Q: I was wondering how much air bags have changed since all the fuss about them hurting people quite some time back. You now seem to find cars with a half dozen or so. What’s changed?

— Karen Thomas

A: Your letter provided the opportunity for me to take a fresh look at Supplemental Restraint System technology, and there is much to report!

Air bags first appeared domestically on some Ford and GM vehicles in the 1970s, fell out of favor, and then came into wide use in the early 1990s, following Chrysler’s lead. With general success and some problems, driver-side and passenger-side air bags became required on all passenger cars and light trucks in the late 1990s.

I recall also the concerns many folks had on those early vehicles because the systems employed a powerful one-size-fits-all strategy that could be dangerous to smaller statured adults, children and unbelted occupants. Rule changes eventually were made to allow car makers to innovate and create systems with dual inflation modes, size and shape variation, and methods to determine and respond to occupant size and position. Newer advanced air bags respond to an occupant classification system, and have been required since 2006. Early occupant classification systems had some bugs, misidentifying items placed on the seat and unusual passenger situations, but that’s largely been cleared up.

Current vehicles may be equipped with up to almost a dozen air bags; driver and passenger multistage front, side curtain, torso, knee and center. A diverse array of sensors keeps track of seat occupancy and position, occupant position and weight, vehicle speed, vehicle yaw, as well as impact severity and direction, among others to insure the best level of protection is offered. During a crash, this information is calculated in an instant and the correct air bags are deployed or not. Should a passenger be slumped against the door, for example, the torso bag on that side would not be deployed, and a child in the front seat — under 13 years of age are still not recommended — should be detected and the front bag will not deploy. Data are collected and stored in the event of a crash, helping to replay the sequence of events.

Seat belt pretensioners are also employed to immediately snug lap and shoulder belts, helping to position the occupant for proper interaction with the air bag. These may be electrically or mechanically deployed, or use a pyrotechnic device to cinch up the belt.

Just as we are about to say air bags have evolved into an almost perfectly orchestrated safety system, we’re faced with about 8 million air bag inflators that when subjected to humid/moisture conditions may cause too quick a burn of propellant during a deployment and shoot flying metal parts. These air bags may be found in certain 2000-2010 BMW, Chrysler/Dodge, Ford, GM, Saab, Honda/Acura, Mazda, Mitsubishi, Nissan/Infiniti, Subaru and Toyota/Lexus vehicles. Owners can check here to see if they might be affected: www.safercar.gov/Vehicle+Owners.