Lichens are considered an indicator of air pollution. Lichens are a combination of fungi and algae that have a symbiotic relationship. They are able to absorb nutrients and water from the air, making them sensitive to changes in air quality. Lichens are known to be sensitive to air pollutants such as sulfur dioxide and nitrogen oxides. When air pollution is present, the growth and health of lichens can be negatively affected. Therefore, the presence, absence, or condition of lichens can be used as an indicator of air pollution in an area. It is important to note that while lichens can indicate air pollution, they do not provide specific information about the type or source of pollution.