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One of the first times that I realised science could be useful rather than a slog, was (surprisingly enough) in physics.

Look familiar to anyone? As usual in physics, this formula looks singularly unhelpful. But in fact it relates force and time. If you troll through all the formulae behind acceleration, you learn that a force gets weaker when applied for longer.

The useful part? This idea is used in bike helmets, to stop your head from getting such a whack when you fall off your bike.

A bicycle helmet

The helmet has a ‘crumple zone’ made from expanded foam polystyrene, which has lots of tiny air bubbles in it that can be crushed. It takes time to crush these air pockets. The extra time this takes results in less force hitting your head. In other words, a bicycle helmet offers a layer of protection and helps to prevent head injury.

But we also have a ‘crumple zone’ in cars. Several, in fact. Usually there is one at the front and rear of the car, with a rigid cab in the middle. And there’s the airbags, which are a sort of ‘crumple zone’ too.

Crumple zones work by increasing the time it takes for a car and its occupants to come to a stop, causing a decreased force on the occupants. This is the same idea as a bike helmet, on a larger scale.

In a car, the airbag acts like a cushion and lessens the impact of a person hitting the cab. Again, this is related to increasing the time and so decreasing the force of collision.

A car crash test at the Insurance Institute for Highway Safety. Note the airbag has deployed and deflated. Source: Creative Commons, Brady Holt

But people don’t drive around with airbags inflated. The airbag must have some sort of trigger so that it inflates during an accident, before the occupants of the car are thrown around.

According to research from Washington University, for an airbag to be useful in preventing severe injury, it must deploy and inflate within 40 mili-seconds of a crash (0.004 seconds). It must also be able to tell the difference between a serious crash and a minor fender-bender.

This is where chemistry comes in.

Air bags are inflated not by compressed gas, but the products of a chemical reaction. Sodium azide, NaN3, is a chemical that releases nitrogen gas (N2) when it is ignited. A sensor causes ignition of sodium azide at the start of a severe crash, causing the airbag to quickly fill with nitrogen gas.

However, harmful sodium (Na) metal is also produced in this process. To prevent this from being an issue, additives such as silicon dioxide are used to react with this sodium metal and produce harmless by-products.

A test car crash, complete with model person. Note the air bag has deflated. Source: Creative Commons, Pava

The airbag only works to cushion impact if it has inflated and is deflating when the person hits it. The impact of hitting the airbag while it is inflating has been compared to hitting a stone.

Some fatalities have been caused by not wearing a seatbelt so that the person was too close to the airbag when it inflated. Many minor cuts and bruises have been caused by airbag impact.  Lots of research is being done by pyrotechnic companies to maximise the safety of airbags.

In the 21 years between 1987 when airbags were made compulsory and 2008, it has estimated that 25 782 lives have been saved by frontal airbags (US Department of Transportation).

Airbags have been shown to be effective not only in the number of lives saved but also in preventing a large number of life-threatening and debilitating injuries such as skull fractures and brain injuries.

Airbags and seatbelts together ensure the safety of car drivers and passengers. Use them both! Surely preventing a fractured skull is worth getting a bruise.


Bicycle Helmet Safety Institution: http://www.bhsi.org/stats.htm#effectiveness

Article: N Engl J. Med 1969; 320: 1361-7

US Department of Transportation brochure, available at http://www.safercar.gov/Air+Bags

Washington University in St Louis website: http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html