Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Do humans give off radiation?

Category: Health      Published: July 17, 2013

thermal image of person
Humans glow mostly in the infrared portion of the electromagnetic spectrum. Public Domain Image, source: NIST.

Yes, humans give off radiation. Humans give off mostly infrared radiation, which is electromagnetic radiation with a frequency lower than visible light. This effect is not unique to humans. All objects with a non-zero temperature give off thermal radiation. And because a temperature of exactly absolute zero is physically impossible, all objects give off thermal radiation.  We have to be careful here. Thermal radiation is not the exact same thing as infrared radiation. "Thermal radiation" is all the electromagnetic waves given off by an object because of its temperature, and includes radio waves, infrared waves, and even visible light. Infrared waves are only one part of thermal radiation. The two tend to get confused because most of the thermal radiation is infrared radiation for temperatures that are comfortable to humans. As an object gets hotter, the peak of its thermal radiation shifts to higher frequencies. The sun is hot enough that most of its thermal radiation is emitted as visible light and near infrared waves.

Thermal radiation only transports heat and indicates the temperature of its source. Different people at different times give off different amounts of radiation. But these differences just indicate who is hotter, and not who is fatter, taller, sadder, or more saintly. Thermal images of a person captured using an infrared camera just indicate the temperature of the person's skin, and cannot be used to diagnose diseases happening below the skin. Clothes tend to block infrared radiation, so a man with his shirt off emits more radiation than when it is on. Anybody who has tinkered with an infrared camera can attest to this fact.

Infrared radiation is non-ionizing and therefore cannot give you cancer. It's a good thing, because the rocks, trees, chairs, tables, and walls around us are constantly flooding us with a barrage of infrared radiation. For the most part, humans do not emit other forms of radiation besides thermal radiation. People often eat trace amounts of radioactive minerals that occur naturally, and they therefore give off tiny amounts of other types of radiation. For instance, Brazil nuts and bananas contain higher amounts of radioactive elements. But the amounts are too small to be noticeable or have an effect on their health. If a person has a medical scan taken that requires drinking radioactive contrast, he will emit more radiation than normal for a few hours until the contrast decays, but the amounts are still too small to have an effect on everyday life and health.

Topics: body heat, infrared, light, radiation, temperature, thermal imaging, thermal radiation