For many years, measurement of body temperature in routine medical practice was limited to oral, rectal and axillary sites. Recent introduction of infrared non-contact thermometers for the auditory canal requires the establishing of temperature relationships between the ear and more traditional thermometry sites. Since an auditory canal is exposed to the environment, the infrared readings from it are influenced by ambient temperature. A linear model of thermal gradients in the vicinity of an ear canal allows us to find simple formulas connecting temperatures taken from the ear with those from traditional core sites like bladder or pulmonary artery, in addition to rectal and oral. The formulas contain environment coupling coefficients. Their values have been found experimentally by measuring body temperatures from subjects in a walk-in environmental chamber and from multiple clinical studies. The derived coefficients are used in the Thermoscan™ PRO-1 Instant Thermometer to calculate core, oral and rectal equivalent temperatures.