Air pollution poses a major threat to human and environmental health in many parts of the world. Pollutants with nitrogen and sulfur can be particularly harmful to natural ecosystems. Critical loads of atmospheric deposition help decisionmakers identify levels of air pollution harmful to ecosystem components. USDA Forest Service scientists at the agency's Pacific Northwest Research Station modeled relationships between nitrogen and sulfur deposition and epiphytic macrolichens. They examined how these pollutants affect species richness, abundance, and diversity of ecologically important lichen. They created eight manager-relevant lichen metrics for quantifying the ecological risk from nitrogen and sulfur deposition across U.S. forests. Preventing exceedance of lichen critical loads can help managers and regulators meet mission operational goals to protect biodiversity and sustain the health and productivity of forests. Protecting lichens also supports direct and indirect ecosystem services related to food, fiber, hunting, recreation, pharmaceuticals and traditional uses.