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The Paso del Norte region is an ideal location to examine the challenges and potential benefits of cross-border efforts to mitigate air pollution. The region is located midway along the US-Mexico border and comprised of portions of two US and one Mexican states: Texas, New Mexico, and Chihuahua (see Figure 1). The region is also an “air shed” or “air basin,” meaning that due to the area’s meteorological and geographic characteristics, its inhabitants share the same air mass. In addition, these same characteristics separate the region’s air mass to some degree from neighboring areas (CEPAARB 2009 and Environment Canada undated). Because they occupy this one air basin, inhabitants in all three states effectively breathe the same air, the quality of which is affected by activities on both sides of the border. Throughout the region, meteorological and geographic conditions combine with both natural and man-made emission sources to impact air quality.

In the Paso del Norte region, air quality monitoring began in the 1970s and 1980s, and air quality was soon identified as a problem due to several violations of federal air quality standards. These violations got the most attention in 1990 when El Paso was declared in violation of air quality standards for three of the six criteria pollutants. Although El Paso had the most obvious air pollution problem, it was clear that the Mexican and New Mexico air basin neighbors were also contributing to what was ultimately a shared problem. Starting with El Paso’s violation of multiple air standards in 1990, the next ten years saw measurable improvement in the region’s air quality. El Paso eventually came into …