Sulfur Dioxide (SO2)
SO2 is a pungent gas produced by burning sulfur-containing fuels. It causes respiratory irritation and forms acid rain by reacting with atmospheric moisture.
Reviewed by Hayden Williams. Last reviewed 2026-05-01. Unit: ppb.
SO2 has a sharp matchstick smell detectable at low ppb. It is a strong respiratory irritant, can trigger asthma attacks within minutes of exposure, and is a major precursor for secondary PM2.5 and acid deposition. SO2 emissions have dropped dramatically in most developed countries thanks to coal phase-outs and low-sulfur fuel rules, but remain elevated near oil refineries, copper/nickel smelters, and active volcanoes.
Health effects
Triggers asthma attacks rapidly. Long-term exposure linked to bronchitis, reduced lung function, and increased respiratory hospital admissions. Acute high-concentration exposure (industrial accidents, fumigation events) can be fatal.
Asthmatic individuals show airway constriction at SO2 concentrations as low as 200 ppb during exercise. Long-term residential exposure near smelters and refineries is associated with increased rates of bronchitis and asthma. SO2 is also the main precursor of acid rain, which damages forests and freshwater ecosystems.
Vulnerable groups
Asthmatics (rapid acute response), people living near refineries or smelters, COPD patients, communities near active volcanoes.
Common sources
- Coal-fired power plants
- Oil refineries
- Copper, nickel, lead smelters
- Heavy fuel oil ships (international shipping)
- Active volcanoes (e.g. Hawaii, Iceland)
- Industrial heating with high-sulfur fuels
Regional context
SO2 has crashed in EU/US/Japan due to flue-gas desulfurization and coal retirements. Still elevated in industrial India (smelter belts), parts of China, the Persian Gulf (refining), Russian/Norilsk metallurgical centers, and downwind of active volcanoes. Marine SO2 hotspots near major shipping lanes have improved since IMO 2020 fuel sulfur cap.
Regulatory thresholds
How SO2 is measured
UV fluorescence is the reference method. The technique excites SO2 molecules with UV light and measures emitted fluorescence. Passive diffusion tubes for monthly averages are common in network monitoring.
How to reduce your exposure
- Avoid living downwind of active smelters or refineries if possible
- HEPA purifiers do not remove SO2 - activated carbon filters can
- Outdoor exercise unaffected in most regions, but check during local industrial events
- Support coal phase-outs and low-sulfur fuel regulations
Trends
Global SO2 down ~70% in EU and US since 1990. Down ~50% in China since 2007. Still problematic in industrial pockets globally. Marine SO2 dropped sharply after the IMO 2020 0.5% sulfur cap on bunker fuel.
Cities where SO2 matters most
Live readings for cities historically affected by elevated SO2:
Frequently asked
What does SO2 smell like?
Sharp, like a struck match. The human nose detects it in the low-ppb range, well below acutely harmful levels. A sudden sulfurous smell often signals a refinery or industrial release event.
Has SO2 declined?
Massively in most developed economies. Coal phase-outs and low-sulfur fuel rules dropped global SO2 emissions by half since 1990 in the EU and US. Still elevated in industrial pockets and near active volcanoes.
Does SO2 cause acid rain?
Yes. SO2 oxidizes in the atmosphere to sulfate aerosol and sulfuric acid, which deposits as acid rain. Acid rain damaged forests across Europe, North America, and East Asia until SO2 controls cut the source.
Sources + further reading
Track SO2 live across 217 cities on atmos.today. See methodology for how readings are sourced and calculated.