Strength and size are not the only factors affecting safety when the passenger is overweight. Use of the seat belt can also be a problem for these travelers. The F.A.A. recommends wearing seat belts throughout the flight, though use is required only for takeoffs and landings.

Dietrich Jehle, a professor of emergency medicine at the University at Buffalo who conducted a study of more than 300,000 serious automobile accidents, said that very overweight drivers faced an increased risk of death in a severe crash and that they were 67 percent less likely to be wearing seat belts, possibly for reasons of comfort.

Dr. Jehle said obese air travelers may also be less likely to wear seat belts. Unbelted passengers are at risk of injury and can be a mechanism for injury to others, Dr. Jehle said. “Force is mass times acceleration, and when someone is heavier and unbelted, there’s that much force that is being applied.”

He said both airlines and car companies needed to address the unique challenges of protecting overweight people. “Since a third of the population is obese, we need to be doing some of our crash testing with obese dummies,” he said.

Dr. Salzar said seat belts should also be tested to ensure that they could restrain heavier individuals. “You’d be amazed at how the large person blasts through that restraint,” he said.

In airplane economy seats, Mr. Ozawa said the proximity to other passengers created a higher likelihood that the heavier passenger would become a hazard by colliding with those sitting nearby. The back of the seats may not be strong enough and the spaces between seats wide enough to protect passengers from the impact of heavier passengers behind or beside them, he said.

AmSafe, an Arizona company that is one of the biggest manufacturers of airliner seat belts, air bags and child restraints, declined requests for comment.