Bio-indicators 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, have also been used as bio-indicators—one example was to monitor radioactive fallout from the Chernobyl nuclear reactor accident in 1986.