Airbag Residues

Various residues (small particles) can attach to a person's body or clothing. These can be detected and analyzed to provide clues as to the person's involvement in a particular incident. Evidence of involvement in a motor vehicle crash can be provided by the detection of the residue released when an airbag is deployed.

An airbag is essentially a pillow (albeit a specialized one) that is normally concealed. It is triggered to expand by the force of a collision.

Airbags first appeared commercially in automobiles in the 1980s. At that time, they were located only on the driver's side, and were concealed in a compartment in the middle portion of the steering wheel. Airbags have been standard equipment on the front passenger side of cars since the 1998 model year and on the trucks since the following year. Some cars now additionally offer side-mounted airbags that offer protection in the event of a side collision.

The deployment of airbags depends on momentum, which is the product of the mass and velocity of an object. When a car is moving forward, both the vehicle and all the objects inside the car, including the passengers, will tend to keep moving forward.

When the vehicle is rapidly stopped, as occurs in a collision, the passengers' momentum will continue to carry them forward. Typically, passengers are restrained by seatbelts. But, in a high-speed accident, or when a passenger is not buckled in, the forward movement can be so great that the passenger is propelled forward at great speed. The result can be catastrophic injury.

It is this injury that airbags are designed to prevent. The airbag inflates when an accelerometer detects a force equal to running into a brick wall at 10–15 miles per hour. This force causes a switch in the accelerometer to close, allowing the flow of electricity. This current activates the sensor, which in turn activates the airbag's inflation system.

It is the inflation system that proves to be of forensic significance. A gas generator inside the airbag contains sodium azide (NaN₃), potassium nitrate (KNO₃) and silica (SiO₂). Upon the signal from the sensor, heat is supplied, which is necessary to decompose NaN₃. A series of three chemical reactions occurs. In the first reaction, the breakdown of NaN₃ produces nitrogen gas (N₂) and sodium (Na). The sodium then reacts with KNO₃ to generate potassium oxide (K₂O), sodium oxide (Na₂O) and yet more nitrogen gas. Finally, the potassium and sodium oxides react with SiO₂ to produce alkaline silicate (glass).

The latter reaction is a safety measure. Potassium oxide and sodium oxide are very reactive substances and it is dangerous to have them as the end products of the airbag inflation.

Prior to inflation, the NaN₃ and KNO₃ are solids, typically pellets. Their subsequent reactions produce a large amount of nitrogen gas, which bursts the nylon or polyamide airbag from its storage compartment at upwards of 200 miles per hour to approximately the size of a couch decorative pillow.

The entire process, from detection of an impact to the deflation of the airbag, is approximately 170 milliseconds; literally, in the blink of an eye. The inflation of the airbag coincides with the forward movement of the driver or passenger, so by the time a person contacts the airbag, deflation has begun. In this way, the bag acts as a cushion and dissipates the passenger's forward momentum. Without this exquisite timing, a person would contact a rock-hard bag that could cause as much injury as a high-speed collision with the car's windshield.

During deflation, the nitrogen gas vents through very small holes in the bag. It is during this deflation that other contents of the bag can also be released. This residue contains alkaline silicate, but most typically consists of cornstarch or talcum powder. The latter two compounds are included to keep the airbag soft and lubricated while stored.

The deflation can produce a fine powdery cloud of residue that will settle on clothing, vehicle surfaces, hair, and exposed skin. The residue is dissipated very quickly from the airbag. Once the bag has deflated, no more residue is released.

The airbag residue is slightly corrosive and so can be a mild respiratory and skin irritant. The red skin that can result from surface contact with the residue or the mild airway and lung irritation produced upon breathing in the residue can be other forensic clues of a person's involvement in a vehicle collision.

Because of this irritation, forensic investigators wear gloves and goggles when near a deployed airbag and wash with soap and water afterwards.

see also Accident reconstruction; Automobile accidents; Death, cause of; Trace evidence.