Critical loads as a policy tool for protecting ecosystems from the effects of air pollutants

Framing the effects of air pollutants on ecosystems in terms of a "critical load" provides a meaningful approach for research scientists to communicate policy-relevant science to air-quality policy makers and natural resource managers. A critical-loads approach has been widely used to shape air-pollutant control policy in Europe since the 1980s, yet has only rarely been applied in the US. Recently, however, interest in applying a critical-loads approach to managing sulfur and nitrogen air pollutants in the US has been growing, as evidenced by several recent conferences, a new critical-loads sub-committee within the National Atmospheric Deposition Program, and nascent efforts by several federal agencies to apply critical loads to land management. Here, we describe the critical-loads concept, including some of its limitations, and indicate how critical loads can better inform future air-pollutant control policy in the US.