Air bags can prevent crash fatalities


November 16, 1993|By Dr. Genevieve Matanoski | Dr. Genevieve Matanoski,Contributing Writer

Today women make more than 50 percent of the car-purchasing decisions in this country, and women want safe cars. This is part of the reason why in this model year, fully 90 percent of the cars will have air bags. Statistics have shown that drivers and passengers are most safe wearing both a seat belt and having air bags in their cars.

Professor Susan Baker, co-director of the Injury Prevention Center at the Johns Hopkins School of Hygiene and Public Health, has studied the effectiveness of air bags for more than 15 years. Because there has been some concern about the use of these safety devices, I asked her to explain how they work and what they do.

Q: Why has it taken so long for car manufacturers to make air bags a standard feature in most automobiles?

Q: As early as 1968, testing on air bags had been largely completed and by the early 70s, they were available in certain expensive cars. At that time, car manufacturers did not advertise or promote their use. Yet, the

original cost was comparable to that of a vinyl roof.

A: How effective are air bags in preventing fatalities?

The Insurance Institute for Highway Safety (IIHS) reports that air bags do prevent fatalities. Researchers at the institute have amassed data from the years 1985 to 1992. What they found, based on approximately 18,000 driver deaths in front and front-angle crashes of air-bag equipped cars, was that there were 24 percent fewer deaths than they would have expected in cars with seat belts only.

The reduction in drivers' deaths varies by size of the car. Deaths were reduced by 37 percent in large cars, 17 percent in mid-size cars and 26 percent in small cars.

Q: What actually happens when an air-bag equipped car has an accident?

Air bags operate in response to a sensor system. The sensors are triggered by a frontal crash and measure the magnitude of the crash instantaneously.

RF When triggered, the sensors signal inflaters to fill the bags with

harmless gas. Within a fraction of a second, the inflated air bags restrain car occupants and then quickly begin deflating to further the cushioning effect.

The force of impact necessary to trigger inflation is the equivalent to the impact of hitting a solid barrier at 10-12 miles per hour.

Each car model varies in the number and location of the sensors. The most common system uses two to four electromechanical sensors located in a vehicle's front end. These are either ball-in-tube or spring-mass types.

In these systems, either the ball or the mass moves in proportion to crash severity.

Once the sensor determines that the crash merits air-bag deployment, an electrical contact signals the bag to inflate.

Q: What actually is inside the bag?

A: There are a variety of different inflaters in today's air bags. However, nearly all are pyrotechnic. This means that they burn sodium azide to produce the harmless nitrogen that inflates the bags.

Q: Can a person get hurt if the air bag hits them when it is inflating?

A: Yes. There have been documented injuries but they are rarely severe, and when weighed against the possible consequences of a major crash accident, seem to constitute a reasonable risk.

Dr. Genevieve Matanoski is a physician and epidemiologist at the Johns Hopkins School of Hygiene and Public Health. She is a founding director of the school's Institute for Women's Health Research and Policy.

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