Information on human thermal physiology is essential to understand if we are to use thermography in any form for human performance or medical applications. Body temperature is a fundamental trait, connected to the execution of physiological actions, for example, movement, growth, exercise, cooling down and warming up, immune response and resistance and much more.
Thermal imaging is under the spotlight right now due to the current COVID-19 pandemic. Thermal cameras are being used to detect high temperatures quickly, and non-intrusively in crowds, groups and individuals and can locate/identify subjects with high temperatures above the optimal level. Although this application is a great tool to help to identify elevated temperatures when used by the right individuals, it does NOT make it a diagnostic tool. The use of Infrared for this purpose is used only as an indicator of temperature, it is not being used to diagnose any condition.
A human is endothermic. Endotherms use their metabolism to regulate their own temperature and to maintain it at its optimal level. This is called thermoregulation. The bodies thermostat is in the hypothalamus, found in the inferior portion of the Diencephalon, the posterior part of the forebrain. The hypothalamic neurons (nerve cells) monitor blood temperature and receive input from peripheral thermoreceptors. Thermoreceptors are nerve endings that respond to heat or cold; they can be found in the skin and also in the mucous membrane of the mouth and throat. The hypothalamus initiates the body’s cooling or heating mechanisms when needed, shivering or sweating may occur respectively. Hypothalamic centres also induce fever.
Fever is a rise in body temperature above the normal range, as is seen in some cases of COVID-19. Normal oral temperature ranges are 37 °C/98.6 °F or rectal temperature of 37.2°C/99°F. Fever is generally accompanied by shivering, increased metabolic rate and oxygen demand can occur. Shivering from fever happens not because of the bodies fall in temperature, but because the setpoint is driven by pyrogen, a substance that is produced by a bacterium or virus, for example, COVID-19, which may produce fever when introduced into the blood.