Sulfur Dioxide (Sulfur Dioxide)

What is Sulfur Dioxide?

Sulfur dioxide (SO2) is a colorless gas with a strong, suffocating odor similar to a just-struck match. It is produced when sulfur-containing fuels like coal and oil are burned, and during various industrial processes. SO2 is both a direct respiratory irritant and a precursor to other harmful pollutants.

In the atmosphere, SO2 can react with water vapor and other compounds to form sulfuric acid, sulfate particles, and other sulfur-containing compounds. These secondary pollutants contribute to acid rain, visibility impairment (haze), and fine particulate matter (PM2.5) pollution.

SO2 levels have decreased significantly in the United States since the 1970s due to Clean Air Act regulations, particularly requirements for sulfur scrubbers on power plants. However, localized high concentrations can still occur near industrial facilities, ports, and areas with heavy diesel traffic.

Major Sources of Sulfur Dioxide

The primary sources of sulfur dioxide in outdoor air include:

• Coal-fired power plants - Historically the largest source, now reduced through emissions controls
• Petroleum refineries and oil processing facilities
• Metal smelters - Extracting metals from sulfide ores releases SO2
• Industrial boilers and furnaces burning high-sulfur fuels
• Ships using high-sulfur marine fuels (bunker fuel)
• Paper and pulp mills - The "rotten egg" smell near mills is sulfur compounds
• Natural sources - Volcanic eruptions can release massive amounts of SO2

Health Effects of Sulfur Dioxide Exposure

According to EPA research, exposure to sulfur dioxide can cause a range of health effects depending on concentration and duration of exposure.

Short-Term Health Effects

• Irritation of nose, throat, and airways causing burning sensation
• Coughing, wheezing, and chest tightness
• Difficulty breathing and shortness of breath
• Bronchoconstriction (narrowing of airways) in people with asthma
• Asthma attacks - SO2 can trigger severe reactions within minutes
• Eye irritation and tearing
• Aggravation of existing respiratory and cardiovascular disease

Long-Term Health Effects

• Chronic respiratory symptoms in populations near industrial sources
• Reduced lung function from repeated exposure
• Development or worsening of asthma
• Increased respiratory infections
• Contribution to cardiovascular disease through PM2.5 formation
• Environmental damage including acid rain, forest damage, and lake acidification
• Damage to buildings, monuments, and infrastructure from acidic deposition

AQI Breakpoints for Sulfur Dioxide

The Air Quality Index (AQI) translates Sulfur Dioxide concentrations into a standardized scale from 0 to 500:

Who is Most at Risk?

Certain groups are more vulnerable to the health effects of sulfur dioxide exposure:

• People with asthma - SO2 can trigger attacks even at low concentrations
• Children with asthma are particularly sensitive to SO2 exposure
• People with other respiratory diseases like COPD or chronic bronchitis
• People with cardiovascular disease
• Communities living near power plants, refineries, or industrial facilities
• Port communities exposed to ship emissions
• Outdoor workers in industrial areas

How to Protect Yourself

When Sulfur Dioxide levels are elevated, take these precautions to reduce your exposure:

• Check air quality alerts, especially if you live near industrial sources
• People with asthma should be aware that SO2 can trigger rapid-onset attacks
• Limit outdoor activities when SO2 levels are elevated in your area
• If you smell sulfur (like a struck match), move to an area with cleaner air
• Keep windows closed when industrial emissions or poor air quality is reported
• Support policies requiring emissions controls on power plants and industrial facilities
• If you have asthma and live near SO2 sources, work with your doctor on an action plan
• Consider air quality when choosing where to live - check proximity to industrial facilities

Check Local Air Quality

Monitor Sulfur Dioxide levels in your area using our latest EPA AirNow readings. Knowing current conditions helps you make informed decisions about outdoor activities and protect your health.