Rosekind said the agency and auto industry are still trying to determine precisely what is causing Takata's inflators to explode, but action needed to be taken immediately.
Takata's air bags use ammonium nitrate to inflate in a crash. But the chemical, which can be used to make bombs, is unstable. So far, testing has found that airborne moisture can get into the inflators and cause the ammonium nitrate to burn hotter than it should, Rosekind said.
He urged car owners who get recall notices in the mail should immediately make an appointment to get their cars fixed. Owners can key in their vehicle identification number at https://vinrcl.safercar.gov/vin/
to see if their car is part of the recall. The number is stamped on the dashboard near the driver's side windshield and also can be found on state auto registration documents. It may take weeks before all the identification numbers are entered into the database.