A photo from the Insurance Institute for Highway Safety shows a crash test of a 2002 Honda CR-V, one of the models subject to a recall of air bags that can blast out deadly shrapnel in a crash. (Uncredited/AP)

More than 30 million cars and trucks nationwide are equipped with dangerously defective air bags, congressional officials say, a number that raises questions about whether the U.S. auto industry can handle what could become the largest recall in history.

Federal safety authorities have recalled only 7.8 million vehicles over the defect in a few states, a limited action that lawmakers said Thursday was vastly insufficient to address what they deemed “a public safety threat.”

Two senators demanded a much broader recall that would cover every affected vehicle nationwide. But a recall of that magnitude — including best-selling models from Honda, Toyota, GM, Chrysler and six other companies spanning 2002 to 2007 — could prove far greater than the industry has ever managed.

Manufacturing that many replacement parts could take years and present a variety of logistical nightmares. Dealerships could quickly become overwhelmed by the demand, auto safety experts said. This year, GM recalled a total of 30 million vehicles for faulty ignition switches and other problems, and months later it is struggling to make the repairs.

The defective air bags, made by Japanese manufacturer Takata, can rupture and blast out metal shards, particularly in humid conditions, government officials have said. While the rate of reported incidents is low, linked to four deaths and more than 100 injuries so far, their grisly severity has spurred an urgent debate about the matter in Washington.

Major automakers are recalling millions of vehicles due to potentially defective front passenger air-bag inflators that can rupture and spray metal shrapnel. (Reuters)

Driving her Honda Accord on Christmas Eve in 2009, Gurjit Rathore, a 33-year-old Virginia mother, was struck in the neck by pieces of an exploding air bag and bled to death in front of her three children, according to a lawsuit filed by her family.

Takata controls more than 30 percent of the world’s air-bag market, triggering worries that the recall could grow in the United States and elsewhere. Meanwhile, automakers say new air bags are already in short supply.

Honda said it does not have enough parts to immediately fix the more than 5 million Accords, Civics and other vehicles with defective air bags. For a temporary fix, Toyota is instructing its dealerships to disable air bags and attach notes on vehicles’ glove boxes warning against riding in the passenger seat.

“It would take potentially years for this to be addressed. That is what is scary about this,” said Karl Brauer, a senior analyst at Kelley Blue Book. “You could have tens of millions of dangerous vehicles on the road.”

In a letter Thursday to the National Highway Traffic Safety Administration, Sens. Richard Blumenthal (D-Conn.) and Edward J. Markey (D-Mass.) said they were “increasingly troubled and alarmed” by regulators’ public guidance. Markey’s office put the number of affected vehicles at 30 million by examining auto registration and other federal transportation data.

“The information available to us indicates no factual basis for distinguishing between states or regions of the country regarding the potential severe danger of this defect,” they wrote. “Replacement parts are ‘essential to personal safety’ for all drivers whether they live in New England or Florida.”

Regulators pushed back, saying a nationwide recall would divert a limited supply of replacement air bags “from those at demonstrated risk” in areas with long-term humidity and heat. They added that they were urging Takata and other suppliers to boost production.

“We have taken an aggressive and relatively unprecedented step by forcing a regional recall on limited information,” Transportation Department spokesman Brian Farber said in a statement, “and we will not rest until we know the full geographic scope of the problem.”

To help car owners check whether their vehicles are the subjects of an active recall, the government set up the Web site www.safercar.gov. But over the past few days, the recall search function has been down.

Takata spokesman Alby Berman did not return messages seeking comment Thursday. Honda said it “has nearly completed the mailing of notifications regarding the special campaign,” and Toyota, the largest carmaker in the world, said it has intensified efforts to reach customers “in certain geographic areas that appear to warrant immediate action.”

But in a letter to regulators last week, Markey and Blumenthal criticized the “arbitrary geographic boundaries” of automakers’ “puzzling and inconsistent” recall. While Subaru and BMW have announced nationwide recalls for Takata air bags, Honda, with millions more affected vehicles, limited its messaging to a group of U.S. islands and Southern states.

“Manufacturers are all over the lot” in deciding which areas were affected, said Clarence Ditlow, executive director of the Center for Auto Safety. “How is it that one manufacturer has a bigger danger area than another?”

The limited regional recall means owners of defective cars in non-recall states will neither be notified of the danger nor qualify for free repairs. At least two of the fatal accidents linked to the defect occurred in areas not included in the recall, including in Virginia and Oklahoma.

Because the cars are tracked only when they’re registered or initially sold, a move or sale to someone in a more humid area would mean the defect could go ignored for years. More than 3 million recalled cars and trucks were listed for sale last year, according to Carfax.

“You can live in upper Wisconsin and register your car there but drive to Florida and spend a heck of a lot of time there. You wouldn’t be traced by the manufacturer or NHTSA,” said Ellen Bloom, senior director of federal policy for Consumers Union, an advocacy group. “To assume that people are registering their cars and limiting their driving to where they registered is pretty shortsighted.”

More than 50 million cars and trucks have already been recalled in what has become the worst year for recalls in U.S. history, with 1 in 5 vehicles on the road at risk of critical defects.

In the case of the Takata air bags, the full range of affected vehicles remains unclear. The most recently updated NHTSA warning Thursday said an “undetermined number” of General Motors vehicles were equipped with defective air bags. The list of affected vehicles has grown by millions of cars since Monday, with many vehicles incorrectly added or excluded from the recall on lists made public by regulators.

Geographic recalls have been criticized as a way for automakers to restrict their repair spending while leaving a subset of drivers at risk. In 2004, the Center for Auto Safety unsuccessfully sued NHTSA, claiming geographic recalls violated the National Traffic and Motor Vehicle Safety Act, which requires that automakers notify all owners of defective vehicles and provide a free remedy.

“It’s totally arbitrary, and they use it to save money and have as narrow a recall as possible,” said Joan Claybrook, a former NHTSA head and longtime traffic safety advocate. “There’s no rationale for it other than to save money.”

Members of Congress have requested briefings with NHTSA officials. Rep. Fred Upton (R-Mich.), chairman of the House Energy and Commerce Committee, which has investigated the General Motors and Toyota recalls, said in a statement Wednesday that “we also need to take a close look at . . . the timeline and scope of the recalls,” adding, “When it comes to vehicle safety, there can be no margin for error.”