After recently issuing highly controversial updates for two of the main criteria air pollutants — fine particulates and ozone — EPA is in the midst of its review of two others. Nitrogen oxides were discussed in the TipSheet of July 23, 2008.  Trailing closely behind in the process is sulfur dioxide.
The current EPA standard for SO2 for a 24-hour period is more than 18 times higher than the World Health Organization guideline. That elevated standard may be one reason why there are only a handful of counties that are in violation. 
In a little over a week, EPA is scheduled to release its Integrated Science Assessment for SO2, which will update the agency's science on the pollutant's health effects. The schedule for that release and the rest of the process, as determined by the courts after EPA fell far behind in meeting the timing requirements of the Clean Air Act, is:
- Sept. 12, 2008: Integrated Science Assessment on health effects for use in determining a primary (health-related) standard
- Dec. 12, 2008: Integrated Science Assessment on public welfare effects (such as environmental degradation) for use in determining a secondary standard
- July 30, 2009: proposed rule for primary standard
- Feb. 12, 2010: proposed rule for secondary standard
- March 2, 2010: final rule for primary standard
- Oct. 19, 2010: final rule for secondary standard
The final rules will obviously be set during the next administration.
See the July 23, 2008, nitrogen oxides TipSheet item  for extensive information about these terms, the process, legal machinations, scientists' concerns, and data sources.
For more EPA information on SO2, see: