In the three years after Michigan repealed a mandatory motorcycle helmet law, deaths and head injuries among bikers rose sharply, according to a recent study.

Deaths at the scene of the crash more than quadrupled, while deaths in the hospital tripled for motorcyclists. Head injuries have increased overall, and more of them are severe, the researchers report in the American Journal of Surgery.

Senior author Dr. Carlos Rodriguez decided to do the study after noticing an abrupt change in the trauma unit at Spectrum Health Hospital in Grand Rapids, where he works.

The first week after the law was repealed in April 2012, he told Reuters Health, "I just could not help but notice the number of patients that had been in motorcycle crashes with no helmet on, which was enormously different in number and volume than we had experienced the weekend before."

The study team looked at records for patients admitted to Spectrum Health Hospital and at state transportation department records of fatalities at crash scenes for the seven-month motorcycle season (April to November) in 2011, before the law was repealed, and for the same period in 2012, 2013 and 2014.

Among the accident victims brought to the hospital, the proportion of riders who had not been wearing a helmet rose four-fold, from 7 percent to 28 percent, the researchers found.

About 10 percent of non-helmeted riders brought to the hospital died there, compared to 3 percent of those who had been wearing helmets.

Among riders who died at the crash scene, the proportion not wearing helmets rose from 14 percent before the law was repealed to 68 percent afterward.

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"These are the kinds of things that we wanted to look at to hopefully make legislators see that this was a mistake," Rodriguez said in an email.

Riders without helmets also drank more alcohol after the law was repealed, based on blood tests at hospital admission

The severity of injuries increased for non-helmeted riders, as well as the number of life-threatening head injuries.

Non-helmeted patients tended to stay longer in the Intensive Care Unit and to need machine assistance with breathing.

Overall, hospital costs averaged $27,760 for non-helmeted riders versus $20,970 for patients who had been wearing helmets.

In the U.S., 19 states and the District of Columbia require all motorcyclists to wear a helmet, while 28 states require only some to wear them and three states have no helmet laws at all.

Since the repeal of Michigan's 35-year-old helmet law, riders can go without a helmet if they are over 21, have had their license for at least two years and purchase at least $20,000 in additional medical insurance coverage, the researchers note.

Dr. Ben Zarzaur, a surgeon who studies motorcycle helmet laws, said that riders may choose not to wear a helmet because they say it is less restricting and they may claim to see or hear better without one.

"The other argument (riders make), based on a flawed study several years ago, is that wearing a helmet can increase the risk for a cervical spine injury," Zarzaur said by email.

He noted, however, that there is no real-world evidence for this claim, while there is plenty of evidence that wearing a helmet reduces the risk of death and head injury.

As the study points out, the cost of injuries and deaths from motorcycle crashes is extremely high, Zarzaur said, adding that taxpayers and other insurance payers often bear this cost. "So, choosing not to wear a helmet has consequences for many more people than just to the person who decided not to wear the helmet."

Although there is an extra insurance requirement in the law, Rodriguez said, that it is rarely enforced.

Rodriguez recommends that all motorcyclists should wear a helmet, because drivers may not see them on the road. "Even if you are the most careful motorcycle driver in the world, if you don't wear a helmet, you are putting yourself at risk for a significant injury," he said.