Air Bag Analysis
Contributed by Glenn Schubert
In some incidents, law enforcement agencies have requested technical assistance from forensic scientists in determining the driver of a vehicle that was stolen and crashed, involved in a hit-and-run accident, or involved in a reckless homicide. In the past, evidence such as hairs, fibers, blood and fingerprints found inside the vehicle were used to help make this determination. Now, examination of the deployed airbags and the occupants clothing can provide some important evidence.
Most airbags use a solid-propellant type of inflator in the driver side and passenger side. Side curtain airbags typically use stored gas for inflation. A sensor in the vehicle detects a sudden deceleration, not necessarily a collision, and sends an electrical current to the detonator in the inflator. This causes a slow detonation of sodium azide or guanadine nitrate pellets in the inflator, resulting in the production of gas as hot as 7000C that inflates the airbag. The airbag inflates in less than 1/20 of a second, at a speed of 200 miles per hour, with the hot gas. The airbag begins to deflate immediately, as the hot gas escapes through vent holes in the back, making it a better cushion and absorbing the impact of the occupant. The hot gas can also leak through small holes from the stitching seams in the front of the airbags. If clothing from the occupants comes into contact with the airbag when it is at or near maximum inflation, the leaking hot gas can cause singe patterns on the front of the clothing, characteristic of the seam patterns. The pattern typically appears as a series of small black dots or smears, 2-3mm apart.
In most automobiles, the driver side airbag is round with round seams, so it will produce arc-shaped singe patterns. The vent holes may cause a singe pattern on the cuffs of the driver’s shirt if he is still holding on to the steering wheel when the airbag deploys. The passenger side airbag is often rectangular in shape, with straight seams, so it will produce a straight singe pattern. This makes it possible to differentiate the singe patterns produced by a driver side airbag from the one produced by a passenger side airbag.
The singeing occurs mainly on the clothing covering the chest and arms of a restrained front seat occupant. Singeing can also occur on clothing covering the abdomen of an unrestrained front seat occupant. The size and seam patterns can differ from one airbag to another, usually by make, model and year. Because there are differences, you can have different class characteristics, thereby including or eliminating certain airbags as the source of a singe pattern. Certain fibers, such as cotton and silk, singe more easily than man-man fibers, making the patterns easier to detect. The singe patterns are difficult to observe on dark colored clothing. Wearing a seatbelt will likely reduce the opportunity for clothing to become singed. De-powered and multistage airbags, now utilized in newer vehicles, will also reduce the chances of seeing a singe pattern on an occupant’s clothing.
Some driver side airbags might utilize corn starch or talc as a lubricant, depending on the sealant used on the interior surface. Passenger side airbags do not have a sealant, so no lubricant material is used. Of course, examinations for other types of trace material transfers, such as hairs and fibers, may also be probative and should also be conducted.
Additional References:
1. Schubert, GD, Forensic Value of Pattern and Particle Transfer From Deployed Automotive Airbag Contact, Journal of Forensic Science, Nov 2005, Vol 5, No 6.
2. Schubert, GD, Forensic Analysis On The Cutting Edge, Chapter 2, Forensic Analysis of Automotive Airbag Contact – Not Just A Bag of Hot Air; Wiley and Sons, 2007.