(CNN) Preventable injuries often lead to death among people with autism, a new study says. They are three times more likely than the general population to die because of injuries, according to the study, published Tuesday in the American Journal of Public Health .

For children and young teens with this developmental disability, the numbers are more striking: They are 40 times more likely to die from injury than the general child population, researchers said. Drowning is the most common fatal injury among children with autism.

People diagnosed with an autism spectrum disorder, which causes challenges with social skills and communication, die at an average age of just 36, noted the researchers. For the general population, life expectancy is 72.

Two motives drove Dr. Guohua Li, senior author of the study and founding director of the Center for Injury Epidemiology and Prevention at Columbia University, to research the relationship between autism and injury.

"First, the prevalence of autism has been increasing," Li said, noting that there are an estimated 3.5 million people living with autism in the US, including about 500,000 children under the age of 15. "Second, there is anecdotal evidence that people with autism are at higher risk of injury."

Actual research to provide hard evidence, though, has been difficult to find.

36 years young

For data, Li and Joseph Guan, a master's student at Columbia, turned to the National Vital Statistics System, which records key demographic characteristics for each deceased person alongside information about the cause of death.

The researchers thumbed through more than 39 million death records filed over a 15-year period ending in 2014. By screening the codes entered onto each death certificate, Li and Guan were able to identify those who had been diagnosed with autism. They pinpointed just 1,367 individuals with a recorded diagnosis of autism, 1,043 of whom were male.

Because autism would not be the direct cause of death, it would be under-reported on death certificates, said Li. Working with the available information, he and Guan calculated mortality using the general U.S. population as a reference.

Yearly deaths for people with autism increased nearly sevenfold during the study period, Li and Guan soon discovered.

They found that more than a quarter (28%) of the people with autism died due to injury -- three times the percentage of the general population -- and that more than 40% of these deaths occurred in their homes or residential institution.

The average age when people with autism died due to an injury was about 29, compared with an average of nearly 55 for the general population.

Suffocation (when oxygen cannot enter the body), followed by asphyxiation (when oxygen is lacking in the body) and drowning caused most deaths. Combined, these three causes accounted for nearly 80% of fatal injuries in people with autism.

"Injury deaths in autistic adults are disproportionately due to asphyxiation and suffocation, and injury deaths in autistic children are much more likely caused by drowning," said Li, who is also a professor of epidemiology at Columbia's Mailman School of Public Health.

"Autistic children aged 14 years and younger are 40 times more likely to die from injury than the general pediatric population," Li said. Specifically, drowning accounts for 46% of all injury deaths among children with autism, which translates to 160 times the chance of dying from drowning compared to other children.

"The risk of drowning in autistic children peaks at age 5 to 7 years," Li said.

He explained that children with the disorder often feel anxiety, and wandering, especially toward water, is one way they seek relief. With 100,000 children newly diagnosed with disorders each year in the US, he added, "the first concrete step parents and caregivers could take to reduce the exceptionally high risk of accidental drowning is to enroll these children in swimming classes."

Importance of water safety

"Nearly 50% of children with autism wander," said Michael Rosanoff, an epidemiologist and director of public health at Autism Speaks, an advocacy organization. "And children with autism are often attracted to water."

Past studies as well as community discussion have suggested that children with autism have water accidents resulting in tragedy more frequently than other children, Rosanoff said. The higher risk of drowning estimated in the study, then, was "not a surprising finding to us, but it reaffirms the importance of teaching water safety to children with autism," he said.

"We haven't seen a study of this magnitude before. It's a very large study, probably the largest study to date on this topic," he said.

People with autism essentially have half the life expectancy of the general population, Rosanoff said: "Let's take a second to let that sink in."

"Conditions like epilepsy, depression, attention-deﬁcit hyperactivity disorder, schizophrenia -- these psychiatric and medical conditions are much more common among individuals with autism," said Rosanoff, who observed that another key point of the study is that autism itself is not a cause of death.

"Rather, autism is often a co-occurring medical and psychiatric condition that is likely to play a role in this premature mortality finding," he said.

"Other studies have shown us, including a recent study from Sweden , that, in fact, for example, premature mortality due to suicide is higher among individuals with autism," Rosanoff said. However, that same result was not found in the current study.

Parents should remember that what this study says is "that injuries are often part of the challenge, and injuries can be preventable," he said.

He pointed out that Autism Speaks ' website offers resources on water safety issues such as equipping children for being in the water and teaching them how to swim, as well as a toolkit for parents of children who wander

Higher rate of people with autism

The Autism and Developmental Disabilities Network estimates the prevalence of autism spectrum disorder at one in every 68 children as of age 8.

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This rate has more than doubled between 2000 and 2012, according to the network, which is a group of programs that is funded by the Centers for Disease Control and Prevention and tasked with estimating the number of children with autism and other developmental disabilities living in the United States.

Risk is not evenly spread throughout the population, though.

The disorder is about four times as common in males as in females, for example. And among infants who have an autistic sibling, the risk of developing the disorder may be as high as one in five, while the risk for infants without an affected sibling is just one in 100.

Prevalence also tends to be higher among non-Hispanic white children and among children of highly educated parents.