Nearly 2 million concussions from sports or play activities occur in U.S. children and teens each year and many receive no treatment, according to a new study.

The estimate is based on 2013 data from emergency-room visits, hospitalizations, doctor-office visits, concussion reports made to high school athletic trainers, and information from previous concussion studies.

But the researchers say the numbers are imprecise, highlighting the need for a concussion surveillance system as recommended by the Institute of Medicine, a nonprofit group that advises the government on public health issues.

The federal Centers for Disease Control and Prevention plans to create a system to track concussion rates and trends in children and adults. In addition to getting a better picture of how big a problem concussions are nationwide, the system may help identify which sports and activities are riskiest for kids.

"It is critical this system includes recreational sources of concussion," the study authors said.

The new study, led by pediatrician Dr. Mersine Bryan of Seattle Children's Hospital, involved kids age 18 and younger. It was published Monday in the journal Pediatrics.

The researchers said data suggest between 1 million and 2 million sports- and recreation-related concussions occur in U.S. kids each year. They estimated that half a million or more kids sought treatment from an athletic trainer or received no treatment at all.

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The study found that treatment in doctors' offices was more common than ER treatment, echoing recent research from Pennsylvania.

A CDC study published last month focused on playground-related concussions and said these injuries have increased in recent years

Other research has also suggested that kids' concussion rates may be rising, but some experts say the apparent trend may simply reflect growing awareness about the potential seriousness of concussions.