Burning coal produces carbon dioxide, sulfur dioxide, nitrogen oxides, particulate matter, and a range of toxic trace metals including mercury, arsenic, and lead. It also leaves behind solid waste in the form of fly ash and bottom ash. Carbon makes up 60 to 90 percent of coal’s mass, so CO2 is by far the largest byproduct by volume, with a single ton of bituminous coal releasing roughly 2,325 pounds of carbon dioxide into the atmosphere.
Carbon Dioxide and Climate Impact
Coal is the most carbon-intensive fossil fuel. Globally, coal accounts for 41 percent of all fossil CO2 emissions, more than oil (32 percent) or natural gas (21 percent). In 2023, total fossil CO2 emissions reached about 10.1 billion metric tons of carbon, and coal’s share continued to rise slightly, up 1.4 percent from the year before.
The exact amount of CO2 per ton varies by coal type. Bituminous coal, the most commonly burned variety, produces about 205 pounds of CO2 per million BTU of energy. Subbituminous coal, which has less energy per ton, actually produces slightly more CO2 per unit of energy: around 214 pounds per million BTU. Either way, generating electricity from coal releases roughly twice the CO2 of natural gas for the same amount of power.
Sulfur Dioxide and Acid Rain
Coal contains between 0.2 and 5 percent sulfur by weight. When burned, that sulfur converts to sulfur dioxide (SO2), a gas that reacts with moisture in the atmosphere to form sulfuric acid. This is one of the primary drivers of acid rain, which damages forests, crops, lakes, and streams. High-sulfur coals from the eastern United States and parts of China and India are especially problematic.
Modern coal plants use scrubber systems, formally called flue gas desulfurization, that can remove 98 to over 99 percent of SO2 before it leaves the smokestack. These systems have dramatically reduced acid rain in countries that mandate their use, but many older plants worldwide still operate without them.
Nitrogen Oxides and Smog
The intense heat inside a coal furnace causes nitrogen in both the coal and the surrounding air to react with oxygen, forming a family of gases collectively called NOx. These gases are a key ingredient in ground-level ozone, the main component of smog. NOx also contributes to acid rain alongside sulfur dioxide.
Pollution control systems for nitrogen oxides are less efficient than sulfur scrubbers. Existing installations on coal plants typically remove 50 to 90 percent of NOx, depending on the technology. That still leaves a meaningful amount escaping into the atmosphere, particularly in regions with heavy coal use and limited regulation.
Particulate Matter
Coal combustion releases tiny particles into the air in two forms. Filterable particles are the visible soot and dust you can see in smoke. Condensable particles start as vapor and solidify only after leaving the smokestack, typically shrinking to less than 1 micron in diameter. These condensable particles can account for 50 to over 90 percent of total particulate emissions from a coal plant’s stack.
The finest particles, called PM2.5 (smaller than 2.5 microns, or about 30 times thinner than a human hair), are the most dangerous because they penetrate deep into the lungs and can enter the bloodstream. PM2.5 from coal tends to carry toxic heavy metals and organic pollutants on its surface due to its high surface area. Sulfate compounds are the most concentrated inorganic component in these fine particles.
Toxic Metals and Trace Pollutants
Coal contains trace amounts of nearly every element on the periodic table, and burning it concentrates and releases many of them. The most concerning include mercury, arsenic, lead, nickel, and chromium. Coal plants also emit hydrogen chloride, hydrogen fluoride, formaldehyde, and dioxins.
Mercury is particularly troublesome because it accumulates in fish and moves up the food chain. Regulations established in the United States in 2012 cut mercury emissions from coal plants by 90 percent, acid gas emissions by over 96 percent, and non-mercury metal emissions (arsenic, lead, nickel) by more than 81 percent. These reductions show what’s possible with modern controls, but they also highlight how much pollution uncontrolled coal burning releases.
Coal Ash: The Solid Waste Left Behind
After coal burns, the noncombustible minerals remain as ash. This comes in several forms: fly ash (the lightweight particles captured from exhaust gases), bottom ash (the heavier material that falls to the bottom of the furnace), and boiler slag. Collectively, these are called coal combustion residuals.
Fly ash is primarily made of silica, aluminum, iron, and calcium, but it also contains trace levels of arsenic, lead, and other heavy metals. In the United States alone, coal plants generate over 100 million tons of coal ash per year. Some of it gets recycled into concrete and road materials, but large quantities end up in disposal ponds and landfills, where heavy metals can leach into groundwater if not properly contained.
Radioactive Elements in Coal Ash
One byproduct most people don’t expect is radioactivity. Coal naturally contains small amounts of uranium, thorium, radium, and other radioactive elements. These aren’t dangerous in raw coal, but burning concentrates them in the ash. Thorium-232 levels, for example, can jump from roughly 10 to 25 becquerels per kilogram in raw coal to around 200 becquerels per kilogram in the ash, an increase of roughly 8 to 20 times.
The radioactivity in coal ash is low-level and not an immediate health threat to people living near power plants. But it does mean that coal ash disposal sites contain measurably elevated radioactivity compared to surrounding soil, which adds another dimension to long-term waste management concerns.
Health Effects of Coal Smoke Exposure
The combination of fine particles, toxic metals, and irritant gases from coal burning has well-documented health consequences. Particulate matter inflames airways, impairs the immune response, and reduces the blood’s ability to carry oxygen. The World Health Organization estimates that household air pollution from burning solid fuels like coal contributes to approximately 2.9 million deaths each year.
Those deaths break down across several conditions: about 32 percent from heart disease, 23 percent from stroke, 21 percent from lower respiratory infections, 19 percent from chronic obstructive pulmonary disease (COPD), and 6 percent from lung cancer. Children are especially vulnerable. Exposure to coal smoke nearly doubles a child’s risk of lower respiratory infection and is responsible for 44 percent of pneumonia deaths in children under five. There is also evidence linking coal smoke to low birth weight, tuberculosis, and cataracts.
These figures reflect household exposure, where people cook or heat with coal in enclosed spaces. But even outdoor exposure near coal plants is associated with elevated rates of respiratory and cardiovascular disease, particularly in communities living downwind of older, less-regulated facilities.