When some of the world’s top automakers discovered a potentially deadly flaw in airbags from their Japanese contractor Takata Corp., the companies recalled millions of cars – but not all. That’s because they chose to target vehicles registered in only the geographic regions with “high absolute humidity.” Now, this selective strategy is being scrutinized by U.S. lawmakers.

Areas with high absolute humidity – defined as “a measure of the amount of water vapor in a specific sample of air,” according to NBC News – include such areas as Florida, Puerto Rico and Guam. Auto industry statistics suggest airbags are more susceptible to fail there. Four fatalities have been linked to the Takata airbags, with the devices prone to exploding during a crash and spraying the driver with metal shards.

Critics of the limited recalls are pushing for the automakers to widen the scope. To limit the recalls to only certain areas is “irresponsible and reprehensible when people living in other states may be equally at risk,” U.S. Sen. Richard Blumental (D-Conn.) told NBC News. Blumenthal and fellow senator Edward Markey of Massachusetts wrote a letter to the National Highway Traffic Safety Administration to issue a nationwide recall and are also planning to seek a geographic recall ban in a federal highway safety bill.

After the fourth death linked to Takata airbags, the NHTSA advised owners of 7.8 million cars in the U.S. to get their airbags checked out and fixed. The advisory covered previous recalls from manufacturers including BMW, Toyota, Honda and General Motors, all of which sourced their airbags from Takata. The NHTSA warning and the automakers’ recalls singled out areas with “high absolute humidity.”

The NHTSA said in a statement that it had taken “an aggressive and relatively unprecedented step by forcing a regional recall on limited information,” according to NBC News. It promised to look into the “full geographic scope of the problem.”