Environment Pollution What Is Coal Ash and How Dangerous Is It? By Tiffany Means Tiffany Means LinkedIn Twitter Writer University of North Carolina at Asheville Johns Hopkins University Tiffany Means is a meteorologist who has worked for CNN, the National Oceanic and Atmospheric Administration, and more. Since 2017, she has worked as a freelance science writer covering natural disasters, the climate crisis, and the environment. Learn about our editorial process Updated August 11, 2021 Fact checked by Elizabeth MacLennan Fact checked by Elizabeth MacLennan University of Tennessee Elizabeth MacLennan is a fact checker and expert on climate change. Learn about our fact checking process Share Twitter Pinterest Email Martin_33 / Getty Images Environment Planet Earth Climate Crisis Pollution Recycling & Waste Natural Disasters Transportation Coal ash refers to the hazardous byproducts of coal combustion at coal-based power plants — namely, fly ash, bottom ash, and boiler slag — which contain toxic materials such as arsenic and lead. It is a highly controversial type of industrial waste, given that the U.S. Environmental Protection Agency (EPA) didn’t begin regulating its disposal until 2015. In its natural state, coal is mildly hazardous. It can emit fine particulate pollution when sitting uncovered in stockpiles or being transported by trains, especially during windy weather conditions. But when coal is combusted or burned — such as in a power plant, when coal is burned in a boiler; heat from the furnace converts boiler water to steam; and the steam spins turbines to turn generators — it releases a hazardous brew of toxic pollutants into the air, including: sulfur dioxide (SO2), which contributes to acid rain and respiratory illnesses, nitrogen oxides (NOx), which contribute to smog and respiratory illnesses, and carbon dioxide (CO2), a primary greenhouse gas that contributes to human-caused global warming. Coal’s non-gaseous remnants, coal ash, contain arsenic, lead, mercury, and other heavy metals that are known to cause cancer, developmental disorders, and reproductive issues. The American Coal Ash Association estimates that in 2019, nearly 79 million tons of coal ash were generated. Consider this along with the fact that, from 1950 to 2015, coal was the largest fossil-fuel-based source of electricity generation in the United States (in 2016, it became the second-largest energy source behind natural gas), and you’ll have some idea of how much coal ash currently plagues the planet. What Are the Byproducts of Coal Ash? Coal ash consists of multiple coal combustion byproducts, including fly ash, flue-gas gypsum, bottom ash, and boiler slag, which accumulate in the bellies of coal-fired power stations. Fly Ash Roughly half of coal’s combustion leftovers take the form of “fly ash,” a light-colored, powdery residue that resembles wood ash. Fly ash is so fine and featherlight that it flies up into a power plant’s smokestacks. In the past, fly ash was released into the air this way, but laws now require that fly ash emissions be captured by filters. Flue-Gas Gypsum Flue-gas gypsum is produced when emission scrubbers inside of a coal power plant's exhaust stacks remove sulfur and oxides from the gas streams. It is the second most common coal combustion byproduct. Bottom Ash As its name suggests, bottom ash is the heavier portion of coal ash. Rather than floating into the exhaust stacks, it clumps together and settles at the bottom of the boiler’s furnace. Bottom ash comprises about 10% of coal ash waste. Boiler Slag The portions of coal ash that melt under the intense heat of combustion and then cool to form glassy, obsidian-like pellets are called boiler slag. Traces of boiler slag can be found in smokestack filters, as well as along the furnace bottom. Exactly How Dangerous Is Coal Ash? An aerial view of a coal ash pit cleanup site. Savannah River Site / Flickr / CC BY 2.0 Coal ash is stored near power plants, both in open-air landfills (“ash pits”) and ponds of water or impoundments (“ash ponds”). The problem with this storage system is contaminants within coal ash can leach into the soil, rivers, lakes, and groundwater. This is especially dangerous for those living next to the over 310 active coal ash pits, as well as the over 735 active coal ash pond disposal sites across the United States. It's so dangerous, in fact, that if you live near a wet ash pond and get your drinking water from a well, you may have as much as a one in 50 chance of getting cancer from drinking arsenic-contaminated water, notes the EPA. A December 2008 coal ash spill in Kingston, Tennessee, which resulted in more than one billion gallons of coal ash slurry damaging homes and flowing into tributaries of the Tennessee River, highlighted the environmental and human health dangers of coal ash. In response, the EPA proposed the “Disposal of Coal Combustion Residuals from Electric Utilities” rule for regulating the disposal of coal ash in June 2010. The EPA finalized the rule under the Obama administration in October 2015, but because the rule designated coal ash as “non-hazardous solid waste,” it wasn’t the win environmentalists had hoped it would be. Although the EPA still labels coal as non-hazardous, this doesn’t negate the scientific fact that coal ash contains hazardous chemical compounds. Nor does it void the Coal Ash Rule’s aim of protecting communities against coal ash toxicity, or of holding companies in violation of coal ash regulations accountable. Can Coal Ash Be Recycled? One option for reducing the amount of coal ash that’s dumped in ash pits and ponds is to recycle and reuse it as other materials. The key to safely recycling such toxic materials is a process known as encapsulation, which binds the coal ash on a molecular level and thus minimizes the leaching of toxic chemicals. Fly ash, for example, binds together and solidifies when mixed with water, making it an ideal ingredient for cement and grout. Encapsulated gypsum is commonly used to create drywall. Similarly, slag and bottom ash have been cleared by the EPA to be used as filler during the construction of roadways and embankments; however, these are unencapsulated uses of coal ash — uses where coal ash still poses some level of risk to the surrounding environment. Clearly, coal ash recycling is an imperfect science. Even so, it sometimes offers the least environmentally problematic option for coal ash disposal. View Article Sources "Coal Ash Basics." Environmental Protection Agency. "Disposal of Coal Combustion Residuals from Electric Utilities Rulemakings." Environmental Protection Agency. "Coal Explained: Coal and the Environment." U.S. Energy Information Administration. Munawer, Muhammad Ehsan. "Human Health and Environmental Impacts of Coal Combustion and Post-Combustion Wastes." Journal of Sustainable Mining, vol. 17, no. 2, 2017, pp. 87-96., doi:10.1016/j.jsm.2017.12.007 "2019 Coal Production Product (CCP) Production and Use Survey Report." American Coal Ash Association. "Electricity Explained: Electricity in the United States." U.S. Energy Information Administration. "Coal Ash, Fly Ash, Bottom Ash, and Boiler Slag." Natural Resources Defense Council. "What Are Coal Production Products?" American Coal Ash Association. "Frequent Questions About the 2015 Coal Ash Disposal Rule." Environmental Protection Agency. "Human and Ecological Risk Assessment of Coal Combustion Wastes." Environmental Protection Agency. "EPA Response to Kingston TVA Coal Ash Spill." Environmental Protection Agency. "Coal Ash Rule." Environmental & Energy Law Program at Harvard Law School. "Coal Ash Reuse." Environmental Protection Agency.