An increasing number of children under age 6 are being exposed to marijuana, according to a new study. Three-quarters of cases involve children who ingest the drug in the form of brownies, cookies and other foods containing marijuana.

Researchers at Nationwide Children’s Hospital in Columbus, Ohio investigated marijuana exposures among children under age 6 in the United States using data from the National Poison Data System. They report 18.5 percent of exposures required admission to a health care facility. More than 75 percent of cases involved children under the age of 3, Time reports.

Some children in the study experienced coma, decreased breathing or seizures. Levels of THC, the main psychoactive ingredient in marijuana, can be especially high in marijuana food products, the researchers noted.

Between 2006 and 2013, they found a 147.5 percent increase in marijuana exposure in children under age 6. The exposure rate jumped 610 percent in states where marijuana was legalized for medical purposes before 2000.

While the total number of reported exposure cases between 2000 and 2013 was not large—1,969 children—the quick rise in the rate of exposure is a cause for concern, the researchers wrote in the journal Clinical Pediatrics.

“Any state considering marijuana legalization needs to include child protection in its laws from the very beginning,” said senior author Gary Smith.

Co-author Henry Spiller said the high rates of marijuana ingestion are most likely due to the popularity of marijuana-laced treats. “Very young children explore their environments by putting items in their mouths, and foods such as brownies and cookies are attractive,” he noted in a news release.

The researchers recommend that packaging for commercially available marijuana products be child-resistant and not see-through. Marijuana products in households should be kept up, away and out of sight of children, preferably in a locked cabinet, they said.