Heavier people are more likely to be killed or seriously injured in car accidents than lighter people, according to new research.
That could mean car designers will have to build in new safety features to compensate for the extra hazards facing overweight passengers. In the US, car manufacturers have already had to redesign air bags so they inflate to lower pressures, making them less of a danger to smaller women and children. But no one yet knows what it is that puts overweight passengers at extra risk.
A study carried out in Seattle, Washington, looked at more than 26,000 people who had been involved in car crashes, and found that heavier people were at far more risk. People weighing between 100 and 119 kilograms are almost two-and-a-half times as likely to die in a crash as people weighing less than 60 kilograms.
And importantly, the same trend held up when the researchers looked at body mass index (BMI) - a measure that takes height as well as weight into account. Someone 1.8 metres tall weighing 126 kilograms would have a BMI of 39, but so would a person 1.5 metres tall weighing 88 kilograms. People are said to be obese if their BMI is 30 or over.
The study found that people with a BMI of 35 to 39 are over twice as likely to die in a crash compared with people with BMIs of about 20. It is not just total weight, but obesity itself that's dangerous.
While they do not yet know why this is the case, the evidence is worth pursuing, says Charles Mock, a surgeon and epidemiologist at the Harborview Injury Prevention and Research Center in Seattle, who led the research team. He thinks one answer may be for safety authorities to use heavier crash-test dummies when certifying cars as safe to drive.
Crash tests normally use dummies that represent standard-sized males weighing about 78 kilograms. Recently, smaller crash-test dummies have also been used to represent children inside crashing cars. But larger and heavier dummies are not used, the US National Highway Traffic Safety Administration in Washington DC told New Scientist.
The reasons for the higher injury and death rates are far from clear. Mock speculates that car interiors might not be suitably designed for heavy people. Or obese people, with health problems such as high blood pressure or diabetes, could be finding it tougher to recover from injury.
Richard Kent, an expert in impact biomechanics at the University of Virginia, thinks the new research has established a legitimate connection between obesity and severe injury or death. Because the research used BMI data, it has not confused taller (and therefore heavier than average) people with those who are overweight.
People who are obese might also be at risk because seat belts do not hold them as securely in a crash. "For example, a large amount of [fat] tissue between the restraint system and the bony thorax acts much like a winter coat: it introduces "slack" into the restraint system and decreases its performance," Kent says.
Journal reference: Accident Analysis and Prevention (vol 34, p 221)