the propellant, can allow in moisture over time, and that the parts maker has not adequately tested their airtightness. When the propellant combusts violently, the inflaters can overpressurize and rupture, sending hot metal fragments into the car's cabin.
Despite mounting evidence of the effect of high humidity, the root cause for the air bags' failure has remained a mystery for Takata and automakers. Two parallel investigations, one commissioned by Takata and another by a consortium of automakers, are studying a range of possible reasons.
In his written testimony, Kennedy said, "Takata's best current judgment is that the potential for rupture is related to long-term exposure, over a period of several years, to persistent conditions of high heat and high absolute humidity."
A person with knowledge of studies and tests conducted at the Fraunhofer Institute in Germany, which Takata hired to research the problem, said that so far, investigators had focused on the properties of the propellant and problematic inflater components, but had not yet reached any definite conclusions.
The person said that manufacturing variability and exposure to heat and moisture are conditions faced by any air bag, and did not alone explain the string of violent ruptures involving Takata air bags. The main problem came back to the use of ammonium nitrate propellant, he said.
Takata was set to argue that whatever the root causes, they are so complex and happen over such a long period of time that identifying them would have been beyond the scope of testing standards set by the automakers it supplies.
But behind the scenes, Takata engineers have for years struggled to tame the ammonium nitrate in the propellant, proposing in patents remedies like stabilizing the compound with potassium, reducing the effects of moisture by adding desiccants, or coating the propellant with paraffin.
In one patent application, filed in October 2006, Takata engineers explained that moisture could seep into the propellant both during manufacturing and once the inflater was placed inside the car.
Stress tests showed that without adding moisture-absorbent material to the propellant, the inflater "ruptured due to extremely high internal pressures," the patent application said.
Other air bag makers have said they use different compounds in their propellants.
Engineers at Autoliv, the Swedish-American air bag maker, say in their own patents that they found ammonium nitrate's sensitivity to temperature changes rendered the compound "unacceptable" for typical air bags, and that keeping ammonium nitrate sufficiently dry "is generally impractical for most manufacturing situations."
The problems with the air bags date back almost 15 years, when drivers began lodging complaints about them with federal regulators.