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

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

But the researchers say that 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 problem nationwide, the system may help identify which sports and activities are riskiest for kids.

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

The researchers said data suggest that 1 million to 2 million concussions related to sports and recreation occur in American children each year. Using surveys of athletes who were asked whether they had experienced head bumps and concussion symptoms but didn’t report them during the season, the researchers estimated that half a million or more kids received no treatment, Bryan said.

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

A CDC study published last month said playground-related concussions also have increased in recent years.

Other research has 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.