April 22, 1970: The first nationwide Earth Day celebration is organized by Sen. Gaylord Nelson and Dennis Hayes. It creates a national political presence for environmental concerns. Millions of Americans demonstrate for air and water cleanup and preservation of nature.
April 22, 1915: The use of poison gas in World War I escalates when chlorine gas is released as a chemical weapon in the Second Battle of Ypres. Forevermore, chlorine is not considered a viable alternative disinfectant in Europe.