In the United States and most other countries, primary air quality standards are designed to protect human health and are based on concentrations of pollutants in the air. However, from the perspective of ecosystem health, a more appropriate metric for the impacts of nitrogen (N) and sulfur (S) pollutants is the total atmospheric deposition (or “load,” in kg⋅ha−1⋅y−1) of N and S, because ecosystem effects are more strongly determined by cumulative annual loading than by short-term atmospheric concentrations. The European Commission has adopted the concept of a “critical load,” defined as the load of a pollutant “below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” (1). In PNAS, Payne et al. (2) provide new information on the impact of N deposition on European grassland ecosystems and illustrate some of the important complexities involved in quantifying and implementing critical loads.