Coal ash, a common and unregulated byproduct of coal-fired power plants, has been found to contain high levels of radioactive contaminants, placing at great risk the countless number of people who live near such dumps—including the thousands of North Carolina residents affected by the 2014 Dan River spill.

The study, led by scientists with Duke University and published in the September 2 edition of Environmental Science and Technology, found that “levels of radioactivity in the ash were up to five times higher than in normal soil, and up to 10 times higher than in the parent coal itself because of the way combustion concentrates radioactivity.”

Researchers said their discovery raises concerns because coal ash, also known as fly ash, disposal sites are not yet regulated, nor are they monitored for radioactivity. “We don’t know how much of these contaminants are released to the environment, and how they might affect human health in areas where coal ash ponds and landfills are leaking,” said Avner Vengosh, professor of geochemistry and water quality at Duke’s Nicholas School of the Environment.

According to the study, when coal is burned, naturally occurring radium isotopes “become concentrated in the coal ash residues, and the lead-210 becomes chemically volatile and reattaches itself to tiny particles of fly ash. This causes additional enrichment of radioactivity in the fly ash.”