Scientists hired by the auto industry have determined that multiple factors -- including moisture and high humidity -- can cause some Takata air bags to inflate with too much force and hurl shrapnel at drivers and passengers.
The Independent Testing Coalition, which has been investigating the cause for the past year, announced its findings Tuesday.
Air bags made by Japan's Takata Corp. have caused at least 10 deaths and 139 injuries worldwide. The exact cause of the problem has eluded investigators for more than a decade, although more recent probes have focused on Takata's use of a mixture of ammonium nitrate and other chemicals to create a small explosion and inflate the air bags in a crash.
The rocket science company Orbital ATK, which was hired by the coalition, determined that three factors, working together, can cause the air bags to explode.
An ammonium nitrate compound that doesn't include a moisture-absorbing substance is more likely to explode after long-term exposure to heat and moisture, the company found. Takata does not include a moisture-absorbing substance. Orbital ATK also found that Takata's inflator assembly might allow moisture to seep in under very humid conditions.
The coalition said its findings apply to around 23 million of the 28 million Takata air bag inflators that have been recalled by the U.S. government. All of those air bags use specially formulated ammonium nitrate without a drying agent.
David Kelly, a former acting administrator of the National Highway Traffic Safety Administration and the coalition's project manager, said that while humidity and ammonium nitrate have been discussed as factors, this is the first time scientists have determined all the factors that combine to cause the explosions. Kelly also said the design of the inflator is coming under more scrutiny than it has in the past.
Kelly said determining exactly what was causing the explosions was a critical step. Now, he said, investigators will focus on the performance of all of the inflators that are being used as replacement parts in ongoing recalls of Takata air bags. He said owners whose vehicles have been recalled should continue to get their air bags replaced while the investigation continues.
"If you don't have the root cause, you're just throwing stuff up on the wall," he said. "You may never get to a situation where you can have an end game."
Takata has given multiple explanations for the problem, including quality control problems at manufacturing facilities and exposure of the air bags to high humidity.
Frustrated by the numerous explanations and the slow pace of the investigation, 10 of Takata's customers -- Toyota, BMW, Fiat Chrysler, Ford, General Motors, Honda, Mazda, Mitsubishi, Nissan and Subaru -- formed the Independent Testing Coalition and hired Orbital ATK in February of last year. Orbital makes rocket propulsion systems, small arms ammunition, warhead fuses and missile controls.
Kelly said he's not sure how long the next phase of the investigation will take. Scientists need to replicate the behavior of air bags over a period of several years, which will take time, he said.
In a statement Tuesday, Takata said the coalition's results are consistent with its own testing. Takata said age and long-term exposure to heat and high humidity appear to be significant factors in cases where inflators have malfunctioned. The company said it is cooperating with the coalition and the U.S. government.
The faulty inflators are used in both driver and passenger-side air bags. Globally, about 50 million inflators are subject to recall. U.S. safety investigators have said that the number of recalls is certain to grow as more tests are done.
Analysts say there could be 50 million or more Takata inflators in U.S. cars and trucks that haven't been recalled yet. Takata must prove to U.S. regulators that the inflators are safe or all of them will be recalled starting in 2018.