MONTPELIER, Vt. — At ski races across the United States, competitors are starting to abandon a type of wax many count on for speed amid concerns it contains toxic chemicals that threaten human health and could reach streams and other critical groundwater sources. The International Ski Federation, the governing body for international skiing, announced plans to ban the use of fluorinated waxes in all disciplines next season. These waxes contain perfluoroalkyl and polyfluoroalkyl substances, collectively known as PFAS.