That beautiful park you walk through could be hiding dangerous levels of lead. So, too, could the soil under the play equipment where your children slide and swing.

New research in the United States finds that urban parks built on sites where waste was incinerated could be hotspots for lead.

“We found that city parks and playgrounds built on the site of a former waste incinerator can still have greatly elevated levels of lead in their surface soils many decades after the incinerator was closed,” study co-author Daniel Richter, a professor of soils at Duke University School of the Environment, said in a university news release.

Lead exposure has been linked to potential long-term health problems, especially in children. It can affect the brain and nervous system, slow growth and development, and cause learning and behavioral problems.

Cities across the United States and Canada burned trash in municipal incinerators for decades. Most were closed in the 1970s because of pollution concerns.

To study whether this led to continued soil contamination in land repurposed as parks, the researchers collected and analyzed surface soil samples from three city parks in Durham, N.C. The sites held incinerators that closed in the early 1940s.

Samples collected from a two-acre section of one park had lead levels over 2,000 parts per million. That’s more than five times higher than the current U.S. Environmental Protection Agency (EPA) standard for safe soils in children’s play areas.

At a second park, soil mostly contained low lead levels, “but about 10% were concerning and a few were very high,” Richter said.

Samples from the third park contained levels of soil lead below the current EPA threshold. They “presented no cause for concern,” Richter said.

He called for increased monitoring because of these significant variations.

“Determining where contamination risks persist, and why contamination is decreasing at different rates in different locations, is essential for identifying hotspots and mitigating risks. Many cities should mobilize resources to do widespread sampling and monitoring, and create soil maps and, more specifically, soil lead maps,” Richter said.

“That’s where we really need to go,” Richter said. “Not just in Durham but in hundreds of other cities where parks, as well as churches, schools and homes, may have been built on former waste incinerator and ash disposal sites.”

About half of all U.S. and Canadian cities burned solid waste between the 1930s and 1950s, according to historic surveys.

“These incinerators burned all kinds of garbage and trash, including paint, piping, food cans and other products that contained lead back then,” Richter said.

That leftover ash was often covered with a thin layer of topsoil or spread around parks, new construction or other urban spaces.

“Historical surveys indicate a lack of appreciation for the health and environmental hazards of city-waste incinerator ash. Back then, they didn’t know what we do now,” he said.

New technology makes sampling and monitoring more feasible, Richter noted.

“There’s been a lot of interest in mitigating lead exposure in cities, but most until now has been focused on reducing risks within the home. Our study reminds us that risks exist in the outdoor environment, too,” he said.

The findings were published online Sept. 11 in Environmental Science & Technology Letters. The U.S. National Institute of Environmental Health Sciences partly funded the research.