The author's father was riding down a narrow country road in a Model T which came around a corner and met a Buick. His father hit the windshield with his head and the glass broke. The only long term damage was a scar on his cheek. Years later, the author was riding with his father coming down from Northern Michigan in early Spring 1940, when we came to a long curve which was ice covered and strewn with several cars which had collided or slid off of the road. We crept by and there was a grisly scene of a woman with her head through the windshield and another woman holding her hand to her forehead and covering her eye. It made a lasting impression on this then 6 year old. Such events were not uncommon going into the 1960’s. The advent of seat belts reduced such sights as did the use of special glass in windshields known as safety glass. However something more was needed to protect passengers. One could dream of having some form of padding protecting the driver and passengers from being injured from collisions with the windshield, dash and steering wheel. That dream has now been pretty well met but there are some associated problems.

Let’s examine an idealized system. At the moment of impact a pillow is deployed between the driver and passengers and any potential hard surface quicker than the driver and passengers would be thrown forward and the seat belts would be tightened so no one is thrown around. There is firm support for the head and neck and back seat passengers do not collide with the seats in front of them. If one calculates how fast all this should occur it is far less than one second. Mechanical devices can not meet that requirement alone but ones driven by an explosive charge could. But, does such an explosive exist which meets some other criteria?

Explosives are normally set off by some form of detonator or detonating device. Explosives are sensitive to physical shock, friction, impact and spark as well as to elevated temperatures. A candidate explosive must pass the requirement that none of the conditions will be encountered to initiate except at the time of an accident. The candidate explosive must not cause a fire or shrapnel and not injure the persons it is going to protect. The creating of a big bang could rupture ear drums. It can not be too powerful.

Explosive power can be related to the velocity of detonation and detonation pressure. These factors are a function of the energy released by the chemical reaction and the mean molecular weight of the gases generated. For all practical purposes the explosive is limited to having only the elements carbon, hydrogen, nitrogen, oxygen, lithium and sodium. CHNO based explosives will need to be balanced to generate only nitrogen gas, carbon dioxide and water. Unfortunately, for the purposes of the airbag deployment, carbon hydrogen and oxygen in the proper proportions will prove too energetic. One is left with only nitrogen as the propellant's gas. There are only two known compounds which might fit our requirements and these are lithium azide (LiN₃) and sodium azide (NaN₃) . For practical purposes it all comes down to sodium azide which is relatively stable but can be exploded to furnish molecular nitrogen and elemental sodium. It meets almost all our requirements. The problem is that elemental lithium or sodium is not benign. They will both react when in contact with oxygen or with water. The products with water are hydrogen and the metal hydroxide. The products with oxygen are the related oxides which would react with water to furnish the highly caustic hydroxide. Fortunately the propellant mixture contains potassium nitrate an silica present in the amounts necessary to convert the sodium and potassium to the related, non-volatile silicates.  The amounts are small because the volume of nitrogen is large, but not all that small if inhaled or gotten into the eyes. Respiratory distress is a possibility as is caustic irritation of the eyes. The writer recommends that eyes exposed to air bag deployment be carefully irrigated with water at room temperature.  ( See Sight )

Other concerns relate to accidental initiation of the propellant charge while performing victim extraction from wrecked motor vehicles. Initiation is accomplished by discharging an electrical capacitor. Until the capacitors have lost their charges, there is a danger of an accidental explosive initiation. That poses a danger to the trapped and to the rescuers. Cutting the battery cables will not disarm the capacitors and this approach will take at a minimum of 20 minutes. The only safe approach will be to remove the capacitors from the circuits. Here one needs to have the appropriate vehicle service manual. Not all armed vehicles are the same.

See also:           Worker Safey
                        Airbag Chemistry
                        How Airbags Work
                        Emergency Rescue Guidelines For Air Bag Equipped Vehicles