A team of Harvard researchers found elevated levels of radioactivity on air particles measured downwind of fracking sites around the United Staes, according to a study published earlier this month in the journal Nature Communications.

The radiation levels they discovered were well below public health limits, but the authors warn they could induce “adverse health effects to residents living close” to fracking sites.

The study’s authors looked at data from the EPA’s radiation monitoring system, collected from 2001 to 2017, finding increased radiation on particles 12 miles (20 km) to 31 miles (50 km) downwind of fracking sites. Those close to shallower conventional wells, meanwhile, showed virtually no increases.

Learn more: StateImpact Pennsylvania > Report: Radiation levels higher downwind of fracking sites