RESEARCH STARTER

Sulfur oxides

Sulfur oxides are chemical compounds comprised of sulfur and oxygen, primarily sulfur dioxide (SO₂) and sulfur trioxide (SO₃). These gases are significant air pollutants that can adversely affect human and animal health, largely due to their role in the formation of acid rain. Sulfur dioxide is the predominant contributor to acid rain, which can cause considerable environmental damage, including harm to forests, freshwater resources, and buildings. While both natural sources, such as volcanic eruptions, and human activities contribute to sulfur oxide emissions, industrialized nations are responsible for a significant proportion, with human activities accounting for about 95% of sulfur oxides released in these regions.

Sulfur dioxide is a colorless, toxic gas known for its strong odor and is linked to respiratory problems, particularly in vulnerable populations like children, the elderly, and those with pre-existing health conditions. Short-term exposure can lead to exacerbated asthma symptoms and increased hospital visits for respiratory illnesses. Over recent decades, significant reductions in sulfur dioxide emissions have been achieved in the United States, reflecting ongoing efforts to improve air quality. Understanding sulfur oxides is crucial for recognizing their environmental and health impacts, particularly in industries reliant on fossil fuels.

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DEFINITION: Chemical compounds containing sulfur and oxygen

Sulfur oxides emitted into the Earth’s atmosphere pollute the air and have negative effects on human and animal health. Sulfur dioxide is the major contributor to acid rain, which causes several types of environmental damage.

Sulfur oxides occur both naturally and as a result of human activities. On a global basis, natural sources, such as volcanoes, contribute about the same amount of sulfur oxides to the atmosphere as human activities. In industrialized nations, such as those of Europe and the Americas, however, human activities contribute 95 percent of the sulfur oxides emitted. The two most important sulfur oxides are sulfur dioxide (SO2) and sulfur trioxide (SO3). SO2 is a colorless, dense, toxic, nonflammable gas with an intense odor; SO3 is a liquid.

In 2005, the US Environmental Protection Agency (EPA) estimated that anthropogenic (human-caused) emissions of SO2 amounted to more than 14 million tons. By 2023, that number had decreased to 1.7 million tons. Of these, most were the result of fossil fuel combustion at power plants and other industrial facilities. Other sources included electricity generation, industrial processes, and smaller amounts from non-road equipment, on-road vehicles, fires, waste disposal, residential wood combustion, and solvent use.

SO2 is an air pollutant and a lung irritant. Scientific evidence links short-term exposures to SO2 (5 minutes to 24 hours) with adverse respiratory effects, including bronchoconstriction (tightening of the bronchi in the lungs) and increased asthma symptoms. Studies also show a connection between short-term exposure to SO2 and increased visits to hospital emergency rooms and hospital admissions for respiratory illnesses, particularly in at-risk populations: children (whose lungs are still developing and have elevated breathing rates while playing), asthmatics, and the elderly (who may also have compromised lung function). Sulfur oxides can react with other compounds in the atmosphere to form small particles that can penetrate deep into the lungs and cause or worsen respiratory diseases, such as emphysema and bronchitis, and aggravate existing heart disease.

Acid rain, or acid snow, sleet, or fog, is a direct result of the method that the atmosphere uses to clean itself. Tiny droplets of water in the atmosphere continuously capture suspended particles and gases; SO2 and oxides of nitrogen are then chemically converted to sulfuric and nitric acids. SO2 is the major gas that produces acid rain. Both SO2 and SO3 contribute to the formation of acid rain, but SO3 is found in the atmosphere at much lower concentrations than SO2.

Industrial acid rain is a substantial problem in Europe, China, Russia, and areas downwind from them because sulfur-containing coal is burned to generate heat and electricity in these parts of the world. The use of tall smokestacks to reduce local pollution contributes to the formation of acid rain considerable distances downwind of the original emissions by releasing acid-forming gases high into the atmosphere. Acid rain has adverse impacts on forests, freshwater resources, soils, and the human-built environment, killing insects and aquatic life and causing damage to buildings. It also affects human health in ways similar to gaseous SO2.

The EPA’s air-quality standard for SO2 is designed to protect against exposure to all sulfur oxides. SO2 is of greatest concern and is used as the indicator for the larger group of sulfur oxides because emissions that lead to high concentrations of SO2 generally lead to the formation of other sulfur oxides. Annual average ambient SO2 concentrations across the United States have decreased by more than 94 percent since 1980, and by the twenty-first century, all areas of the United States had met the EPA’s standard for SO2.


Bibliography

Christiani, David C., and Mark A. Woodin. “Urban and Transboundary Air Pollution.” In Life Support: The Environment and Human Health, edited by Michael McCally. MIT Press, 2002.

McKinney, Michael L., et al. “Air Pollution: Local and Regional.” In Environmental Science: Systems and Solutions. 6th ed., Jones and Bartlett, 2017.

Tiseo, Ian. "Annual Sulfur Dioxide (SO₂) Emissions in the United States from 1970 to 2023." Statista, 14 Mar. 2024, www.statista.com/statistics/501303/volume-of-sulfur-dioxide-emissions-us/. Accessed 28 Aug. 2025.

Vallero, Daniel. Fundamentals of Air Pollution. 5th ed., Academic Press, 2014.

Full Article

DEFINITION: Chemical compounds containing sulfur and oxygen

Sulfur oxides emitted into the Earth’s atmosphere pollute the air and have negative effects on human and animal health. Sulfur dioxide is the major contributor to acid rain, which causes several types of environmental damage.

Sulfur oxides occur both naturally and as a result of human activities. On a global basis, natural sources, such as volcanoes, contribute about the same amount of sulfur oxides to the atmosphere as human activities. In industrialized nations, such as those of Europe and the Americas, however, human activities contribute 95 percent of the sulfur oxides emitted. The two most important sulfur oxides are sulfur dioxide (SO2) and sulfur trioxide (SO3). SO2 is a colorless, dense, toxic, nonflammable gas with an intense odor; SO3 is a liquid.

In 2005, the US Environmental Protection Agency (EPA) estimated that anthropogenic (human-caused) emissions of SO2 amounted to more than 14 million tons. By 2023, that number had decreased to 1.7 million tons. Of these, most were the result of fossil fuel combustion at power plants and other industrial facilities. Other sources included electricity generation, industrial processes, and smaller amounts from non-road equipment, on-road vehicles, fires, waste disposal, residential wood combustion, and solvent use.

SO2 is an air pollutant and a lung irritant. Scientific evidence links short-term exposures to SO2 (5 minutes to 24 hours) with adverse respiratory effects, including bronchoconstriction (tightening of the bronchi in the lungs) and increased asthma symptoms. Studies also show a connection between short-term exposure to SO2 and increased visits to hospital emergency rooms and hospital admissions for respiratory illnesses, particularly in at-risk populations: children (whose lungs are still developing and have elevated breathing rates while playing), asthmatics, and the elderly (who may also have compromised lung function). Sulfur oxides can react with other compounds in the atmosphere to form small particles that can penetrate deep into the lungs and cause or worsen respiratory diseases, such as emphysema and bronchitis, and aggravate existing heart disease.

Acid rain, or acid snow, sleet, or fog, is a direct result of the method that the atmosphere uses to clean itself. Tiny droplets of water in the atmosphere continuously capture suspended particles and gases; SO2 and oxides of nitrogen are then chemically converted to sulfuric and nitric acids. SO2 is the major gas that produces acid rain. Both SO2 and SO3 contribute to the formation of acid rain, but SO3 is found in the atmosphere at much lower concentrations than SO2.

Industrial acid rain is a substantial problem in Europe, China, Russia, and areas downwind from them because sulfur-containing coal is burned to generate heat and electricity in these parts of the world. The use of tall smokestacks to reduce local pollution contributes to the formation of acid rain considerable distances downwind of the original emissions by releasing acid-forming gases high into the atmosphere. Acid rain has adverse impacts on forests, freshwater resources, soils, and the human-built environment, killing insects and aquatic life and causing damage to buildings. It also affects human health in ways similar to gaseous SO2.

The EPA’s air-quality standard for SO2 is designed to protect against exposure to all sulfur oxides. SO2 is of greatest concern and is used as the indicator for the larger group of sulfur oxides because emissions that lead to high concentrations of SO2 generally lead to the formation of other sulfur oxides. Annual average ambient SO2 concentrations across the United States have decreased by more than 94 percent since 1980, and by the twenty-first century, all areas of the United States had met the EPA’s standard for SO2.


Bibliography

Christiani, David C., and Mark A. Woodin. “Urban and Transboundary Air Pollution.” In Life Support: The Environment and Human Health, edited by Michael McCally. MIT Press, 2002.

McKinney, Michael L., et al. “Air Pollution: Local and Regional.” In Environmental Science: Systems and Solutions. 6th ed., Jones and Bartlett, 2017.

Tiseo, Ian. "Annual Sulfur Dioxide (SO₂) Emissions in the United States from 1970 to 2023." Statista, 14 Mar. 2024, www.statista.com/statistics/501303/volume-of-sulfur-dioxide-emissions-us/. Accessed 28 Aug. 2025.

Vallero, Daniel. Fundamentals of Air Pollution. 5th ed., Academic Press, 2014.

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