Power plants release a wide range of pollutants into the atmosphere, and the specific mix depends heavily on the fuel being burned. Coal-fired plants are the most prolific emitters, sending out sulfur dioxide, nitrogen oxides, carbon dioxide, particulate matter laced with heavy metals, and trace amounts of mercury. Natural gas plants produce far less of most conventional pollutants but still release CO₂ and unburned methane. Even “clean” energy sources like geothermal and waste-to-energy facilities have their own atmospheric signatures, from hydrogen sulfide to dioxins. Understanding these emissions matters because they affect air quality, climate, and human health in overlapping but distinct ways.
The Big Three From Fossil Fuel Combustion
When any fossil fuel burns, three categories of pollutant dominate the exhaust: sulfur dioxide (SO₂), nitrogen oxides (NOₓ), and carbon dioxide (CO₂). Coal is the worst offender for all three because it contains more sulfur and nitrogen-bearing compounds than natural gas, and its carbon content per unit of energy is higher. SO₂ forms when sulfur impurities in the fuel react with oxygen during combustion. Once in the atmosphere, SO₂ oxidizes further into sulfate aerosols, tiny particles that scatter sunlight and contribute to acid rain. Aircraft measurements of coal-plant plumes have found that most of this atmospheric oxidation happens close to the plant, with only about 0.2% per hour converting beyond ten kilometers downwind.1Atmospheric Environment. The oxidation of sulfur dioxide to sulfate aerosols in the plume of a coal-fired power plant In Indian coal plant plumes studied during COVID lockdowns, the sulfate-to-soot ratio was far higher than from diesel trucks or biomass burning, because modern coal combustion burns fuel efficiently enough that relatively little black carbon or organic material escapes the stack.2npj Climate and Atmospheric Science. Rapid growth and high cloud-forming potential of anthropogenic sulfate aerosol in a thermal power plant plume during COVID lockdown in India
NOₓ emissions work differently from SO₂. They form primarily from the high temperatures inside the combustion chamber, where nitrogen in the air (and in the fuel itself) reacts with oxygen. Once released, NOₓ interacts with volatile organic compounds and sunlight to produce ground-level ozone, the main ingredient in smog. Research on power plant plumes in the eastern United States has shown that the ozone impact of any single plant depends on its NOₓ output, the proximity of other emission sources, and how much sunlight and organic chemistry is available in the surrounding air.3Journal of Geophysical Research: Atmospheres. Effects of changing power plant NOx emissions on ozone in the eastern United States: Proof of concept On hot, sunny days with light winds, a single large coal plant can measurably elevate ozone concentrations for many kilometers downwind.
CO₂ is the dominant greenhouse gas from any fossil fuel plant, but it is not a conventional “pollutant” in the way SO₂ or particulate matter is. It does not directly irritate lungs or corrode buildings. Its damage is climatic: it traps heat in the atmosphere over decades to centuries. A coal plant emits roughly twice as much CO₂ per kilowatt-hour as a natural gas combined-cycle plant, which is a major reason the electricity sector has been shifting toward gas where possible.
Particulate Matter and Fly Ash
The visible haze around an old, uncontrolled coal plant comes largely from particulate matter, especially fly ash. These are tiny solid or semi-solid particles carried up the flue with the exhaust gases. Their composition reads like a geology lesson: glassy spheres of silicon, aluminum, iron, and calcium, along with residual unburned carbon. Laboratory analysis of coal fly ash from both western and eastern U.S. coals has found that particles smaller than 2.5 micrometers in diameter account for more than 80% of the total particle count, though the larger particles carry most of the mass.4Fuel Processing Technology. Investigation of primary fine particulate matter from coal combustion by computer-controlled scanning electron microscopy The finest fraction, below one micrometer, is dominated by unburned carbon particles, tiny spheres bonded together in chain-like clusters.
The chemical fingerprint of fly ash varies with the coal being burned. Western U.S. coals, which tend to be lower in sulfur but rich in calcium, produce ash dominated by calcium-silicon-aluminum glass phases. Eastern coals, richer in pyrite, generate iron-silicon-aluminum glass instead.4Fuel Processing Technology. Investigation of primary fine particulate matter from coal combustion by computer-controlled scanning electron microscopy Electron microscopy of fly ash from coal-fired plants has confirmed that more than 95% of particles in both the fine and coarse fractions are smooth mineral spheres, with sulfur layered on the surface of many particles.5Atmospheric Environment. Characterization of individual fly ash particles emitted from coal- and oil-fired power plants A portion of this fly ash is magnetic, consisting of iron oxide phases including ferrospinel and hematite mixed with silica and alumina.6PubMed Central. Magnetic Fractions of PM 2.5 , PM 2.5–10 , and PM 10 from Coal Fly Ash as Environmental Pollutants
Why does particle size matter so much? Particles below 2.5 micrometers (called PM2.5) can penetrate deep into the lungs and even enter the bloodstream. They are the fraction most strongly linked to respiratory and cardiovascular disease. Larger particles tend to settle out of the atmosphere quickly and are filtered by the nose and throat, but the ultrafine fraction can travel hundreds of kilometers and linger in the air for days.
Mercury and Other Trace Metals
Coal contains trace amounts of mercury, and burning it sends that mercury into the air. Stack measurements at coal-fired plants in Japan found total mercury concentrations in flue gas ranging from roughly 0.4 to 1.1 micrograms per cubic meter, with more than 99.5% of the mercury in gaseous form rather than attached to particles.7Science of The Total Environment. Mercury emissions from a coal-fired power plant in Japan In Chinese coal plants, the dominant form leaving the stack is elemental mercury, which accounted for roughly two-thirds to more than 90% of total mercury depending on the plant and its pollution controls.8Atmospheric Chemistry and Physics. Mercury emission and speciation of coal-fired power plants in China
Elemental mercury is especially concerning because it can drift for thousands of kilometers before settling out, contaminating remote lakes and oceans far from any coal plant. Once deposited in water, microorganisms convert it to methylmercury, a potent neurotoxin that accumulates up the food chain into fish. This is the main reason pregnant women are warned about certain types of seafood, and coal plants are the single largest industrial source of airborne mercury in many countries.
Plants equipped with modern pollution controls can cut mercury significantly. In one ultra-low-emission plant studied in China, the selective catalytic reduction system oxidized elemental mercury, making it easier for downstream scrubbers to capture. The wet flue gas desulfurization system removed the most mercury overall, followed by the electrostatic precipitator and then a wet electrostatic precipitator, which together brought stack emissions well below what older plants release.9Fuel. Study on the mercury emission and transformation in an ultra-low emission coal-fired power plant Mercury is not the only trace metal of concern. Depending on the coal source, arsenic, lead, cadmium, chromium, and selenium can all appear in the flue gas or ride along with fly ash particles.
What Natural Gas Plants Add to the Mix
Natural gas combined-cycle plants are often called “cleaner” than coal, and for SO₂ and particulate matter, the difference is dramatic: gas contains almost no sulfur or ash. But gas plants still produce CO₂ (though less per unit of electricity than coal) and NOₓ. Their more surprising emission, and one that has gotten increasing attention, is methane, the primary component of the natural gas itself.
Methane is a greenhouse gas roughly 80 times more potent than CO₂ over a 20-year time horizon. Any unburned methane that escapes the stack or leaks from fuel-handling equipment chips away at the climate advantage gas has over coal. Aircraft measurements over U.S. natural gas power plants found that the fraction of fuel escaping unburned from the stack ranged from near zero to about 0.2%, with an average emission factor close to EPA estimates during normal operation.10PubMed. Observations of Methane Emissions from Natural Gas-Fired Power Plants However, during startup, one facility showed an emission factor roughly 80 times the normal value, with about 2.5% of fuel throughput escaping as methane.
Facility-wide methane losses can be even larger than what comes out the stack. A separate study using downwind aircraft measurements found average methane emission rates from gas power plants of about 140 kilograms per hour, which was 21 to 120 times higher than what those facilities had self-reported. The methane was more strongly correlated with water vapor than with CO₂, suggesting that much of it came from non-combustion equipment like compressors, valves, and pipelines rather than from incomplete burning of fuel.11PubMed. Assessing the Methane Emissions from Natural Gas-Fired Power Plants and Oil Refineries Mobile measurements at a gas-fired plant in Seoul confirmed this pattern, identifying underground pipeline leaks and intermittent pressure-relief venting as major emission hotspots that far exceeded the facility’s own bottom-up estimates.12Atmospheric Measurement Techniques. Methane quantification of LNG gas-fired power plant in Seoul, South Korea
Waste-to-Energy Plants and Dioxins
Waste-to-energy (WTE) facilities burn municipal solid waste to generate electricity, and their emission profile is different from fossil fuel plants. The most notorious pollutants are dioxins and furans, chlorinated organic compounds that form when materials containing chlorine (like PVC plastics or treated wood) are incinerated at certain temperatures. Dioxins are extremely persistent in the environment and toxic at very low concentrations, affecting the immune, neurological, and reproductive systems.13PubMed Central. Assessing dioxin emissions change in the transition from landfilling of MSW to waste-to-energy
WTE plants also emit SO₂, NOₓ, particulate matter, hydrogen chloride (HCl), carbon monoxide, and trace heavy metals including mercury, cadmium, and lead. An inventory covering all 481 WTE plants operating in China in 2020 calculated emission factors for each of these pollutants based on hundreds of thousands of monitoring values.14Environmental Science & Technology. Air Pollutant Emission Inventory of Waste-to-Energy Plants in China and Prediction by the Artificial Neural Network Approach The good news is that modern combustion and flue gas cleaning have slashed dioxin output. In France, total dioxin emissions from all municipal solid waste incinerators dropped from 435 grams of toxic equivalent in 1997 to just 1.2 grams by 2008, a decline of more than 99%, with all facilities operating well below the EU limit of 0.1 nanograms per cubic meter.15PubMed. Dioxin emissions from municipal solid waste incinerators (MSWIs) in France
Geothermal Plants and Hydrogen Sulfide
Geothermal power plants tap heat from underground, so they burn no fuel. But they are not emission-free. The steam and hot water they bring to the surface contain dissolved gases, most importantly hydrogen sulfide (H₂S), a toxic gas with a strong rotten-egg smell. Non-condensable gases in geothermal steam also include CO₂ (though far less per kilowatt-hour than fossil fuel combustion) and sometimes trace amounts of ammonia and boron.
H₂S is the primary air quality concern. Dispersion modeling of 32 geothermal plants in Tuscany, Italy, found that H₂S concentrations can reach as high as 1,100 micrograms per cubic meter near the stacks but drop rapidly with distance, diluting along the prevailing wind direction.16PubMed. Modelling of hydrogen sulfide dispersion from the geothermal power plants of Tuscany (Italy) Similar modeling for a plant in West Java confirmed the same pattern: H₂S is concentrated close to the facility and poses localized rather than regional risk.17IOP Conference Series: Earth and Environmental Science. Simulation of Hydrogen Sulphide Dispersion from Geothermal Power Plant Operations in Bandung Regency, West Java Many modern geothermal plants use H₂S abatement systems that convert the gas to elemental sulfur or inject it back underground, but in areas with high geothermal activity and many plants, the cumulative emissions can still affect local air quality and cause nuisance odors for nearby residents.
Nuclear Plants and Water Vapor
Nuclear power plants produce electricity by splitting atoms rather than burning fuel, so they emit no SO₂, no NOₓ, no CO₂, and no particulate matter during operation. What you see billowing from the iconic cooling towers is water vapor, nothing more. However, nuclear reactors do release trace amounts of radioactive gases and aerosols, including isotopes of krypton, xenon, tritium, and iodine. These leave the plant through carefully filtered ventilation systems at levels that are tightly regulated.
Comparative dosimetry studies have found that the radiation exposure to the public from a nuclear plant’s airborne effluents is far lower than that from a coal plant. Coal naturally contains trace radioactive elements like uranium, thorium, and their decay products, which concentrate in fly ash during combustion. One study calculated that the effective dose from external exposure to a coal plant’s gaseous emissions was roughly 40 times higher than the corresponding dose from a nuclear plant’s stack releases.18Hindawi / Science and Technology of Nuclear Installations. Public Health Effects of Radioactive Airborne Effluents from Nuclear and Coal-Fired Power Plant That comparison surprises most people, but it reflects how dilute radioactive material is in nuclear exhaust versus how concentrated it becomes in coal ash.
How Pollution Controls Change the Picture
Modern coal and gas plants bear little resemblance to the uncontrolled smokestacks of the mid-20th century, thanks to a layered system of pollution controls. Electrostatic precipitators charge incoming particles and collect them on metal plates, capturing the vast majority of fly ash before it reaches the atmosphere. When paired with a low-temperature economizer (which cools the flue gas and improves particle collection), electrostatic precipitators can achieve removal efficiencies above 99.7% for PM2.5 and 99.2% for the even finer PM1.0 fraction.19Proceedings of the Combustion Institute. Measurement of particulate matter and trace elements from a coal-fired power plant with electrostatic precipitators equipped the low temperature economizer
For SO₂, the standard technology is flue gas desulfurization, commonly called a “scrubber.” Wet scrubbers spray a limestone or lime slurry into the exhaust gas, where it reacts chemically with SO₂ to form calcium sulfate (gypsum). Pilot-scale work on dual-loop spray scrubbers has shown that higher inlet SO₂ concentrations actually improve the scrubber’s relative efficiency, and the main resistance to absorption lies on the liquid side of the reaction rather than the gas side.20Chemical Engineering Research and Design. Pilot-scale experiment and simulation optimization of dual-loop wet flue gas desulfurization spray scrubbers For NOₓ, selective catalytic reduction injects ammonia into the flue gas, which reacts with nitrogen oxides over a catalyst to produce harmless nitrogen and water. Running these systems under ultra-low emission limits requires precise ammonia dosing; too little leaves NOₓ in the exhaust, while too much leads to ammonia slip and deposits that can foul downstream equipment.21Environmental Progress & Sustainable Energy. Static mixing retrofit for Selective Catalytic Reduction in a 4 × 600 MW coal‐fired power plant under ultra‐low emission constraints
The combined effect of these technologies has been enormous. In the United States, the Acid Rain Program achieved large reductions in SO₂ and NOₓ from power plants, although follow-up research has shown that the environmental damage from acid and nitrogen deposition is harder to reverse than originally expected and may require even deeper emission cuts.22PubMed Central. A fresh look at the benefits and costs of the US acid rain program
Public Health Consequences Downwind
Emissions from power plants do not stay at the plant. SO₂ converts to fine sulfate particles, NOₓ contributes to ozone formation, and mercury accumulates in ecosystems. For communities living downwind, the health effects are tangible. A modeling study of power plants near Washington, D.C., estimated that reducing their emissions would prevent roughly 240 premature deaths, 60 cardiovascular hospital admissions, and 160 pediatric asthma emergency room visits per year in the surrounding population.23PubMed Central. The importance of population susceptibility for air pollution risk assessment: a case study of power plants near Washington, DC Those numbers come from concentration-response relationships linking PM2.5 and ozone to mortality and morbidity, applied to the population exposed.
In China, analysis of what would happen if available agricultural biomass replaced coal in power plants projected simultaneous reductions in NOₓ, SO₂, PM10, PM2.5, and CO₂ emissions, with SO₂ alone dropping by over 115,000 tons.24PubMed. Significant co-benefits of air pollutant and CO2 emission reduction from biomass energy utilization in power plants in China This kind of modeling underscores a point that often gets lost in climate debates: conventional air pollutants from power plants cause immediate, local harm in addition to the long-term, global harm of CO₂. Cutting emissions addresses both problems at once.
What Carbon Capture Adds to the Exhaust
Carbon capture and storage technology aims to intercept CO₂ before it leaves the plant and store it underground. But the capture process itself introduces a new class of emissions that would not otherwise exist. The most common approach uses amine solvents to chemically absorb CO₂ from the flue gas. Those solvents are somewhat volatile, and traces escape into the atmosphere through the absorber exhaust. At a pilot carbon capture plant at the Łaziska coal-fired power plant in Poland, the main contaminants in the cleaned gas leaving the absorber were ammonia (about 12 parts per million by volume), the amine solvent AMP (about 20 ppm), and acetone (about 1 ppm).25International Journal of Greenhouse Gas Control. Amine emissions and CO2 purity from carbon capture pilot plant at the Łaziska coal-fired power plant
The bigger concern is what happens to amines once they are in the atmosphere. Amine compounds can react with nitrogen oxides in the air to form nitrosamines and nitramines, which are potent carcinogens. A worst-case-scenario assessment of monoethanolamine (MEA) capture found that amine deposition near a hypothetical plant could exceed toxicity limits for aquatic organisms by a factor of three to seven, and the secondary formation of nitrosamines from degradation products like diethylamine could approach or exceed safety limits for drinking water.26International Journal of Greenhouse Gas Control. Worst case scenario study to assess the environmental impact of amine emissions from a CO2 capture plant That does not mean carbon capture is a bad idea, but it does mean that trading CO₂ for amine emissions requires careful management, including water wash systems to scrub amines from the exhaust before it is vented.
Cooling Towers and Water Vapor Plumes
The most visible emission from many power plants, fossil or nuclear, is the white plume rising from cooling towers. This is overwhelmingly water vapor, not smoke. Wet cooling towers work by evaporating water to remove waste heat from the steam cycle, and the warm, moisture-laden air rises and condenses into a visible plume as it meets cooler ambient air. On cold or humid days, these plumes can extend for kilometers and sometimes trigger localized fog or light drizzle downwind. Modeling of combined-cycle plants has examined both the vapor plume itself and the “drift,” tiny liquid droplets that carry dissolved minerals from the cooling water into the surrounding air. The drift is typically a very small fraction of total water loss but can deposit trace chemicals on nearby surfaces over time.
Some plants use dry or hybrid cooling systems that eliminate or reduce the visible plume, mainly in water-scarce regions or near airports where fog could be a safety hazard. The tradeoff is reduced thermal efficiency and higher construction cost. For the casual observer, learning that the dramatic white plume is just steam and not pollution is often the single biggest correction to their mental image of power plant emissions.