What is sulfur dioxide?
Sulfur dioxide (SO2) is a gas composed of one sulfur atom and two oxygen atoms, and one of the family of sulfur-oxygen gases known as sulfur oxides (SOx). It dissolves into water very easily, turning it acidic. SO2 is colorless and known for having a pungent, irritating odor described to be like that of a just-lit match; the lingering smell of fireworks; or occasionally, rotten eggs.
Where does sulfur dioxide come from?
Sulfur oxides are emitted when sulfur-containing fuel is burned. The largest sources of sulfur in the atmosphere are the burning of fossil fuels by power plants and copper smelting. Some other examples are locomotive engines, ships, and off-road diesel equipment operated with fuels containing sulfur; natural gas and petroleum extraction from the ground; and oil refining processes.
Erupting volcanoes can be a significant natural source of sulfur dioxide emissions.
How is sulfur dioxide measured?
Chemical concentrations in the air are measured by units of the substance (milligrams, micrograms, nanograms, picograms) per volume of air as cubic meters or cubic feet. Sulfur dioxide is typically expressed as parts per billion (ppb).
Sulfur dioxide (SO2) sensors work using a fluorescent reaction analyzer. An air sample is pulled in with a vacuum pump, first through a filter to remove particles, and then through a scrubber that removes gases that would interfere with the SO2 reading. The scrubbed sample goes into a reaction chamber where it is irradiated with an ultraviolet light at 214nm. Sulfur dioxide absorbs UV radiation at 200-240nm. After absorption, the molecule starts to emit fluorescent (light-producing) photons of about 350nm. That fluorescence is measured using a photomultiplier and the signal is converted to a usable number representing the concentration of sulfur dioxide in the sample.
What are the standards for sulfur dioxide in the United States?
National Ambient Air Quality Standards (NAAQS) for pollution specify a maximum amount of a pollutant to be present in outdoor air.
EPA’s national ambient air quality standards (NAAQS) for SO2 are designed to protect against exposure to the entire group of sulfur oxides (SOx). SO2 is the component of greatest concern and is used as an indicator for the larger group of sulfur oxides. Control measures that reduce sulfur dioxide may have the important co-benefit of reducing the formation of particulate sulfur pollutants, like fine sulfate particles.
Currently, the EPA has 1 hour and 3 hour secondary standards in place for sulfur dioxide.
| Pollutant | Standard Type | Mass per Volume | Terms | Date Established |
| SO2 | 1 hour | 75 ppb | Not to be exceeded in an hour more than 1% of the time, as averaged over a three year period | 22 June 2010 |
| SO2 | 3 hour | 50 ppb | Not to be exceeded more than once a year, as averaged over a 3 hour period | 30 April 1971 |
How does sulfur dioxide affect people?
Short term exposure to sulfur dioxide can irritate the eyes and harm the respiratory system. It causes coughing, mucus secretion, and aggravates conditions such as asthma and chronic bronchitis.
Long term exposure to persistent levels of sulfur dioxide can have a more lasting effect on health. Lung function changes were seen in workers exposed to low levels over a span of 20+ years. However, these workers were also exposed to other chemicals, so health effects may not have resulted from SO2 alone.
Exposure to high levels of sulfur dioxide can be life threatening. Levels above 1 parts per million (ppm) results in increased incidence of respiratory symptoms and disease, decreased lung function, and increased risk of death. Exposure to extreme levels of sulfur dioxide at 100 ppm is considered immediately dangerous to life and health. Burning of the nose and throat, breathing difficulties, and severe airway obstructions occurred in miners who breathed sulfur dioxide released as a result of an explosion in a copper mine.
Who is most at risk for adverse health effects from sulfur dioxide?
Elderly people and people with cardiovascular or respiratory issues are at an elevated risk of the effects of sulfur dioxide. Controlled human exposure and epidemiological studies show that people with asthma, especially children, are more likely to experience adverse responses with SO2 exposure, compared with the non-asthmatic population. Effects at levels near the 1 hour standard (75 ppb) are those of asthma exacerbation, including bronchial constriction with respiratory irritation symptoms like wheezing, shortness of breath, and a tight feeling in the chest.
How does sulfur dioxide affect the environment?
When SO2 combines with water, it creates sulfuric acid, which is the main component of acid rain. Acid rain accelerates erosion and causes corrosion on paints and building materials.
Sulfur oxides and acid rain contribute to the acidification of surface water, and soil. This acidification causes a variety of issues that harm vulnerable aquatic and terrestrial systems, including slowed growth and injury to forests and the localized extinction of fish and other species. SOx also promotes chemical reactions that facilitate the accumulation of mercury in water and soil. This can lead to elevated levels of mercury in food, which in turn increases risk of adverse health effects in humans due to mercury ingestion.
Animal studies also show respiratory effects from breathing in sulfur dioxide. Animals exposed to high concentrations of SO2 showed decreased respiration, inflammation of the airways, and destruction of areas of the lungs.