In the study of air pollution, a critical load is defined as "a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge". Air pollution research in relation to critical loads has focused on nitrogen and sulfur pollutants. After these pollutants are emitted into the atmosphere, they are subsequently deposited into ecosystems. Both sulfur and nitrogen deposition can acidify surface waters and soils. As added acidity lowers the pH of water, fish and invertebrate health are negatively impacted. Sulfur and nitrogen, as acidifying agents, may change soil nutrient content by removing calcium and releasing toxic aluminum, further impacting plants and animals. Nitrogen deposition can also act as a fertilizer in the environment and alter the competitive interactions of plants, thereby favoring the growth of some plant species and inhibiting others, potentially leading to changes in species composition and abundance. The deposition of nitrogen contributes to nutrient enrichment in freshwater, coastal, and estuarine ecosystems, which may cause toxic algal blooms, fish kills, and loss of biodiversity. Air pollutants impact essential ecosystem services such as air and water purification, decomposition and detoxification of waste materials, and climate regulation. When deposition is greater than the critical load of a pollutant for a particular location, it is considered a critical load exceedance, meaning the biota are at increased risk of ecological harm. Some components of an ecosystem are more sensitive to deposition than others; therefore, critical loads can be developed for a variety of ecosystem components and responses, including (but not limited to) shifts in diatoms, increases in invasive grass species, changes in soil chemistry, decreased forest health, altered and reduced biodiversity, and lake and stream acidification. The history, terminology, and approach used to calculate critical loads differ by region and country. The differences between approaches used by European countries and in the U.S. are discussed below.
