For months, the problem of defective airbags made by the Japanese supplier Takata has festered in the auto industry as recalls have mounted and automakers have searched for the reason that the airbags can explode violently, sending pieces of metal into the cabin.

On Wednesday, Toyota and Nissan sharply escalated the recalls, adding 6.5 million vehicles worldwide to the 25 million already deemed to contain potentially dangerous components. At least six deaths and more than 100 injuries have been linked to the defect.

Toyota said tests carried out on vehicles with Takata-made airbags, including some recovered from junkyards, had shown that the steel inflaters, the parts that contain the propellant and can burst apart, were sometimes not airtight. In those cases, the propellant can become destabilized, raising the risk of rupture. Among the tests Toyota conducted were CT scans, like those used in hospitals to see inside the human body, to check the components’ integrity.

“Certain types of airbag inflaters were found to have a potential for moisture intrusion over time,” the company said in a statement.