Federal Standards - Sulphur Dioxide

Sulfur dioxide pollution
Sulfur dioxide (SO2) is a harmful gas emitted by power plants, refineries, and diesel engines. The U.S. Environmental Protection Agency (EPA) has proposed a new limit on sulfur dioxide in the air—the first such change in 38 years.

EPA is proposing a new national air quality standard that will target the one-hour spikes in sulfur dioxide emissions. EPA is also proposing to remove the existing standards that target daily and annual average levels of SO2. These standards, called the National Ambient Air Quality Standards, define the amount of an outdoor air pollutant that is safe to breathe. These standards set enforceable goals for air pollution cleanup programs.

The American Lung Association supports an hourly standard of 50 ppb, 99th percentile. The American Lung Association has long advocated for adding a 5-minute standard. In fact, the Lung Association won a court decision in 1998 to require EPA to review the need for a 5-minute standard. However, 1-hour standard would be a major step forward toward protecting the health of the public. A stringent short-term standard is needed to protect the health of people with asthma and others. The American Lung Association opposes revocation of the current daily and annual average standards. The American Lung Association supports the expansion of the proposed monitoring network to assess air quality near 2,000 high emissions sources that emit more than 100 tons per year.

What is sulfur dioxide?
Sulfur dioxide (SO2) is a gaseous air pollutant composed of sulfur and oxygen. SO2 forms when sulfur-containing fuel such as coal, oil, or diesel is burned. Sulfur dioxide also converts in the atmosphere to sulfates, a prime component of fine particle pollution in the eastern U.S.

Where does sulfur dioxide come from?
Each year manmade sources in the U.S. emit 15 million tons of sulfur dioxide. The largest sources of sulfur dioxide emissions are electricity generation, industrial boilers, and other industrial processes such as petroleum refining and metal processing. Diesel engines are another major source, including old buses and trucks, locomotives, ships, and off-road diesel equipment. The plume from a coal-fired power plant touches down at ground level (downwash) during high wind conditions or is trapped during atmospheric inversions, leaving high concentrations of SO2 in the community near the plant.

High concentrations may also occur during start-up, shutdown, upsets, and malfunctions of pollution control equipment. 
Ports, smelters, and other concentrated sources of sulfur dioxide also cause high concentrations of emissions nearby. 
SO2 control programs that are based on trading schemes (such as the acid rain control program) do not protect against localized hotspots in the vicinity of power plants or other major sources. 

What are the health effects from sulfur dioxide pollution?
Sulfur dioxide causes a range of harmful effects on the lungs, as the EPA’s most recent review of the science concluded:

  • Wheezing, shortness of breath and chest tightness and other problems, especially during exercise or physical activity. 
  • Continued exposure at high levels increases respiratory symptoms and reduces the ability of the lungs to function. 
  • Short exposures to peak levels of SO2 in the air can make it difficult for people with asthma to breathe when they are active outdoors.
    • Rapid breathing during exercise helps SO2 reach the lower respiratory tract, as does breathing through the mouth. 
  • Increased risk of hospital admissions or emergency room visits, especially among children, older adults and people with asthma.

Those at greatest risk include:

  • Children
  • Adults aged 65 and older
  • People with heart or lung disease
  • People with asthma who are active outdoors

Millions of people are unprotected by the current standards. Kids with asthma should be able to play outside whenever and wherever they can, knowing that the air quality standards are set to protect them. Adults with asthma should be free to work outdoors or participate in outdoor recreation without fear that air pollution concentrations will trigger asthma exacerbations.

A stringent short-term SO2 standard would trigger reductions in millions of tons per year in sulfur dioxide emissions, lowering health risks for millions of people. Reductions in sulfur dioxide can also save lives. The gas sulfur dioxide changes into deadly fine particles in the atmosphere, a form of pollution that shortens life. EPA estimates reducing SO2emissions by a million tons per year could save between 4,700 to 12,000 premature deaths each year, by 2020.

EPA summarized its findings on SO2 in the Integrated Science Assessment. That assessment concludes:

“Collectively, the human clinical, epidemiologic, and animal toxicological data are sufficient to conclude that there is a causal relationship between respiratory morbidity and short-term exposure to SO2.” (emphasis added) 

EPA found that the studies provided “clear and convincing evidence” of harm leading to “respiratory symptoms, lung function, airway inflammation, AHR [airway hyperreactivity], and ED [emergency department] visits and hospitalizations.”

  • Clinical studies provide clear evidence for harm to people with asthma who breathed high levels of SO2 while they were exercising. These studies found that these individuals suffered a decline in lung function and an increase in respiratory symptoms, even after only a 15-minute exposure of 200 ppb and greater. People with asthma suffered increased airway resistance after several minutes of breathing SO2 at concentrations of 100 ppb under conditions of exercise, when exposed to SO2 via a facemask, which approximates mouth-breathing, such as occurs during exercise or with respiratory infections.
  • Epidemiological studies provide convincing evidence of increased respiratory symptoms in children at far lower concentrations than the clinical chamber studies. The community health studies find effects at current ambient concentrations, well below the level of the current 24-hour current NAAQS of 140 parts per billion (ppb).
    • A large multi-city study linked previous day SO2concentrations with morning respiratory symptoms in 8 urban areas where median 3-hour average SO2 levels ranged from 17 ppb to 37 ppb.
    • Inner city children with asthma suffer from declines in lung function following exposure to higher daily concentrations of sulfur dioxide.
    • Present day concentrations of SO2 are also implicated in increased emergency department visits and hospitalizations for respiratory causes among children and older adults.
    • A study in Bronx, New York found that asthma hospitalizations in children climbed as hourly sulfur dioxide concentrations increased. Hospitalizations began to rise at hourly concentrations greater than 9 ppb, with a sharp increase an concentrations greater than 40 ppb. 
  • Reducing SO2 levels are linked to an immediate gain in life expectancy, according to evidence from a study that examined the health impact of drops in sulfur dioxide.

These include millions of people. For example, there are an estimated 23.3 million people, including 7.0 million children with asthma in the U.S. 

Contact: cleanair@lungcolorado.org