Adults traveling with babies may have no idea how dangerous it is to allow infants and toddlers to fly on commercial airline flights as "lap children."

The Federal Aviation Administration and the airlines don't require babies and children under age 2 to travel in child safety seats, primarily for cost reasons.

In August 2005, the FAA said, "Analyses showed that if forced to purchase an extra airline ticket, families might choose to drive, a statistically more dangerous way to travel."

At that time, FAA administrator Marion Blakely said, "Statistics show that families are safer traveling in the sky than on the road."

But the FAA has acknowledged the inherent danger: While most parents would do anything for their child — including holding on for dear life in an airborne emergency — the simple fact is they can't always hold on to the child.

That's because commercial aircraft are designed to withstand tremendous G-forces, but humans are not. And therefore a 25-pound baby could easily weigh three or four times that amount when a parent is struggling to hold on to it during an emergency, let alone dealing with impact, smoke or fire.

In addition, a baby strapped inside a parent's seat belt can be crushed by the parent's weight during an emergency.

These laws of physics have been proven time and again, in the most heartbreaking of circumstances. In several cases, lap children have been severely injured and killed in accidents that were survivable.

In the 1989 crash landing of United Airlines Flight 232 near Sioux City, Iowa, for example, the accident report from the National Transportation Safety Board noted that of the four lap children on the plane, "the mothers of the infants in seats 11F and 22E were unable to hold on to (them)."

The NTSB added mandatory child safety seats to its "Most Wanted" list of FAA improvements in May 1999 (although it was removed in 2006). Other experts such as the American Academy of Pediatrics, the Association of Flight Attendants and the National Air Disaster Alliance have strongly concurred.

The best way to ensure that a baby or small child will be safe while flying is to strap him or her into a safety seat. This means that parents need to purchase a seat for every member of their traveling party, regardless of age or size.

There was a time when parents could be fairly certain of a nearby empty seat, so they could bring a safety device aboard and cabin staff would place it on an adjoining seat without purchasing a ticket. But with passenger load factors at all-time highs, the days of stretching out next to an empty middle seat are long gone.

Not all safety devices are created equal. The FAA suggests that parents make sure their restraint device is certified for use on aircraft.

For guidelines, visit www.faa.gov/passengers/fly_children/crs/index.cfm. And most airlines restrict where safety seats can be placed (exit rows are not allowed).

Some carriers such as Southwest offer discounted tickets for infants and kids. Parents should carry a copy of their child's birth certificate, since some airlines require proof of age.