Bioindicators are physiological, chemical, or behavioral changes that occur in organisms as a result of changes in the environment. Bryophytes of the genus Hypnum are particularly sensitive to pollutants, especially sulfur dioxide. As a result, most bryophytes are not found in cities and industrial areas. Mosses and liverworts, especially Hypnum cupressiforme and Homalotecium serieceum, were used as bioindicators to monitor radioactive fallout from the Chernobyl reactor accident in 1986.