Lichens are used as an air pollution indicator. Lichens are symbiotic organisms composed of a fungus and an algae or cyanobacteria. They are sensitive to air pollutants such as sulfur dioxide, nitrogen dioxide, and heavy metals. When air quality is poor and pollution levels are high, lichens tend to suffer and become less abundant or disappear. Therefore, their presence or absence is a good indicator of the air quality in a given area. They are often used in biomonitoring programs to assess the health of ecosystems and to identify areas with high levels of air pollution. It is important to note that while lichens can indicate overall air pollution levels, they cannot provide detailed information about specific pollutants.