|US SO2 emissions by source sector in 2005. Source: EPA. Click to enlarge.|
For the first time in nearly 40 years, EPA is proposing to strengthen the nation’s sulfur dioxide (SO2) primary air quality standard to protect public health. Power plants and other industrial facilities emit SO2 directly into the air. Exposure to SO2 can aggravate asthma, cause respiratory difficulties, and result in emergency room visits and hospitalization. People with asthma, children, and the elderly are especially vulnerable to SO2’s effects.
EPA is taking comment on a proposal to establish a new national one-hour SO2 standard, between 50 and 100 parts per billion (ppb). The existing primary standards were 140 ppb measured over 24-hours, and 30 ppb measured over an entire year. The Agency also is taking comment on alternative levels for the 1-hour standard up to 150 ppb.
The proposal addresses only the SO2 primary standards, which are designed to protect public health. EPA will address the secondary standard—designed to protect the public welfare, including the environment—as part of a separate proposal in 2011.
This new proposed standard is designed to protect against short-term exposures ranging from five minutes to 24 hours. Because the revised standards would be more protective, EPA is proposing to revoke the current 24-hour and annual SO2 health standards.
EPA estimates that the revised standards would yield health benefits valued between $16 billion and $100 billion. Those benefits would include reduced hospital admissions, emergency room visits, work days lost, cases of aggravated asthma and chronic bronchitis, among others.
The proposal also includes a new “form” for the SO2 standard. The form is the air quality statistic that is compared to the level of the standard to determine if an area meets the standard.
For the new 1-hour SO2 standard, EPA is proposing that the form be a 3-year average of the 4th highest daily maximum 1-hour average concentration in a year, or its equivalent (a 3-year average of the 99th percentile of the annual distribution of daily maximum 1-hour average concentrations). The 99th percentile and the 4th highest daily maximum in a year will be the same in a year if all days are monitored; the percentile form would account for days that lack monitoring data.
EPA is also proposing changes to the ambient air monitoring and reporting requirements for SO2. These proposed requirements are expected to result in a minimum of 348 SO2 monitoring sites. EPA estimates that at least one-third of the SO2 monitors already in operation may meet the proposed siting requirements. The proposal also would change the Air Quality Index to reflect the revised SO2 standards. This change would improve states’ ability to alert the public when short-term SO2 levels may affect their health.
Sulfur dioxide is one of a group of highly reactive gasses known as “oxides of sulfur.” The largest sources of SO2 emissions are from fossil fuel combustion at power plants (66%) and other industrial facilities (29%). Smaller sources of SO2 emissions include industrial processes such as extracting metal from ore, and the burning of high–sulfur fuels by locomotives, large ships, and non-road equipment.
EPA first set National Ambient Air Quality Standards (NAAQS) for SO2 in 1971, establishing both a primary standard to protect health and a secondary standard to protect the public welfare. The last review of the SO2 NAAQS was completed in 1996 and the Agency chose not to revise the standards. Annual average SO2 concentrations have decreased by more than 71% since 1980.
In the last review, EPA also considered, but did not set, a five-minute SO2 NAAQS to protect asthmatics at elevated ventilation rates from bronchoconstriction and other respiratory symptoms associated with 5-10 minute peak concentrations of SO2. The decision not to set a five-minute standard in 1996 was challenged successfully by the American Lung Association and remanded back to EPA in 1998; no formal action with regard to the remand has been taken until this proposal.
Under a judicial consent decree, EPA must complete this review of the primary SO2 standard by 2 June 2 2010. The public comment period will be open for 60 days after the proposal is published in the Federal Register. The agency will hold a public hearing on 5 Jan 2010 in Atlanta.