The normal body temperature is 37.0°C (range 36.6–37.4°C). Core temperature refers to the temperature of blood supplying vital organs, such as the brain, heart and lungs and is the most reliable and generally used measure in clinical settings. Core temperature is measured deep within the body (e.g. rectum, bladder) and requires special thermometers. It is generally impractical to measure core temperatures in the field, so heat or cold illness should always be considered when a person displays relevant symptoms and signs in a setting where heat or cold injury could occur.