CHICAGO (AP) – As many as 2 million concussions from sports or play activities occur in U.S. children and teens each year and many receive no treatment, a new study suggests.

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

But the researchers say the numbers are not as precise as they should be, 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 the size of the concussion problem nationwide, the system may help identify which sports and activities are riskiest for kids.

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 concussions related to sports and recreation occur in U.S. kids each year. Of those, they estimated that half a million or more kids received no treatment, using surveys of athletes who were asked if they had experienced head bumps and concussion symptoms but didn’t report them during the season, Bryan said.

Bryan said some concussions might have been counted twice if kids got treatment in more than one setting but that the researchers tried to account for that.

A CDC study published last month said playground-related concussions have also 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.