What goes up must come down. Air pollutants eventually deposit into ecosystems, causing damage. The amount of pollution that can be tolerated before harm occurs is known as the CRITICAL LOAD.
What is a critical load?Air pollution emitted from a variety of sources may eventually deposit out of the air into ecosystems.
A CRITICAL LOAD is the quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment are not expected to occur according to present knowledge.Critical loads can be developed for any type of air pollutant, but most commonly they are developed for nitrogen or sulfur compounds emitted by vehicles, powerplants, industry, agriculture, and other sources. More »
How are critical loads calculated?Scientists calculate critical loads using ecosystem modeling, observations, or experiments. A critical load is specific to a certain resource, such as alpine plant diversity, and a defined effect, such as a 10 percent loss in diversity.
How is the National Park Service developing and using critical loads?The National Park Service is dedicated to resource protection and recognizes critical loads as valuable tools for evaluating and communicating ecosystem condition. We are encouraging and supporting critical loads research in parks and are participating in multiagency efforts to advance the science of critical loads. One of these efforts, the Critical Loads Ad-Hoc Committee (CLAD), includes the National Park Service, the Environmental Protection Agency, the Forest Service, and the U.S. Geological Survey. CLAD is sponsoring several critical loads projects in parks, including Shenandoah, Great Smoky Mountains, Rocky Mountain, and Glacier National Parks.