Even though airbags were historically designed for unbelted passengers, now they're supposed to work in combination with seat belts. The seat belt holds you in position while the airbags provides a softer, more cushioned stopping agent. (Seat belts are also needed in side and rear collisions, and in rollover accidents, where air bags don't inflate.)
One of the problems of not being belted arises from where the air bag hits you. They're most effective when they strike the torso. Says Roy Alson, an emergency medicine physician in Winston-Salem, N.C., if it hits the head and neck, it can hyperextend the neck and cause significant injuries.
And that's the calculus that puts smaller people at risk: "The serious and sometimes fatal injuries that we have seen have been injuries to the head and neck in smaller individuals," Alson says.
Many solutions have been proposed to the problem: shutting off the bags, taking some of the steam out of them, or giving the bags enough brains to "see" who's in the seat. There's talk of using the classic half-solution to bad design: warning labels. Some experts have even mentioned that the kids should go in the back seat.