HARRISBURG — Pennsylvania state environmental regulators released a study Thursday on radiation levels in oil and gas industry wastewater and byproducts that they say shows little potential for direct radiation exposure to workers and the public. But it recommends a follow-up study to determine whether radiation can build up in the environment.

The 200-page study began in 2013 after questions arose about the radioactivity in the huge volumes of wastewater and solid waste coming out of the ground in Pennsylvania’s natural gas drilling boom.

State officials had said drilling waste had triggered radiation monitors at landfills but cautioned that very low levels of naturally occurring radioactive materials were found in the drilling waste.

Since the study began, researchers from Duke University have reported finding high levels of radiological elements in sediments downstream of a treatment facility on Western Pennsylvania’s Blacklick Creek, a tributary of the Allegheny River, that had handled drilling wastewater.

The study by Pennsylvania’s Department of Environmental Protection was peer-reviewed, it said, and included sampling at well sites, wastewater treatment plants, landfills and roads treated by oil and gas wastewater.

“There is little potential for additional radon exposure to the public due to the use of natural gas extracted from geologic formations located in Pennsylvania,” it said.

However, it also cautioned that a buildup of radiological elements in the environment is possible from spills and at facilities that treat the waste.

In addition, filter residue from wastewater treatment facilities will require safe long-term disposal.