Critical loads are typically expressed as deposition loading rates of one or more pollutants in amount per area per year (., kilo- grams per hectare per year (kg/ha/yr)). Critical loads are based on changes to specific biological or chemical indicators such as species composition of a given ecosystem (., grassland) or biotic community (., understory plants or tree-dwelling lichens) or acid neu- tralizing capacity (ANC) in soils, streams or lakes. Because different sensitive receptors (., forest soils, high elevation lakes, species of lichen) or species may have varying sensitivities to air pollutant loads, multiple critical loads can be used to describe a continuum of impacts.
