This book addresses the relationships between air pollution in the western United States and trends in the growth and condition of Western coniferous forests. The West is defined in this case as the eleven conterminous states of California, Oregon, Washington, Idaho, Nevada, Arizona, New Mexico, Utah, Colorado, Wyoming, and Montana. Approximately one-third of the West is forested, primarily by coniferous forest types.
United States Environmental Protection Agency, "Western Forests and Air Pollution" (1992). Pollution. Paper 1.