As required by the Clean Air Act, EPA is reviewing its standard for one of the six main criteria air pollutants, nitrogen dioxide (NO2). As part of its review process, the agency announced on July 14, 2008, the release of its Integrated Science Assessment (ISA) on the health effects of nitrogen oxides (for which NO2 serves as an indicator). The last time a similar assessment for NO2 was released was 1993. Reviews are supposed to be completed every five years.
- EPA press release.  Integrated Science Assessment for Oxides of Nitrogen — Health Criteria  (Final Report), includes a link to the July 11, 2008, Federal Register notice about this Report.
NO2 is generated primarily through emissions from combustion sources such as vehicles, power plants, and industrial engines of various types, as well as biomass burning. Natural sources include wildfires and lightning. NO2 and other nitrogen oxides have direct health impacts, such as respiratory damage and premature death, and also contribute to the formation of ground-level ozone and fine particulates, two other significant criteria air pollutants.
The release of the ISA (formerly called an Air Quality Criteria Document) comes midway through a new review process that was developed by the Bush administration in 2006 and 2007. Critics of the revised process say that it politicizes decisions more than before, and reduces the input of scientists. EPA says it makes the process more efficient. See the views of the agency's own Clean Air Scientific Advisory Committee,  which has a variety of "serious concerns" about the new process.
Without the impetus of a lawsuit, the NO2 review process and issuance of a new standard — or confirmation of the current standard — could have dragged on for many more years. However, the Center for Biological Diversity and others filed a lawsuit in 2005, and an ensuing consent decree of Nov. 19, 2007, available here, established the following schedule:
- July 11, 2008: ISA on health effects for use in determining a primary (health-related) standard
- Dec. 12, 2008: ISA on public welfare effects (such as environmental degradation) for determining a secondary standard
- May 28, 2009: proposed rule for primary standard
- Dec. 18, 2009: final rule for primary standard
- Feb. 12, 2010: proposed rule for secondary standard
- Oct. 19, 2010: final rule for secondary standard
Among the documents to be released during the next couple of years will be a risk and exposure assessment, which will include consideration of the ISA just released, followed by a policy assessment (which replaces the former Staff Paper). The formal rulemaking process follows the release of these documents. A graphic of the generic timeline for the next steps, as well as other information on the new process, is here. 
The Integrated Science Assessment provides considerable new scientific information about the occurrence of nitrogen oxides and their health effects. For instance, the old science left EPA unable to make a definitive conclusion that short-term exposure to NO2 caused a variety of respiratory problems, whereas the ISA says the evidence is "sufficient to infer a likely causal relationship." The old science offered no evidence about links with cardiovascular diseases, whereas the ISA says there is now mixed evidence of such linkages for short-term exposures. For all types of deaths, the old science offered no evidence, but the ISA says the evidence now provides "positive and generally robust associations between ambient NO2 concentrations and risk of nonaccidental and cardiopulmonary mortality" for short-term exposures. For long-term exposures, the ISA notes that there are a few more studies than before, but the science remains limited, making definitive conclusions difficult.
The ISA says that people likely to be more vulnerable to exposure to nitrogen oxides include children, the elderly, people who already have respiratory problems, and people who spend much time on and near busy roadways (including those in buildings near such roads).
The current EPA standard is 0.053 parts per million for the annual arithmetic mean. This is equivalent to 100 micrograms per cubic meter. The World Health Organization guideline is much lower, at 40 ug/m3, and the WHO also has a short-term, 1-hour guideline of 200 ug/m3. WHO says there is evidence of health impacts below its guidelines, though the complications of studying nitrogen oxides make it difficult to say what the lowest concentration is below which adverse impacts don't occur.
Annual average concentrations in some cities, such as Chicago, Los Angeles, New York, and Phoenix, can be about 2/3 of EPA's current annual standard, and many other US cities routinely exceed the WHO guideline. EPA acknowledges that 1-hour peaks can exceed the WHO 1-hour guideline in some settings, such as areas with high vehicle traffic. Within an urban area, ground-level concentrations can be highly variable. The current monitoring network may not capture these variations very well.
As the review process goes forward, you can track NO2 levels of concern to your audience here.  In general, higher concentrations tend to occur throughout much of the eastern half of the country, in much of California, and in larger urban areas in the West.