Mark Rosekind, administrator of the National Highway Traffic Safety Administration, announced Wednesday that Takata has agreed to recall another 35 million to 40 million air-bag inflators. (Evan Vucci/AP)

Federal regulators have determined what is causing Takata air bags — installed in millions of U.S. vehicles — to explode and on Wednesday ordered that their recall be expanded and accelerated.

After 11 deaths, more than 100 injuries and the recall of 24 million vehicles, three independent studies have shown that Takata air bags are prone to explode after they are degraded by age and hot, humid weather.

“The science clearly shows that these inflaters become unsafe over time, faster when exposed to humidity and variations of temperature,” said Mark Rosekind, administrator of the National Highway Traffic Safety Administration (NHTSA).

The conclusion had been anticipated after a pattern emerged in which most of the fatalities and injuries occurred in cars that were more than seven years old and kept in humid regions where temperatures fluctuated between warm and hot.

Front and side Takata air bags are installed in a wide range of cars made by 14 U.S. and foreign automakers. Already, 28.8 million of the air-bag inflaters are under recall, and NHTSA on Wednesday added 35 million to 40 million more to the list. Each air bag has its own inflater, and most vehicles have two or more air bags.

The Japanese firm is the world’s largest manufacturer of air bags, and replacements are in such short supply that many U.S. drivers who have received recall notices are being told it will take weeks or months before their vehicles can be repaired.

“People who receive notification that there is a remedy available for their vehicle should act immediately to have their inflater fixed,” Rosekind said. “All vehicle owners should regularly check SaferCar.gov for information about any open safety recall on their vehicle and what they can do to have it fixed free of charge.”

Takata agreed to an amended consent order this week that would add up to 40 million of its air bags to the recall, NHTSA said in a statement. These recalls will continue through December 2019.

Unlike most air-bag makers, Takata uses ammonium nitrate to trigger a small explosion that inflates the air bag when the vehicle strikes something.

But in some vehicles — particularly those that were several years old and kept in regions with high humidity — the ammonium nitrate burns too fast, causing the chemical’s container to explode and spray drivers and passengers with metal shrapnel.

Takata says it will phase out the use of ammonium nitrate in its air-bag systems by 2018. That means that some drivers may have to have their air bags replaced twice, once with a newer ammonium nitrate bag and a second time when air bars that don’t use the problematic chemical become available.

“This recall schedule ensures the inflaters will be recalled and replaced before they become dangerous, giving vehicle owners sufficient time to have them replaced before they pose a danger to vehicle occupants,” Rosekind said. “NHTSA will continue to evaluate all available research and will act quickly to protect safety.”

He said the recall would be carried out in phases, based on the age of the inflaters and the degree to which they have been exposed to the high humidity and fluctuating high temperatures that accelerate the degradation of the chemical propellant.

“NHTSA’s aggressive actions in 2015 means this recall is already a year ahead of where it would have been if the agency had waited for this research,” Rosekind said. “As a result, all of the most dangerous inflaters responsible for the deaths and injuries are already under recall.”

The recall expansion does not apply to a modified version of the Takata air bags. Those air bags contain a chemical desiccant that absorbs moisture.

NHTSA said there are about 53 million Takata inflaters in use that don’t have the drying agent. About 32 million have the drying chemical.

Under the consent order amended this week, Takata agreed to do additional research into the safety of the desiccated inflaters. Unless they are proven safe, Takata will be required to recall them, too.

Last year, NHTSA said Takata would face a fine of up to $200 million, $70 million for mishandling the recall of the air-bag inflaters, and an additional $130 million if it further violated its settlement agreement with NHTSA.

“Today’s action is a significant step in the U.S. Department of Transportation’s aggressive oversight of Takata on behalf of drivers and passengers across America,” Transportation Secretary Anthony Foxx said. “The acceleration of this recall is based on scientific evidence and will protect all Americans from air bag inflaters that may become unsafe.”

Huma Hanif, 17, became the 10th American to die from an exploding Takata air bag March 31, when one malfunctioned after her car hit another vehicle at an intersection in suburban Houston. The auto dealer and Honda said they had sent six air-bag recall notices to Hanif’s family, but family members said they had not received the notices.

Among the automakers that have had some models recalled are Acura, Audi, BMW, Chevrolet, Chrysler, Dodge, Ford, General Motors, Honda, Infiniti, Mazda, Mercedes-Benz, Mitsubishi, Nissan, Pontiac, Saab, Saturn, Subaru, Toyota and Volkswagen.