Coal ash is mildly radioactive. It contains naturally occurring radioactive elements found in coal, but burning concentrates them, making coal ash up to five times more radioactive than normal soil and up to ten times more radioactive than the original coal. While these levels are low compared to what most people picture when they hear “radioactive,” the details matter, especially for communities living near coal ash disposal sites.
Why Burning Coal Concentrates Radioactivity
Coal forms over millions of years from organic material buried underground, and it naturally absorbs trace amounts of radioactive elements from surrounding rock. These include radium-226, thorium-232, potassium-40, and polonium-210. In raw coal, the concentrations are low enough to be unremarkable.
When coal burns in a power plant, the organic material combusts away as gas and heat, but the mineral content stays behind as ash. This process works like reduction in cooking: the same amount of radioactive material now occupies a much smaller volume. Researchers at the International Atomic Energy Agency measured this concentration effect and found enrichment factors that vary widely depending on the element and the power plant. Radium-226 concentrates by a factor of 5 to 15. Thorium-232 concentrates 1 to 10 times. Polonium-210, a particularly hazardous element, can concentrate 30 to 200 times in fly ash compared to the fuel that was burned. The exact numbers depend on the type of coal and how the plant operates.
How It Compares to Background Levels
Soil everywhere on Earth is slightly radioactive. The practical question is how coal ash compares. Research from Duke University’s Nicholas School of the Environment found radioactivity levels in coal ash up to five times higher than in normal soil. The EPA has found that high-end radium activity in coal ash can be nearly 10 picocuries per gram (pCi/g) above typical background soil. For context, some Superfund and state cleanup programs use a threshold of about 5 pCi/g of radium above background as a standard that triggers action. The median radioactivity of fly ash and bottom ash already falls close to that threshold, sitting around 4.3 pCi/g above background.
The EPA describes coal ash as “only slightly more radioactive than the average soil in the United States” and has stated that the radiation levels are small enough that no special precautions are needed for general handling. That said, “slightly above average” across millions of tons of material stored in unlined ponds and landfills creates a different risk picture than a thin layer of ordinary dirt.
The Real Health Concern Isn’t Just Radiation
Radioactivity in coal ash gets attention, but it’s one piece of a larger problem. Coal ash also contains arsenic, cadmium, lead, and mercury. When the EPA assessed cancer risk from coal ash mixed into surface soil, it flagged a combined risk from ingesting arsenic and radium together, plus direct exposure to gamma radiation from radium. The radioactivity and the toxic metals reinforce each other.
People living near coal ash sites can be exposed through multiple routes. Windblown dust is the most obvious: fine and ultrafine particles from dry ash ponds can be inhaled, and research has shown these tiny particles can pass through the nasal pathway directly into the bloodstream and brain. Contaminated groundwater is another pathway, particularly for communities that rely on well water. Skin contact with contaminated soil or settled dust adds a third route. Chronic exposure to this combination of particulate matter and heavy metals causes persistent inflammation throughout the body. For children, the stakes are higher because several metals in fly ash are neurotoxins, and exposure during early development can disrupt brain growth and lead to lasting neurological problems.
Coal Ash in Building Materials
Roughly 40 to 50 percent of coal ash produced in the U.S. gets recycled into products like concrete, blended cement, structural fill, roofing granules, and blasting grit. If you’ve driven on a highway or walked through a parking garage, you’ve likely been near concrete containing coal fly ash. The EPA has evaluated this use and concluded that the natural radiation in these recycled products is low enough that no precautions are needed. Once coal ash is locked into concrete, the radioactive particles are encapsulated and don’t become airborne or leach into water the way they can from an open disposal pond.
What Regulations Cover Coal Ash Radioactivity
The EPA regulates coal ash disposal under the Coal Combustion Residuals (CCR) rule, which sets standards for landfills and surface impoundments. When the agency assessed whether radon gas escaping from buried coal ash posed a risk, it found the emissions were indistinguishable from natural background soil and dropped that concern from further review. The bigger regulatory issue is what happens when coal ash mixes with surface soil over time: the accumulation can push radium levels and cancer risk above cleanup thresholds used at contaminated sites around the country.
A 2024 rule update specifically addressed legacy coal ash ponds, many of which were built decades ago without modern liners or monitoring. These older sites represent the highest risk because they allow coal ash to leach into groundwater and erode into surface soil, creating the conditions where radioactivity and heavy metals accumulate together in the environment around them.