Sulfur dioxide enters the atmosphere from two broad categories: natural events like volcanic eruptions and human activities like burning fossil fuels. Human sources dominate, with coal combustion alone responsible for nearly 50% of all human-caused sulfur dioxide worldwide. But volcanoes, industrial smelting, oil refining, and even indoor cooking all play a role.
Volcanoes and Other Natural Sources
Volcanoes are by far the largest natural source. They collectively release 20 to 25 million tons of sulfur dioxide into the atmosphere each year, according to satellite measurements cataloged by NASA. This happens both during dramatic eruptions and through the quieter, continuous exhaling that many volcanoes do year-round, venting water vapor mixed with sulfur dioxide, carbon dioxide, hydrogen sulfide, and heavy metals. A single large eruption can temporarily rival months of industrial output from an entire country.
Beyond volcanoes, smaller natural contributions come from geothermal vents, hot springs, and the decay of organic matter in soils and wetlands. Ocean microorganisms also release sulfur compounds that convert to sulfur dioxide in the atmosphere. These sources are relatively minor compared to volcanic output, but they form part of the planet’s natural sulfur cycle.
Coal and Oil: The Biggest Human Sources
Burning sulfur-containing fuels is the dominant human source of sulfur dioxide, and the numbers make the hierarchy clear. In 2022, coal combustion accounted for nearly 50% of all human-caused sulfur dioxide emissions globally, while fuels derived from crude oil contributed another 25%. Natural gas, by contrast, produces essentially zero sulfur dioxide when burned.
Among coal types, bituminous coal is the biggest contributor at 37.8% of total emissions, with lignite coal adding another 9.2%. On the oil side, heavy fuel oil accounts for 19.3%, diesel oil for 5.6%, and lighter oils for a smaller share. The electricity generation sector expanded rapidly in the second half of the 20th century and now accounts for roughly half of all sulfur dioxide emissions on its own, largely because so many power plants worldwide still burn coal.
Metal Smelting and Industrial Processes
Extracting metals from their ores is another significant source. Copper smelting is a prime example: copper ore is made of copper sulfide, so heating it to separate the metal releases large quantities of sulfur dioxide as a byproduct. The same chemistry applies to lead and zinc smelting, where sulfide-based ores are roasted at high temperatures. Satellite imagery from NASA has tracked sulfur dioxide plumes drifting from major smelting facilities around the world, sometimes visible from space.
Other industrial processes contribute as well. Petroleum refineries release sulfur dioxide from several points in their operations, particularly from fluidized catalytic cracking units (which break heavy oil into lighter products), sulfur recovery units, and the flares and heaters used throughout the refining process. Natural gas processing plants also emit sulfur dioxide when they “sweeten” raw gas by removing hydrogen sulfide, a toxic sulfur compound found in unprocessed natural gas.
Shipping and Transportation
Ocean-going ships were once one of the dirtiest sources of sulfur dioxide on the planet. Marine vessels traditionally burned heavy fuel oil with a sulfur content as high as 3.5%, producing enormous plumes over shipping lanes. In 2020, the International Maritime Organization slashed the allowable sulfur content in ship fuel to 0.50% (and just 0.10% in designated emission control areas near coastlines). That rule was forecast to cut overall sulfur oxide emissions from ships by 77%, a reduction equivalent to 8.5 million metric tons per year.
On land, diesel vehicles and equipment contribute a smaller but meaningful share. Diesel fuel contains sulfur that converts to sulfur dioxide during combustion, though regulations in most developed countries now require ultra-low-sulfur diesel, which has dramatically reduced this source over the past two decades.
Indoor Sources From Cooking and Heating
Sulfur dioxide is not just an outdoor pollutant. Burning solid fuels indoors for cooking or heating can create surprisingly high concentrations. Research in China’s Yunnan Province found that households burning smokeless coal had indoor sulfur dioxide levels reaching a median of 907 micrograms per cubic meter, with some kitchens exceeding that. For context, the World Health Organization’s 24-hour guideline for outdoor sulfur dioxide is 40 micrograms per cubic meter.
The type of stove matters enormously. Ventilated stoves that channel exhaust outside produced detectable sulfur dioxide in only about 31% of measurements among smoky coal users. Open firepits, on the other hand, produced detectable levels 73% of the time, with a median concentration of 234 micrograms per cubic meter. Kerosene heaters and coal stoves without chimneys pose similar risks. This is primarily a concern in parts of the world where solid fuel burning for household energy remains common.
How Global Emissions Have Shifted
Global sulfur dioxide emissions have changed dramatically over the past several decades, though not in a simple downward line. Emissions in North America and Europe have dropped steeply since the 1970s and 1980s, driven by clean air regulations, scrubbers on power plant smokestacks, and a shift toward lower-sulfur fuels. The United States, for instance, has cut its sulfur dioxide output by more than 90% from peak levels.
But emissions in parts of Asia, the Middle East, and Africa rose during the same period as industrialization accelerated. China’s emissions surged through the early 2000s before declining as the country installed pollution controls on its coal plants. India and several other rapidly developing economies have seen continued growth. The net result is that the geographic center of sulfur dioxide pollution has shifted eastward, even as the global total has declined from its late-20th-century peak. Tracking data maintained through the Community Emissions Data System covers the full industrial era from 1750 to 2022, with the next update expected in 2026.