NEW YORK (CBSNewYork/AP) — Two U.S. senators are calling on regulators to issue a nationwide recall of cars with faulty airbags made by Takata Corp., questioning why automakers have been allowed to limit recalls to only certain locations with high humidity.

Democratic Sens. Richard Blumenthal of Connecticut and Edward Markey of Massachusetts made the demand in a letter to Transportation Secretary Anthony Foxx, who oversees the National Highway Traffic Safety Administration.

The letter delivered Thursday also asks Foxx to encourage automakers to provide drivers with free loaner cars if parts aren’t available.

EXTRA: List Of Cars Included In Airbag Recall

Airbag inflators made by Takata can rupture, causing metal fragments to fly out in a crash. Safety advocates say the problem has caused four deaths and multiple injuries. So far automakers have recalled about 12 million vehicles worldwide due to the problem.

But in the U.S., many automakers have limited the recall to high-humidity areas in southern states. Takata has recommended that automakers conduct recalls only in Florida, Hawaii, Puerto Rico and the U.S. Virgin Islands, areas with high “absolute” humidity. Some automakers have widened their recalls to include a few more states, but cars in many states with high humidity levels have been excluded.

Takata and the National Highway Traffic Safety Administration are investigating the impact of prolonged absolute humidity, which is a measure of the moisture content in the air, on chemicals that propel the airbags in a crash. They’re looking into whether moisture in the air can cause the chemicals to explode with too much force, causing metal parts to fracture.

The senators’ letter said they are alarmed by confusing advice being issued by NHTSA, the government’s highway safety watchdog, and by the “glacial pace” of the agency’s response. They’re also concerned that manufacturers are limiting the recalls to just a few states.

“Almost any area of the country may have high heat and humidity for an extended period of time, including the Northeast. The lines they’ve drawn are completely senseless and arbitrary,” Blumenthal said in an interview. “It would be laughable except it’s deadly serious in its consequences.”

Takata airbags have been the subject of recalls for several years, but U.S. regulators didn’t open an investigation into them until June. No firm cause of the problem has been identified, and the agency has said it will order a wider recall if necessary.

This week, NHTSA issued rare warnings to owners of nearly 8 million recalled cars with the Takata airbags to get them repaired as soon as possible.

The agency urged people to check their vehicle identification numbers on the NHTSA website to see if they’ve been recalled, but the website crashed Monday night and still isn’t working fully. Deputy NHTSA Administrator David Friedman apologized for the mistakes and said the crash likely was caused by a software change.

Car owners can also find recall information by checking their automaker’s site.

The Takata-related recalls so far cover certain vehicles made by BMW, Chrysler, Ford, General Motors, Mazda, Honda, Mitsubishi, Nissan, Subaru and Toyota.

Messages were left Thursday seeking comment from Foxx, NHTSA and Takata.

For more information, visit www.safercar.gov.

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