What is sulfur dioxide?

Sulfur dioxide is a colorless gas that smells strong, suffocating, and pungent. It is the most common form of sulfur oxide.¹
Sulfur dioxide belongs to a group of highly reactive gases called sulfur oxides. Sulfur oxides react easily to form several harmful compounds, such as:

• sulfuric acid
• sulfurous acid
• sulfate particles

Where does sulfur dioxide come from?

Sulfur dioxide is generated by any industrial activity that uses materials containing sulfur to generate electricity, though it can also be produced by vehicles through fuel combustion.
Power plants are the largest source of sulfur dioxide in the United States, India, China, and Europe.^2,3 Smelting is the largest source in Canada.⁴ A smelter complex in Russia and a coal mining province in South Africa are the largest emitters of sulfur dioxide in the world.⁵

Additional sources of sulfur dioxide include:⁶

• coal combustion
• oil combustion
• industrial boilers
• diesel engines
• volcanoes
• oceans

Nearly all sulfur dioxide emissions in the world are produced by human activity. Less than 2 percent of emissions come from natural sources.
Despite the low global emissions of natural sources, satellite tracking of sulfur dioxide emissions found they can be significant. According to a 2017 study published in Scientific Reports, volcanoes emit around 63 kilotons of sulfur dioxide per day.⁷

How does sulfur dioxide affect our health?

Short-term sulfur dioxide exposure can have many negative health impacts, including:⁸

• nasal mucus
• choking
• irritation to the ears, eyes and throat
• wheezing
• chest tightness
• shortness of breath

Long-term exposure to sulfur dioxide can lead to more serious problems, such as:

• respiratory illness
• alterations in the lungs’ defenses
• increased cardiovascular disease

Breathing complications are more common in children, older adults, asthmatic people, or people who are active outdoors.⁹ People who live downwind from volcanoes may also be exposed to dangerous levels of sulfur dioxide.

A 2010 study published in Environmental Research found that sulfur dioxide was linked to daily mortality in four Asian cities.¹⁰

What is sulfur dioxide used for?

Sulfur dioxide has numerous industrial and commercial uses, such as:¹¹

• food additives
• lubricant
• adhesive
• disinfectant
• refrigerant
• bleach

A 2012 study published in Gastroenterology and Hepatology From Bed to Bench noted that between 3 and 10 percent of adult asthmatics may experience dermatological, gastrointestinal, and respiratory symptoms when exposed to sulfite additives.¹²

In addition, a 2017 study published in PLOS One demonstrated that sulfites in preservatives could inhibit beneficial gut bacteria.¹³

What are the environmental impacts of sulfur dioxide?

By itself, sulfur dioxide can damage the environment. Adverse effects of sulfur dioxide emissions include:¹⁴

• respiratory problems for both humans and livestock
• damage to vegetation
• damage to buildings and materials

Sulfur dioxide is a precursor to acid rain, which can cause acidification of lakes and soils as well as accelerate the deterioration of buildings.

When sulfate particles are combined with other compounds like ammonia, they can become particulate matter, or PM2.5. PM2.5 impacts the environment in many of the same ways that sulfur dioxide does.

Is sulfur dioxide a greenhouse gas?

Sulfur dioxides are considered indirect greenhouse gases, along with nitrogen oxides, carbon monoxides, and non-methane volatile organic compounds (VOCs).¹⁵ An indirect greenhouse gas has an effect on atmospheric warming through either chemical reaction or changing the Earth’s capability to balance radiative energy.¹⁶

Sulfur dioxide is a contributor to aerosols, which can absorb solar radiation and warm the atmosphere or cool the atmosphere by creating cloud droplets that reflect sunlight.

What is being done to reduce sulfur dioxide emissions?

Power plants are the biggest global contributor to sulfur dioxide emissions. Power plant sulfur dioxide control strategies have reduced emissions by about 80 percent between 2005 to 2015 in the eastern United States.^17,18

The Ozone Monitoring Instrument (OMI) onboard the National Aeronautics and Space Administration’s (NASA) Aura satellite observed global changes in sulfur dioxide and nitrogen dioxide levels starting in October 2004. At that time, Pittsburgh, Pennsylvania and the Ohio Valley emitted the most sulfur dioxide in the U.S. from coal-fired plants. The region demonstrated dramatic declines in emissions over the ten-year period.

Investigators suggest that the decline followed the use of flue gas de-sulfurization, fuel switches from coal to natural gas, and some older coal power plants shutting down. Market-based cap and trade programs as well as state and federal regulations have also encouraged emission control technologies.

Data from the satellite, checked against surface monitoring stations, found that much of Canada’s sulfur dioxide emissions had also significantly decreased between 2005 to 2014.¹⁹ The Canadian government credits regulations and international agreements, such as the Canada-United States Air Quality Agreement, with improving air quality. The exception to the downward trend in sulfur dioxide emissions is Canada’s oil sands in Alberta, which has remained a constant emissions hotspot visible from space.²⁰

Reductions in urban emissions will not necessarily remove all sulfur dioxide pollutants from the air.²¹ A 2015 article published in the Proceedings of the National Academy of Sciences of the United States of America suggested that sulfur dioxide will continue to interact with rurally-generated methane to form particulate pollution, though at substantially lower rates than fossil fuel production.

Government regulations have improved air quality in some places, but there remains work to be done. Many cities around the globe bear a financial and humanitarian burden from severe air pollution.

Learn more through our Cost of Air Pollution counter to see how clean air saves lives and preserves economic prosperity.