A recalled Takata air-bag inflator is shown after it was removed at the AutoNation Honda dealership service department in Miami. (Joe Skipper/Reuters)

Federal officials involved in the largest auto recall in history are taking steps to ensure that the vehicles most at risk for deadly air-bag explosions receive replacement parts first.

With 29 million cars already under recall and the number expected to grow to 42 million vehicles, replacements for air-bag inflaters made by the Japanese firm Takata cannot be produced fast enough to meet demand.

In all, 19 automakers are subject to the recall, and with replacement inflaters in short supply, the National Highway Traffic Safety Administration (NHTSA) said Friday that top priority should be given to vehicles with inflaters most likely to fail.

To ensure that, NHTSA revised its recall order to set a deadline schedule under which the cars are to get replacements.

“It accelerates the remedy to the highest-risk vehicles immediately,” a senior NHTSA official said. He spoke on the condition of anonymity because he was not authorized to speak publicly. “This is a recall that affects almost the entire auto industry.”

As Takata air bags age, particularly in hot, humid climates, they develop a defect that can cause them to explode in a crash, spraying the driver or passengers with metal shrapnel. That malfunction has been blamed for 11 deaths and 180 injuries in the United States.

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 shrapnel.

Replacing the inflaters is complicated because the size and shape of the air-bag container has varied over the years with the make and model of vehicles.

“These are not Lego pieces, where you can snap out a yellow and pop in a red,” the NHTSA official said.

Many vehicles contain more than one air bag, with both front and side bags, so the total number of inflaters expected to face recall is 64 million to 69 million in 42 million vehicles, NHTSA said.

Takata, which has been the world’s largest air-bag maker, has teetered near bankruptcy as the crisis with its product emerged. NHTSA officials said Friday that it is the automakers, not Takata, that are responsible for replacing the faulty products.

In the early going, some vehicles under recall had their inflaters replaced with new Takata inflaters that contained ammonium nitrate. Those replacements, based on the theory it would take several years before they became risky themselves, ultimately will need to be replaced a second time.

Although NHTSA could not provide an exact number, officials said that now most replacements are being made by other manufacturers that do not use ammonium nitrate.

“Takata is designing the box, but other suppliers are supplying the inflaters,” said the NHTSA official. He said most automakers, who fear the taint of Takata, prefer using other inflater suppliers.

To date, NHTSA said, automakers have replaced 12.5 million of the recalled inflaters.

Car owners who have not received recall notices but want to see whether their cars have been recalled can search by vehicle number at safercar.gov/rs/takata/takatalist.html.