Wavelength range and sources
Infrared radiation (IR), also known as thermal radiation, is that band in the electromagnetic radiation spectrum with wavelengths above red visible light between 780 nm and 1 mm. IR is categorized as IR-A (780 nm-1.4 µm), IR-B (1.4-3 µm) and IR-C, also known as far-IR (3 µm-1 mm). Common natural sources are solar radiation and fire. Common artificial sources include heating devices, and infrared lamps used and in the home and in infrared saunas for health purposes. Industrial sources of heat such as steel/iron production also fall into the infrared region. Lasers are a special source of IR emitted over one or more extremely narrow wavelength bands.
IR effects on the body and health implications
IR penetrates the human skin and the eye to various depths ranging from several millimeters by IR-A to superficial absorption of IR-C. Humans have inborn protective aversion responses to pain from high heat and to the bright light that is often also present, so that potentially harmful exposure is avoided. Harmful health effects of IR are due to thermal injury of tissues mediated largely through water molecules but also through changes to protein structure.
The main harmful health effects of high IR exposure are to the eye. The cornea, iris, lens and retina are all highly sensitive to varying degrees of thermal damage. When the cornea absorbs IR radiation with conversion into heat, this is conducted to the lens. Aggregation of lens proteins after repeated exposure to extreme heat can cause lens opacities or cataracts, as are often seen in glass workers and iron and steel workers.
Skin damage due to hyperthermia can occur but depends on the intensity and the duration of IR exposure. If the temperature of the skin is held at 44°C it takes several hours before irreversible damage occurs. This compares with less than a second at surface temperatures of 70°C. Long-term IR exposure of the skin without burning, such as after years of skin exposure to open fires, can cause a red-brown mottling of the skin. However, IR is not thought to play a role in initiating skin cancer.
If the whole body is subjected to high levels of heat, increases in body temperature and physical heat-stress can result. Heat stress needs to be evaluated considering all contributing factors including air movement, temperature and humidity as well as the source of the heat.
Protection recommendations are aimed especially at the skin and relevant parts of the eye, which are at risk from excessive exposure to infrared radiations.
To avoid detrimental health effects from infrared radiation on the eye and skin, such as thermal injuries, ICNIRP provides guidance and recommends limits of exposure. Different limits are recommended depending on the wavelength bands and action spectra. The limits also depend on exposure duration and on the size of the source.