Coal and oil are the two power sources that cause the most pollution, whether measured by carbon emissions, toxic byproducts, or premature deaths. Together they account for 73% of all fossil fuel CO2 emissions globally, releasing roughly 27 gigatonnes of carbon dioxide in 2023 alone. No other energy sources come close to matching their combined damage to air, water, and human health.
Coal: The Single Dirtiest Energy Source
Coal is responsible for 41% of global fossil CO2 emissions, making it the largest single contributor to climate change among all fuel types. It also remains the world’s biggest source of electricity, generating 35% of global power in 2024, a position it has held for more than 50 years.
Per unit of electricity produced, coal releases about 2.31 pounds of CO2 per kilowatt-hour. But carbon dioxide is only part of the problem. Burning coal sends sulfur dioxide, nitrogen oxides, and fine particulate matter into the air, all of which damage lungs and cardiovascular systems. The global mortality rate from coal power is approximately 100 deaths per billion kilowatt-hours produced. That’s more than 250 times deadlier than wind energy and over 2,500 times deadlier than nuclear power, per unit of electricity.
The pollution doesn’t stop at the smokestack. Coal mining contaminates groundwater with heavy metals including arsenic, lead, chromium, iron, and manganese. Rainfall and mine drainage carry these metals into surrounding water systems, where they concentrate over time. Chromium is particularly dangerous. In coal mining regions studied in China, it was the dominant cancer risk factor, contributing over 95% of the total carcinogenic health risk from groundwater contamination. All measured carcinogenic risk values in those areas exceeded the maximum acceptable level set by health agencies.
Coal ash, the waste left after burning, creates another layer of risk. Stored in massive ponds near power plants, it can leak or spill, releasing the same toxic metals into rivers and soil. Communities near coal operations face pollution from extraction, combustion, and waste storage simultaneously.
Oil: High Emissions Across Every Sector
Oil accounts for 32% of global fossil CO2 emissions. While it generates a relatively small share of the world’s electricity, oil dominates transportation and industrial energy use, which makes its total pollution footprint enormous.
When oil is used for electricity generation, it produces about 2.46 pounds of CO2 per kilowatt-hour, actually higher than coal on a per-unit basis. The mortality rate from oil-fired power is roughly 36 deaths per billion kilowatt-hours, lower than coal but still far above any clean energy source.
Oil’s pollution extends well beyond combustion. Extraction operations release methane (a gas roughly 80 times more potent than CO2 at trapping heat during its first 20 years in the atmosphere), along with volatile organic compounds that form ground-level ozone. Refining crude oil produces sulfur compounds, benzene, and other toxic air pollutants that affect workers and nearby communities. Oil spills, whether from tankers, pipelines, or offshore drilling, devastate marine ecosystems and coastlines in ways that can take decades to recover from.
Oil emissions also grew 2.5% from 2022 to 2023, the fastest increase among the three major fossil fuels that year. Projections for 2024 showed continued growth of about 0.9%, driven largely by rising demand for transportation fuels in developing economies.
How Coal and Oil Compare to Other Sources
Natural gas, often marketed as a “cleaner” fossil fuel, does produce less CO2 at the point of combustion than coal or oil. But methane leakage during extraction and transport significantly closes that gap. A Stanford-led study found that methane leaked from U.S. oil and gas operations averages about 3% of total production volume, with some regions losing nearly 10%. The federal government’s estimate of roughly 1% appears to significantly undercount actual emissions.
Biomass and wood burning deserve mention as well. While sometimes classified as renewable, their real-world health impacts are severe and growing. Between 2008 and 2017, the share of premature deaths from stationary sources attributed to biomass and wood burning rose from around 14-17% to 39-47% in the United States. Research from Harvard’s School of Public Health found that gas, biomass, and wood are not clean or healthy alternatives to coal.
Still, none of these sources rival coal and oil in total pollution output. Coal and oil together released more CO2 in 2023 than natural gas, cement production, and all other fossil sources combined. Their dominance in global emissions has persisted for decades, and both continued to grow in 2024.
Why These Two Sources Still Dominate
Coal remains entrenched because it is cheap to mine, widely available on every continent, and provides baseload power that many developing nations depend on. Countries like China and India continue to build new coal plants even as wealthier nations retire theirs. Global coal emissions grew 1.4% in 2023.
Oil’s grip on the global economy is even harder to break. It fuels nearly all road, air, and sea transportation. Replacing oil requires not just new energy generation but entirely new infrastructure: electric vehicles, charging networks, and redesigned supply chains. That transition is underway but far from complete.
The combined effect is stark. At current rates, coal and oil release over 27 gigatonnes of CO2 annually, enough to make meaningful climate targets nearly impossible to hit without dramatic reductions in both. Every other pollution source, from natural gas to industrial agriculture, operates in the shadow of these two.