A variety of air pollutants are emitted into the atmosphere from human-caused and natural emissions sources throughout the United States and elsewhere. These contaminants impact sensitive natural resources in wilderness, including the national parks. The system of national parks in the United States is among our greatest assets. This book provides a compilation and synthesis of current scientific understanding regarding the causes and effects of these pollutants within national park lands. It describes pollutant emissions, deposition, and exposures; it identifies the critical (tipping point) loads of pollutant deposition at which adverse impacts are manifested.