A combination of three factors is the root cause of Takata air bag inflator ruptures linked to at least 10 deaths worldwide, a group of 10 carmakers says.
According to the review by a Utah-based team from Orbital ATK, the cause of the ruptures linked to more than 100 injuries is exposure to humidity, design and manufacturing issues, and use of the volatile chemical ammonium nitrate.
The results of the year-long review come as the National Highway Traffic Safety Administration continues to investigate whether the recalls of nearly 29 million defective Takata inflators in the US should be expanded to include another 70 million to 90 million inflators with ammonium nitrate.
Former managers interviewed by Reuters described "chronic" quality failures at Takata's North American inflator plants, an assessment reflected in dozens of company emails and documents dating back to 2001.
Those problems, the former managers said, make it difficult for the company and regulators to pinpoint which inflators -- among tens of millions -- pose a danger.
When exposed to moisture, ammonium nitrate, which is used to inflate the air bag, can cause the inflator to rupture with deadly force, spraying shrapnel into vehicle occupants.
The review by the carmaker-backed Independent Testing Coalition found the ruptures were caused by a combination of three factors: ammonium nitrate propellant without moisture-absorbing desiccant, long-term exposure to repeated high-temperature cycling in the presence of moisture, and "an inflator assembly that does not adequately prevent the intrusion of moisture".
In a statement on Tuesday (local time), Takata said the group's findings were "consistent" with its own and those of Germany's Fraunhofer Group, which was hired earlier by Takata to conduct additional testing.
David Kelly, a former NHTSA acting administrator who is coordinating the coalition's activities, briefed NHTSA and congressional staff members on Tuesday on the findings. A NHTSA spokesman said the agency was reviewing the findings.
The study only covered inflators that do not have a drying agent, or desiccant. Most of the inflators with ammonium nitrate that have not been recalled have the drying agent.
In a statement on Tuesday, NHTSA cited a consent order issued last November under which "all Takata ammonium nitrate-based inflators must eventually be recalled unless Takata can establish their long-term safety", including those with the drying agent.
The safety agency previously has said that "multiple tens of millions" of Takata inflators may need to be replaced.
The consortium, which includes Honda, Toyota, Ford, General Motors and Fiat Chrysler will next investigate the performance of all inflators being used as replacement parts for current recalls.
Congress has raised concerns that some replacement inflators with ammonium nitrate also will eventually need to be replaced.