US regulators have more than doubled the recall of Takata airbag inflators to include tens of millions more cars, making the order the largest automotive recall in history.

The inflators, meant to save people in the event of a crash, are prone to explode, expelling fragments of metal. Some 63.8m cars are affected in the US – equivalent to more than a quarter of the vehicles on US roads. As of Wednesday morning the inflators were known to have killed a dozen people – 11 in the US and one in Malaysia – and hurt more than 100 others.

At a press conference in Washington on Wednesday afternoon, however, National Highway Transportation and Safety Administration (NHSTA) administrator Mark Rosekind said he had learned from Honda that two more people had been killed by the faulty auto part in Malaysia in the past three weeks.

In Houston, Texas last month, a teenage girl was killed in a minor collision that triggered the airbag, when the Takata inflator exploded.

“Please,” Rosekind asked reporters, “as you report your stories, include this. Vehicle owners who have received notice that there are parts available for their repair should take action immediately.”

Makes and models affected are listed in a special page on the NHSTA website or by vehicle identification number. The models from the expanded recall are not included yet, according to the regulator’s guide, but users can sign up for an alert tailored to the car they own. NHSTA has said the top priority for replacing airbags goes to “high absolute humidity” areas across the American south-east and island territories and encouraged car owners to keep checking the page.

Over time, humid conditions can alter the composition of ammonium nitrate and a trying agent, chemicals that react to deploys of the air bag and increase the likelihood of an explosion. Ruptures are much less likely in new vehicles.

The recall already affects 28.8m cars in the US; the NHSTA said it had fixed 8.17m of the faulty products as of 22 April. The accelerated recall adds between 35m and 40m new cars to the list, Rosekind said.

“I’m going to make this personal: my family has a vehicle with a Takata inflator sitting in the driveway,” said Rosekind. “We are checking weekly to see when those parts are going to be available.”

The defect does not affect all the Takata inflators, but Rosekind urged consumers to try to find “loaner” cars where they could. Ordering companies to provide temporary replacements is beyond his remit.

“I have looked at it every which way,” he said. “NHTSA has no authority to require loaners to be made available.”

Part of the problem lies in creating a design for a new inflator that will not blow up.

“Where we could push, we’ve pushed as quickly as we can with the caveat that we can’t push too hard without running the risk of a new design for an inflator that’s not safe,” Rosekind said.

Stock in the Japanese manufacturer was down 9.25% on Wednesday, following news of the expanded recall. The problem now is that the models of car under recall are so numerous that the replacement of all dangerous parts is likely to take years.

Rosekind said the recall was planned through 2019.