Burning fossil fuels releases carbon dioxide, water vapor, and heat as the primary products of combustion. But the full list is much longer and more consequential. Depending on the fuel type and combustion conditions, burning coal, oil, and natural gas also sends sulfur dioxide, nitrogen oxides, carbon monoxide, particulate matter, heavy metals, and volatile organic compounds into the air.
The Basic Chemistry of Combustion
Fossil fuels are hydrocarbons, meaning they’re built from carbon and hydrogen atoms. When these fuels burn in the presence of oxygen, the carbon bonds with oxygen to form carbon dioxide (CO2), the hydrogen bonds with oxygen to form water vapor, and the breaking of chemical bonds releases energy as heat. That heat is what powers engines, generators, and furnaces.
In a perfect, complete combustion reaction, CO2 and water would be the only chemical outputs. But real-world combustion is never perfect. Fuels contain impurities like sulfur and trace metals, temperatures vary, and oxygen supply fluctuates. These imperfections produce a range of additional pollutants.
Carbon Dioxide and Its Climate Impact
CO2 is the most significant emission by volume and the primary driver of climate change from fossil fuel use. Different fuels produce different amounts. According to the U.S. Energy Information Administration, burning coal (bituminous) releases about 93 kilograms of CO2 per million BTU of energy. Diesel fuel oil releases about 74 kilograms, and natural gas releases roughly 53 kilograms for the same amount of energy. This is why switching from coal to natural gas reduces CO2 output per unit of electricity, though it doesn’t eliminate it.
Carbon Monoxide and Incomplete Combustion
When there isn’t enough oxygen for complete combustion, carbon monoxide (CO) forms instead of carbon dioxide. CO is a colorless, odorless gas that’s toxic to humans because it binds to red blood cells and prevents them from carrying oxygen. Incomplete combustion also produces volatile organic compounds (VOCs), including benzene, toluene, and xylene. These are released from vehicle exhaust, industrial burners, and any situation where fuel doesn’t burn cleanly. Black carbon, commonly called soot, is another product of incomplete combustion and a major component of fine particulate matter.
Sulfur Dioxide and Nitrogen Oxides
Coal and heavy oils contain sulfur as a natural impurity. When these fuels burn, the sulfur reacts with oxygen to form sulfur dioxide (SO2). Nitrogen oxides (NOx), primarily nitric oxide (NO) and nitrogen dioxide (NO2), form when the high temperatures of combustion force nitrogen and oxygen in the air to combine. Both SO2 and NOx are irritating to the lungs on their own, but their larger impact comes from what they do once airborne.
In the atmosphere, SO2 and NOx mix with water, oxygen, and other chemicals to form sulfuric and nitric acids. These fall back to earth as acid rain, which damages forests, acidifies lakes and streams, and corrodes buildings and infrastructure. The EPA identifies fossil fuel combustion as the primary source of these acid rain precursors.
Particulate Matter
Fossil fuel combustion is a major source of fine particulate matter, classified by size. PM10 particles (up to 10 micrometers in diameter) include dust and larger soot fragments. PM2.5 particles (2.5 micrometers or smaller) are far more dangerous because they penetrate deep into the lungs and can enter the bloodstream. According to the WHO, PM2.5 is composed of sulfate, nitrate, black carbon, mineral dust, and ammonia. Combustion of fuels in power plants, industrial facilities, and vehicles is a primary source of these fine particles.
Heavy Metals and Toxic Trace Elements
Coal in particular contains trace amounts of mercury, lead, nickel, and arsenic. These metals are released into the air when coal burns. In the U.S., coal and oil-fired power plants account for 44 percent of all mercury emissions. Mercury is a potent neurotoxin that accumulates in waterways and builds up in fish tissue, eventually entering the human food chain. Globally, industrial pollution from fossil fuel burning accounts for roughly two-thirds of all mercury released into the environment each year, with the remaining third coming from natural sources like volcanic eruptions.
Ground-Level Ozone and Smog
Some of the most harmful pollution from fossil fuels doesn’t come directly out of a smokestack or tailpipe. It forms in the atmosphere through secondary reactions. Nitrogen oxides and volatile organic compounds released during combustion react with sunlight to create ground-level ozone, the main ingredient in photochemical smog. Here’s the chain of events: sunlight breaks apart NO2 into nitric oxide and a free oxygen atom, that oxygen atom combines with O2 to form ozone (O3), and VOCs in the air keep regenerating NO2, which feeds the cycle. The result is a persistent buildup of ozone and other toxic compounds like peroxyacetyl nitrates in the lower atmosphere. Smog is worst on hot, sunny days in urban areas with heavy traffic.
Methane Leakage From Natural Gas
Natural gas is roughly 98 percent methane, a greenhouse gas far more potent than CO2 over shorter time frames. While combustion itself converts most methane to CO2, methane escapes at every stage of the natural gas supply chain: during drilling, processing, pipeline transport, and distribution. Distribution infrastructure alone is responsible for a measurable share of upstream methane emissions. Even the combustion process releases a small fraction of unburned methane, about 0.05 percent of the total greenhouse gas output from burning natural gas. These leaks partially offset the CO2 advantage natural gas holds over coal.
Health Consequences of These Emissions
The combined effect of these pollutants on human health is staggering. A 2023 study published in The BMJ estimated that fine particulate matter and ozone from all sources cause approximately 8.34 million excess deaths globally each year, and roughly 5.13 million of those deaths are directly attributable to fossil fuel emissions. More than half of these deaths involve heart and metabolic conditions: heart disease alone accounts for 30 percent, stroke for 16 percent, and type 2 diabetes for 6 percent. Chronic obstructive pulmonary disease contributes another 16 percent, and lower respiratory infections about 8 percent. Growing evidence links fossil fuel air pollution to arterial hypertension and neurodegenerative diseases like Alzheimer’s and Parkinson’s as well, though the research on those connections is still developing.
The health burden falls disproportionately on people living near power plants, highways, and industrial zones, where concentrations of particulate matter, NOx, and volatile compounds are highest.