Air pollutants move freely across political borders. Because of physical conditions such as topography, geomorphology and weather, border communities share common air sheds or air basins that are characterized by changing wind patterns depending on the season. Wind is the means of transport of air pollutants, and thus any human activity that generates pollutants on one side of the border will have an impact on the other side. Many border residents are currently exposed to health-threatening

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... ls of air pollution by the presence of substances such as particulate matter (PM 10 and PM 2.5 ), ozone (O 3 ), carbon monoxide (CO), nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ). This situation has created concerns on both sides of the border, and the U.S Environmental Protection Agency (EPA) and the Mexico's National Institute of Ecology (Instituto Nacional de Ecologia-INE) have developed regional strategies to improve air quality based on separate but similar national ambient air quality standards. The objectives of this paper are to describe the current state of binational air quality and to analyze what is needed to be done to make the environment of the CA-BC border region sustainable.