Steel that contains a minimum of 10% chromium is called stainless steel, because the chromium reacts with the oxygen in the air to form a microscopic, invisible layer that protects it from the environment. Higher levels of chromium and other elements such as nickel can enhance this protection even more. Stainless steel resists corrosion and staining, maintains its strength at high temperatures, and is easily maintained. It is widely used in items such as automotive and food processing products, corrosive environments such as steam heat exchangers and pipes to transport acids, and medical or health equipment. Depending on the application, stainless steel comes in many grades (chemistries), but the main stainless steel families are 300 and 400 series.