Mark Rosekind, administrator of the National Highway Traffic Safety Administration, said a review of ongoing investigations into the cause determined that a combination of time, temperature fluctuations and moisture make chemicals in the airbags degrade.
"The science now clearly shows these inflators can become unsafe over time," he said.
His remarks came amid a doubling of recall efforts that already represent the largest consumer-product recall in US history. Prior to Wednesday's announcement, 28.8 million inflators were already under recall because some Takata airbags could explode instead of deploying normally during a crash. Takata said Wednesday it would recall an additional 35 to 40 million inflators. The expansion comes as the global supplier and federal regulators agreed on an amendment to a consent order signed in November that spelled out a fine as high as $200 million.
Faulty airbags are responsible for killing 13 motorists and injuring at least 100 more. The two most recent deaths occurred in Malaysia within the past three weeks, Rosekind said.
Takata "misled, obscured and withheld information from NHTSA, consumers and their customers in the automotive industry. We can't lose sight of that." – Mark Rosekind
In the United States, residents who have older cars and live in hot, humid areas are at the greatest risk. Federal authorities have given them priority in a coordinated repair effort, but those efforts won't be completed until 2019.
The findings come from a report written by H.R. Blomquist, a former industry chemist and propellant expert commissioned by NHTSA to evaluate the work of three ongoing investigations into the long-running Takata airbag problems.
Blomquist writes that faulty inflator design allows humid air to slowly enter the inflator, where moisture-sensitive propellant slowly degrades over time due to fluctuations in temperature. When airbags deploy in a crash, "the damaged propellant burns more rapidly than intended, and over-pressurizes the inflator's steel housing causing fragmentation," he wrote.
These conclusions support NHTSA's earlier suspicions that high humidity played a role in the ruptures. Earlier, some manufacturers conducted regional recalls of airbags on vehicles located in the Gulf Coast region. The degradation can occur as early as six years from the date the airbags are manufactured, according to the report.
Degradation can occur in all Takata airbags that use ammonium nitrate as their propellant but do not contain desiccant, a substance that helps absorb moisture. Airbags that do contain desiccant were not part of Wednesday's expanded recall.
Most other airbag manufacturers have shied away from the use of ammonium nitrate in their products and instead use guanidine nitrate, which they believe is a less volatile compound.
Blomquist reviewed three ongoing probes to reach his conclusions. Honda, the automaker most directly affected by the recalls, had contracted with Exponent, an engineering consulting company to study the issue; several other automakers, set up an Independent Testing Coalition to examine the airbags; Takata also had an ongoing review that contributed to his research.
Rosekind said Takata officials have been cooperative in their recent probes, though earlier government orders show the Japanese supplier has "misled, obscured and withheld information from NHTSA, consumers and their customers in the automotive industry," Rosekind said. "We can't lose sight of that."