If the government tries to protect small children by requiring that they use child safety seats on airplanes, more kids could wind up dead, a study says.

That's because some parents likely would travel by car rather than pay for a separate airplane seat for children under 2. And traveling by car is more risky, according to researchers writing in last week's Archives of Pediatric and Adolescent Medicine.

Banning "lap babies" from planes would save 0.4 lives per year, according to the researchers. But the mandate would result in an overall increase in deaths if the proportion of families switching from air to car travel exceeded about 5 percent to 10 percent.

"The airline industry [would] profit from the policy," said Dr. David Bishai, a public health researcher at Johns Hopkins University who wrote an editorial accompanying the study. "I [am] concerned that they will cover over their money-grabbing by saying, 'We're going to save lives.' In reality, they might be costing more lives."

Congress asked the Federal Aviation Administration in 1995 to study the impact of banning lap babies, and the agency came back with the same conclusion: More children would be put at risk because families would drive instead of fly. But in the late 1990s, several airlines announced that they would sell tickets for small children at a discount, said Alison Duquette, spokeswoman for the FAA.

This development "changed the playing field," Duquette said, such that the agency gave notice in 1998 that it was considering making a rule on the subject. That let the FAA start collecting safety data on how well new car seats work in planes.

But the rule-making hasn't gone any further.

"We are still considering issuing a proposal," Duquette said. "But we have not issued a proposal as yet. ... When and if we do that, we would be taking into consideration the costs and benefits, which is essentially what this study looks at."

The Association of Flight Attendants has supported a federal rule for years, arguing that neither flight attendants nor a baby's parents can protect unrestrained children in the event of a crash or severe turbulence. The American Academy of Pediatrics released a policy statement in 2001 supporting a mandate, saying there was no data to support the argument that parents might opt to drive rather than pay for tickets for young children.

But the new report shows otherwise, said Dr. Thomas B. Newman, the University of California San Francisco researcher who conducted the study.

Not many deaths could be prevented by mandating safety seats, Newman said, because there aren't that many deaths involving unrestrained young children in survivable plane crashes. Since the potential benefit per child is low, that makes the cost per life saved high unless the cost per round trip ticket is close to zero.

US Airways has not taken a position on the issue, said David Castelveter, airline spokesman. Parents who want an adjoining seat for a child under 2 can buy it for half the price of their own seat.

Bishai, the editorial writer from Johns Hopkins, suggested that airlines could solve the problem simply by always seating infants' parents next to vacant seats on flights. But Castelveter said such a policy could upset other passengers. Besides, he said, there are fewer vacant seats on flights these days.

A spokeswoman for the Air Transport Association, the trade group for airlines, disputed Bishai's suggestion that the safety-seat idea is a money grab. In fact, the association doesn't support a mandate at this time, since there are concerns about the design of some car seats in airplanes.

Duquette, the FAA spokeswoman, said one point should not get lost in the debate: "We do strongly recommend that parents use a child safety seat that's approved for use in aircrafts."