Coal ash is the waste left behind after coal is burned to generate electricity. It contains concentrated levels of heavy metals and other toxic substances that can contaminate drinking water, pollute the air, and increase cancer risk for people living nearby. The United States produces roughly 100 million tons of it each year, making it one of the largest industrial waste streams in the country.
What Coal Ash Actually Is
When a power plant burns coal, not everything combusts. The mineral matter in coal survives the process and collects as ash in different forms depending on where it ends up in the furnace and pollution control systems. There are four main types.
Fly ash is the most common. It’s an extremely fine, powdery material composed mostly of silica that gets carried upward with exhaust gases and captured by filters before it reaches the smokestack. Because the particles are so small, fly ash is also the type most easily inhaled if it escapes into the air.
Bottom ash is coarser and heavier. These angular particles are too large to float upward, so they collect at the base of the furnace. Boiler slag forms in certain furnace types where bottom ash melts into a liquid state, then cools into smooth, glassy pellets when hit with water. Flue gas desulfurization material is a byproduct of the scrubbers that remove sulfur dioxide from smokestack emissions. It’s either a wet calcium-based sludge or a dry powder, depending on the system.
All four types share the same core problem: burning coal concentrates the trace metals and minerals that were embedded in the original rock. Arsenic, mercury, lead, cadmium, chromium, and selenium all survive combustion and end up in the ash at much higher concentrations than they existed in the raw coal.
How People Get Exposed
Coal ash reaches people through three main pathways: air, water, and soil. The route depends largely on how the ash is stored and whether containment systems are working.
Airborne exposure happens when dry coal ash blows off storage piles, landfills, or trucks as fine dust. The smallest particles, those under 10 micrometers, are the most concerning because they’re small enough to travel deep into the lungs, reaching the tiny air sacs where oxygen enters the bloodstream. From there, toxic metals can affect organs beyond the lungs. Communities downwind of uncontrolled ash disposal sites can inhale this dust daily without realizing it.
Water contamination is the other major pathway. When coal ash sits in unlined storage ponds or landfills, rainwater percolates through it, leaching heavy metals into the surrounding groundwater. People who rely on private wells near these sites may drink water containing elevated levels of arsenic, selenium, or other contaminants for years. Surface water contamination also occurs when ponds overflow or when structural failures send ash directly into rivers and lakes.
The Health Risks
Long-term exposure to coal ash is linked to heart disease, respiratory illness, and cancer. The specific health effects depend on which contaminants are present and how much exposure occurs, but the overall pattern is consistent across studies.
Respiratory diseases like asthma, bronchitis, and chronic lung damage are among the most common effects, driven primarily by inhaling fine particulate matter contaminated with toxic metals. These particles trigger inflammation in the airways and, over time, can cause permanent scarring of lung tissue.
Cancer risk is the most studied concern. A 2025 analysis of U.S. counties found that areas containing or adjacent to coal ash storage sites had significantly higher cancer rates compared to counties farther away, even after adjusting for differences in smoking, drinking, and physical activity levels. Lung cancer showed particularly strong associations with airborne arsenic concentrations and fine particulate matter from coal ash. These findings held up after accounting for the behavioral risk factors that typically drive lung cancer, suggesting the environmental exposure itself is a meaningful contributor.
Arsenic, one of the most abundant contaminants in coal ash, is a known human carcinogen linked to cancers of the lung, bladder, and skin. Lead exposure affects the nervous system and is especially harmful to children’s brain development. Mercury damages the kidneys and nervous system. Chromium in its most toxic form causes lung cancer when inhaled.
What Happens When Storage Fails
Most coal ash in the U.S. has historically been stored in one of two ways: dumped into wet surface impoundments (essentially open ponds) or placed in dry landfills. Both methods have caused documented environmental damage when poorly managed, but wet impoundments carry additional catastrophic risks.
The most dramatic example occurred in Kingston, Tennessee in 2008, when an earthen wall holding back a massive coal ash pond collapsed and released over a billion gallons of toxic sludge into the surrounding community and river system. The spill buried homes, contaminated waterways, and triggered years of cleanup. Selenium, a metalloid that bioaccumulates through the food chain, was found at elevated levels in fish near the spill site for years afterward. Dozens of cleanup workers later became seriously ill or died, with many attributing their conditions to prolonged coal ash exposure.
Even without a catastrophic breach, impoundments routinely leak. Many older ponds were built without liners, allowing contaminated water to seep directly into groundwater. The EPA has documented hundreds of cases where coal ash disposal units have damaged surface water, groundwater, or air quality. Contaminants can migrate underground for miles before showing up in someone’s well.
How Coal Ash Is Regulated
Coal ash occupied a regulatory gray area for decades. It wasn’t classified as hazardous waste, which meant disposal standards were minimal and enforcement was largely left to states. That changed in 2015 when the EPA finalized the first federal rules specifically governing coal ash disposal.
The 2015 rule set requirements for both landfills and surface impoundments, addressing the three main risks: groundwater contamination from leaking, airborne dust from uncovered ash, and structural failure of impoundment walls. It required groundwater monitoring, established structural integrity standards for ponds, and mandated that unlined impoundments showing groundwater contamination either install liners or close.
A significant gap remained, however. The original rule exempted “legacy” sites, meaning inactive coal ash ponds at retired power plants. A federal court struck down that exemption in 2018, and in May 2024, the EPA finalized rules extending groundwater monitoring, corrective action, and closure requirements to these legacy impoundments. Under updated timelines published in early 2026, facilities must complete site evaluations by 2028, install groundwater monitoring systems by 2031, and begin closing contaminated sites by 2032.
These deadlines mean that many legacy coal ash ponds will continue sitting unmonitored for several more years, a timeline that frustrates environmental advocates who point out that contamination doesn’t pause for regulatory schedules.
Coal Ash Recycled Into Building Materials
Not all coal ash ends up in disposal sites. About 35 million tons were recycled in 2021 alone, primarily into two products: concrete and wallboard. Roughly 12.6 million tons went into concrete and grout, where fly ash substitutes for a portion of Portland cement. Another 11.7 million tons of flue gas desulfurization gypsum replaced mined gypsum in drywall panels.
These are considered “encapsulated” uses because the ash gets locked into the finished product, preventing contaminants from leaching into the environment. The EPA has evaluated both applications and concluded that environmental releases from coal ash concrete and wallboard are comparable to, or lower than, releases from the same products made with conventional materials. For unencapsulated uses where coal ash is spread directly on land in large quantities, the rules are stricter. Users must demonstrate that contaminant releases stay below health-based benchmarks.
Recycling coal ash into concrete actually improves the material’s durability and workability while reducing the need for cement production, which is itself a major source of carbon emissions. From a practical standpoint, living or working in a building made with fly ash concrete does not pose a known health risk. The danger comes from coal ash in its loose, uncontained form, sitting in open ponds or blowing off uncovered piles where people can breathe it in or where rain can wash it into the water supply.