Sulfur oxides are one of the six criteria air pollutants regulated by EPA. The agency is supposed to review its standard for this important air pollutant every five years. The last review was completed in 1996.
In response to lawsuits, EPA was scheduled to release its proposed rule for a new primary health-based standard by July 30, 2009. That has been postponed once again, and the new court-ordered date for release of the proposed rule is Nov. 16, 2009, with a final rule due by June 2, 2010.
An essential step in the review and standard-setting process for sulfur oxides, for which sulfur dioxide (SO2) is considered the key indicator, is the development and publication of a Risk and Exposure Assessment. In this process that has also been drawn out, two drafts have been released. The final REA is due for release by the end of July 2009. Its contents will provide numerous clues about where the agency is headed. It should be available at:
It likely will be informative to check on the response of the agency's Clean Air Scientific Advisory Committee to the REA once it's released. CASAC's previous comments are at:
- CASAC advisory reports  (near bottom for SO2).
The American Lung Association's Janice Nolen, 202-785-3355, <email@example.com >, is speculating that the agency may be considering a standard or two for shorter time periods than are currently addressed, in order to address the short-term peaks that can pose a health risk. That might include something along the lines of a 1-hour, or even 5-minute standard. There may also be a 24-hour standard, which is a time period addressed currently. An annual standard, as now exists, may or may not be in the cards. Any of the shorter time periods would likely need a more extensive monitoring network, in order to track the fluctuations that invariably occur.
The current EPA 24-hour standard of 0.14 ppm is about 18 times higher than the World Health Organization's guideline of 20 ug/m3. EPA's annual standard is 0.030 ppm; WHO has no annual guideline, but its 10-minute guideline is 500 ug/m3.
Under the current US EPA standard, only a few counties are in violation. 
Release of the proposed and final rule for the secondary SO2 standard, which targets protection of the environment rather than health protection that is the focus of the primary standard, is trailing each of these dates by several months. It is still scheduled to be released in conjunction with a secondary standard for nitrogen oxides (NOx, for which NO2 is the key indicator), with a proposed rule on Feb. 12, 2010, and a final rule on Oct. 19, 2010.
For more information on the SO2 standard, see the TipSheet of Sept. 3, 2008. 
For more on the NO2 standards, as well as the science and politics underlying both the NO2 and SO2 standards, see the TipSheet of June 24, 2009.