CHICAGO — 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.