Sulfur Dioxide · Canairy

What is sulfur dioxide?

Sulfur dioxide (SO2) is a colorless gas with a sharp, choking odor, like the smell of a freshly struck match. It belongs to a family of gases called sulfur oxides (SOx). In the atmosphere, SO2 reacts with water vapor and other compounds to form sulfuric acid and sulfate particles, both of which contribute to acid rain and to PM2.5.

What does sulfur dioxide smell like?

SO2 has a sharp, pungent smell that most people compare to a just-struck match or burnt fireworks. At higher concentrations it stops registering as a smell and becomes an irritant: stinging eyes, a burning nose and throat, and coughing.

Most people first notice the odor somewhere around 0.3 to 1 ppm (300 to 1,000 ppb), though the threshold varies widely from person to person. That is well above the EPA's 1-hour health standard of 75 ppb, and above the roughly 200 ppb where people with asthma can start to react. In other words, SO2 can reach unhealthy levels long before you smell anything, so smell is not a reliable warning. The nose also tires quickly with continued exposure, so a fading smell does not mean the gas is gone. Monitoring data is the dependable signal, not your nose.

Sources of SO2 emissions

About two-thirds of US SO2 emissions come from burning fossil fuels at power plants, primarily coal-fired and oil-fired units. Another quarter comes from industrial facilities: petroleum refineries, cement kilns, pulp and paper mills, and metal smelters that process sulfur-containing ores like copper, zinc, and lead.

Smaller sources include locomotives, large ocean-going ships, and heavy off-road equipment that burn high-sulfur fuel. US SO2 emissions have dropped roughly 90% since 1990, mostly from coal plant retirements and flue-gas scrubbers, but localized hotspots remain near specific industrial sites.

Natural sources of SO2

Volcanoes are the largest natural source of SO2. An actively degassing volcano can release thousands of tons a day, and it does not need to be erupting: communities downwind of Hawaii's Kilauea know the result as "vog," a haze of SO2 and sulfate particles that can push local levels well above health standards. Geothermal features like fumaroles and hot springs release smaller, steady amounts.

Wildfires also emit some SO2, though far less than they emit fine particles — in smoke, PM2.5 is the main health concern, not SO2. Away from volcanic and geothermal areas, natural sources contribute little to the SO2 people actually breathe; the levels that show up on monitors near cities and industry come overwhelmingly from burning fossil fuels.

Health effects of sulfur dioxide

SO2 hits the airways fast. Even brief exposures of 5 to 10 minutes can narrow the airways (bronchoconstriction) and trigger wheezing, chest tightness, and shortness of breath, especially during exercise. People with asthma can react at concentrations as low as 200 ppb.

Long-term exposure contributes to chronic bronchitis and worsens existing heart and lung disease. SO2 also converts to sulfate particles that lodge deep in the lungs, contributing to the broader health burden of PM2.5. Children, older adults, and people with asthma face the highest risk.

How it's measured and typical levels

SO2 is measured in parts per billion (ppb) for ambient air, or parts per million (ppm) at higher industrial concentrations. One ppm equals 1,000 ppb. It is also one of the five major pollutants that make up the US Air Quality Index, so a high SO2 reading feeds directly into the AQI number your monitoring app reports.

Key benchmarks:

WHO 24-hour guideline: 40 µg/m³ (about 15 ppb)
US EPA NAAQS, 1-hour (primary): 75 ppb
US EPA NAAQS, 3-hour (secondary): 0.5 ppm (500 ppb)
NIOSH workplace ceiling: 5 ppm

Background levels in most US cities now run below 5 ppb. Areas near active coal plants, refineries, or smelters can see 1-hour peaks above 75 ppb. Communities downwind of Hawaii's Kilauea volcano have measured hourly levels above 1,000 ppb during eruptions.

What you can do

If you live within a few miles of an industrial SO2 source, check your state environmental agency's monitoring data and AirNow for daily reports. On days when SO2 is forecast above 75 ppb, people with asthma should limit outdoor activity and keep rescue inhalers handy.

Indoors, SO2 levels are usually lower than outside. Closing windows and running a recirculating HVAC system on high-SO2 days reduces exposure further. HEPA filters do not capture SO2; activated carbon and alkaline-impregnated filters do, though these are uncommon in residential systems.

The biggest reductions come from policy changes, scrubbers on power plants, low-sulfur diesel, low-sulfur marine fuels, not from individual action. Support for those rules matters more for SO2 than for most pollutants.

Sources

This article is for educational purposes only. Canairy does not provide medical advice, diagnosis, or treatment. Talk to a qualified health professional about your specific situation.