Early mechanical refrigerators in the 1800s and early 1900s used toxic or flammable chemicals as refrigerants, including ammonia, methyl chloride, and sulfur dioxide. Dangerous leaks caused several fatal accidents, leading to the development of Freon (a chlorofluorocarbon) in 1928 as a safer alternative. It was later discovered that Freon damages the ozone layer, and it too was eventually phased out.