Differences in the Effectiveness of Frontal Air Bags by Body Size Among Adults Involved in Motor Vehicle Crashes

Abstract

Objective. There is concern that small stature occupants (particularly women) involved in motor vehicle crashes (MVCs) may be at risk of injury or death from frontal air bags, though evidence to substantiate this concern is lacking. We sought to assess how occupant body size (measured through height and weight) affects air bag effectiveness in mitigating the risk of serious injury, after adjusting for important crash factors.

Methods. This was a retrospective cohort study using a national population-based cohort of adult front-seat occupants involved in MVCs as included in the National Automotive Sampling System Crashworthiness Data System database (NASS CDS) from 1995 to 2006. Drivers and front-seat passengers 15 years and older involved in MVCs involving passenger vehicles and light trucks were included in the analysis. The primary outcome was serious injury, defined as an Abbreviated Injury Scale (AIS) score ≥3 in any body region. Multivariable logistic regression models were used to test interaction terms (effect modification) between air bags, body size, and injury. The predicted probability of injury across body sizes was plotted to further illustrate potential differences.

Results. Sixty-nine thousand three hundred eighty-seven adult front-seat occupants during the 12-year period were included in the analysis, of which 9333 (2.3%) were seriously injured. There was no evidence that height or weight modified air bag effectiveness among all crashes (p > .40). In primary frontal collisions, there was some evidence for effect modification by weight (p = .04) but not by height (p = .59). When assessed using air bag deployment, height was a strong effect modifier (p = .0078), but not weight (p = .43). Predicted probability figures confirmed that occupant height modifies the effect of air bag deployment, but there was no similar visual evidence for body weight.

Conclusions. In this sample, we found no consistent evidence that body size modifies the overall effectiveness of frontal air bags. However, among crashes involving air bag deployment, the effect of deployment on injury differs by occupant height, with a relative increase in the odds of serious injury among smaller occupants. In such crashes, the probability of injury with (versus without) deployment began to increase with occupant heights less than 155 cm (5′), reaching a level of statistical difference below 138 cm (4′ 6″).

Keywords:

• Air Bag
• Injury
• Motor Vehicle Crash

This research was presented in abstract form at the 2007 Society for Academic Emergency Medicine annual meeting in Chicago, Illinois.

Notes

∗All proportions and means are adjusted for the NASS CDS sample design (clusters, strata, and weights)

∗∗Percentages refer to column percentages and are rounded to the nearest integer unless < 1%

∗Results are from models without interaction terms for ease in interpretation. When air bag × body size interactions were tested in each model, we found the following: height × air bag (all crashes p = .49; frontal crashes p = .59); height × deployment (p = .0078); weight × air bag (all crashes p = .51; frontal crashes p = .04); weight × deployment (p = .43)

∗∗When air bag deployment was substituted for air bag presence, the odds ratio for injury in all crashes was OR 2.10 (95% CI 1.59–2.77) and in frontal crashes was OR 1.87 (95% CI 1.41–2.49)

†These variables represent fractional polynomial terms. We use “x_1” and “x_2” as variable names of the two-part fractional polynomial terms for clarity. Because statistical significance is measured using nested models, single p values are provided for each two-part term