Photo by dno1967b on Flickr.

Two federal government agencies will withdraw their longstanding claims that bicycle helmets reduce the risk of a head injury by 85%. The decision comes in response to a petition the Washington Area Bicyclists Association (WABA) filed under the federal Data Quality Act.

In 1989, a study in Seattle estimated that helmets prevent 85% of head injuries. Later efforts to replicate those results found a weaker connection between helmets and head injuries, but public health advocates, government web sites, and the news media often present it as fact.

Bad information can cause problems, even when it is promoted with the best intentions. If people think that helmets stop almost all head injuries, consumers will not demand better helmets, and legislators may feel it makes sense to require everyone to wear one. WABA asked two federal agencies to correct the misinformation, and after a lengthy process, they’ve agreed to do so.

How effective are bicycle helmets?

In theory, helmets should absorb the shock from a crash. If your head strikes the ground or a vehicle, your brain could be seriously shaken by the sudden deceleration. With a helmet, the foam around your head forms a cushion.

They can also prevent head fractures by spreading the force of the impact. It’s like the difference between being hit on the head by a rock or a beach ball with the same weight.

It’s hard to tell how often helmets actually prevent head injuries, however. Experiments on people are unethical, so instead researchers collect hospital data on people involved in bicycle crashes.

In 1989, a team of researchers led by Dr. Robert S. Thompson, a preventative care specialist at the Group Health Cooperative of Puget Sound, collected data about cyclists in Seattle who went to area hospitals after a crash. Only 7% of the cyclists with head injuries wore helmets, but 24% of those without head injuries did wear helmets. Their statistical analysis, which was published in the New England Journal of Medicine, estimated that helmets had reduced the risk of a head injury by 85%.

Dr. Thompson’s study was a “case-control study” like those that first found a link between smoking and cancer. There is no true “control” group, but epidemiologists say these studies are good for showing whether something has a good or bad effect on health, though not for quantifying it.

Dozens of researchers sought to replicate the Thompson findings in their own communities. They also found that helmets reduce the risk of head injuries, but less frequently than Thompson’s team found. Some studies even found that helmets increase the risk of neck injuries. If you consider the entire body of research rather than just one study, and look at both head and neck injuries, helmets only reduce the risk of injury by about 15% to 45% .

Nonetheless, public health advocates seized on the 85% estimate as a good way to communicate risk: failing to wear a helmet makes you more than 6 times as likely to experience a head injury. Government websites and newspapers have repeated it to the point where it has become ubiquitous in discussions about bicycle helmets.

Misinformation encourages helmet laws, discourages better helmets

Bicycle safety is one of WABA’s central missions. It requires helmets on all rides that it organizes, and it sponsors the Bicycle Helmet Safety Institute, an independently-funded organization that reviews bicycle helmets and encourages improvements in their design. In the 1990’s, WABA supported proposals to require children under the age of 16 to wear bicycle helmets, which eventually became law.

But WABA draws the line at laws requiring adults to wear helmets. Such laws do little to promote safety, but they discourage bike sharing and other uses of bicycles for short trips.

This year, WABA fought hard against a bill in the Maryland General Assembly that would have required all adults to wear bicycle helmets on any trip, no matter how short. The Maryland Department of Transportation supported the mandatory helmet bill, citing the 85% estimate, while an article about it in the Washington Post cited a figure from the Centers for Disease Control (CDC) that helmets prevent 80% of head injuries. Fortunately, objections from cyclists persuaded the bill’s sponsor not to move forward with it.

Recently, most helmet research has focused on making helmets cool, rather than more protective. Better ventilation and more fashionable designs might encourage more people to buy and wear helmets, but it does not make them safer. Could that be because everyone is assuming that helmets are already 85% effective? Would that change if people thought helmets were less than 50% effective?

WABA pushed agencies to correct the misinformation

Last February, I sent emails to both CDC and NHTSA, pointing out that the 85% estimate is incorrect and providing citations to newer research. Laurie Beck, an epidemiologist from CDC promised to remove the error.

Meanwhile, NHTSA staff told me that they were too busy to discuss the matter, so we made a formal “request for correction” under the Data Quality Act, which requires information on federal web sites to be accurate and supported by appropriate research.

Two months later, NHTSA agreed to remove the 85% estimate from its website. We expect other agencies to follow the lead of NHTSA and CDC, though some may need some encouragement.

This probably won’t be the last we hear this factoid. Some nongovernmental public health advocates have ignored the results of the last 20 years of research and won’t correct their stump speeches simply because the federal government removes an outdated estimate from its websites.

Will NHTSA step forward? The agency funds a lot of data collection efforts, and it clearly seems to think that the public needs to know how effective helmets are. Now that it concedes that it has been propagating the wrong answer for all these years, will it fund the research needed to provide the correct answer?

A version of this post is cross-posted at WABA Quick Release.