Sulfur dioxide (SO₂) is a colorless, gaseous air pollutant composed of sulfur and oxygen, produced when sulfur-containing fuels such as coal, petroleum oil, or diesel are burned. It is also emitted from the combustion of biomass, including wood, peat, crop residues, and prescribed burns.1 Sulfur dioxide is a major air pollutant in many regions worldwide.
Sulfur dioxide exposure has harmful effects on the lungs, causing wheezing, shortness of breath, and chest tightness, which can increase the risk of hospital admissions or emergency room visits, particularly among children, older adults, and individuals with asthma. Sulfur dioxide exposure is linked to heart disease, as well as eye and skin irritation.2
Beyond human health, sulfur dioxide has extensive environmental impacts, contributing to acid rain, reduced crop productivity, soil degradation, and structural damage to buildings and infrastructure. In the atmosphere, sulfur dioxide undergoes chemical reactions that transform it into sulfate aerosols, which scatter sunlight and brighten clouds, enhancing their ability to reflect solar radiation. These processes create a cooling effect, partially offsetting the warming impact of greenhouse gases.3
Changes in the fuel sources contributing to sulfur sulfur dioxide emissions reflect major historical energy transitions. In the 19th century, coal gradually replaced wood as the dominant fuel, followed by a shift toward liquid fuels derived from crude oil in the 20th century.
Today, coal combustion accounts for nearly 50% of global anthropogenic sulfur dioxide emissions, while fuels derived from crude oil contribute another 25%. Heavy oils, such as bunker fuel and residual fuel oil, contain significantly higher sulfur levels than diesel and gasoline, leading to greater emissions. The remaining 25% of emissions originate from industrial and manufacturing processes that involve heating or chemically transforming sulfur-containing materials, releasing sulfur dioxide as a byproduct.
Starting in the late 19th century, sulfur dioxide emissions surged for nearly a century, driven by rapid economic growth fueled largely by fossil fuels. However, growing awareness of the harmful effects on air quality and ecosystems led to a sharp decline. This reduction stemmed from three interconnected strategies: air pollution regulations, fuel switching, and technological advancements.
In the United States, strict measures such as the Clean Air Act (1970) and the Acid Rain Program (1995) required power plants and industrial facilities to cut sulfur dioxide emissions significantly. Similarly, the European Union implemented regulations like the Large Combustion Plant Directive (1988) and the Industrial Emissions Directive (2010) to curb emissions. China, on a different trajectory, saw emissions rise through the early 2000s before experiencing a sharp decline following the introduction of the National Air Pollution Action Plan (2013).
Regulatory actions spurred the widespread adoption of flue gas desulfurization systems (“scrubbers”), which effectively remove sulfur dioxide from power plant exhaust gases. These systems became prevalent in the 1980s and 1990s, particularly in coal-fired power plants. At the same time, the energy sector transitioned toward low-sulfur coal, while cars, trucks, and ships increasingly adopted low-sulfur liquid fuels.
The 19th century saw the residential sector as a major source of sulfur dioxide emissions, primarily from wood and coal combustion in homes. By the early 20th century, industrial activities contributed a growing share. In the second half of the century, electricity generation expanded rapidly, eventually accounting for nearly 50% of total sulfur dioxide emissions in 2022.
The largest historical emitters of sulfur dioxide share a combination of factors: early and extensive industrialization, heavy reliance on fossil fuels (particularly coal for electricity generation), weak regulatory oversight, and inefficient energy conversion technologies (e.g., engines, furnaces, and boilers). The United States accounts for approximately 23% of all-time global SO₂ emissions, followed by China (11%), the United Kingdom (7%), Russia (6%), and Germany (6%).
Global sulfur dioxide emissions peaked in 2005, declining 73% by 2022. Some nations peaked early, including the United States (1973), Canada (1973), Germany (1973), and Japan (1970). Others peaked later, such as Brazil (2000), China (2006), and Saudi Arabia (2016). India has now surpassed China as the world’s largest sulfur dioxide emitter, driven by its continued reliance on coal, while China has enacted stricter pollution controls. Since 2007, China’s emissions have dropped by 75%, whereas India’s have risen by 50%.4
Sulfur dioxide emissions are expected to continue declining as the global transition away from fossil fuels accelerates. Stricter regulations will be instrumental, including the phase-out of coal-fired power plants and the implementation of tighter emissions controls in the power and maritime transport sectors. The growth of solar and wind energy, along with a potential resurgence of nuclear power, will further drive reductions by decreasing dependence on fossil fuels for electricity generation.
1 U.S. Environmental Protection Agency, “Sulfur Dioxide Basics,” retrieved March 8, 2025, https://www.epa.gov/so2-pollution/sulfur-dioxide-basics
2 American Lung Association, “Sulfur dioxide,” retrieved March 8, 2025, https://www.lung.org/clean-air/outdoors/what-makes-air-unhealthy/sulfur-dioxide
3 MIT Climate Portal, “How much global warming has been hidden by the cooling effect from sulfur produced by burning coal and oil?” retrieved March 8, 2025, https://tinyurl.com/58376w9e
4 Li, C., McLinden, C., Fioletov, V., Krotkov, N., Carn, S., Joiner, J., Streets, D., He, H., Ren, X., Li, Z., and Dickerson, R.: India is overtaking China as the world’s largest emitter of anthropogenic sulfur dioxide, Scientific Reports, https://www.nature.com/articles/s41598-017-14639-8
