Japanese experts have issued a warning about polluted drinking water caused by US military bases in Japan's southernmost prefecture of Okinawa. Excessive organofluorine chemical compounds have reportedly been detected in drinking water supplies. This group of chemicals is used in a wide range of industrial processes and is harmful to the body if consumed. It can cause elevated cholesterol, changes to liver function, changes in thyroid hormone levels and reduced immune response. An investigation has not been carried out, because the US has not granted permission.