Infrared (IR) radiation is a form of energy often perceived as heat. It is a natural part of the environment, primarily from the sun, but also generated by artificial devices. The primary concern with infrared radiation is its capacity to cause thermal injury to tissues.
Placement on the Electromagnetic Spectrum
Infrared radiation occupies the electromagnetic spectrum between visible light and microwaves. Its wavelengths range from approximately 780 nanometers to 1 millimeter, making it invisible to the human eye. Because the energy of its photons is low (less than 1.6 electron volts), IR is classified as non-ionizing radiation. This means it lacks the energy to directly remove electrons from atoms or molecules, the mechanism by which X-rays and ultraviolet light damage DNA.
The infrared spectrum is divided into three sub-regions based on wavelength: Near-infrared (IR-A, 780 nm to 1.4 µm), Mid-infrared (IR-B, 1.4 to 3 µm), and Far-infrared (IR-C, 3 µm up to 1 mm). These wavelength differences correspond to varying penetration depths and biological effects on the skin and eye.
Mechanisms of Thermal Interaction
The fundamental way infrared radiation affects human tissue is by being absorbed and converted into kinetic energy, which is felt as heat. When IR photons strike the body, they cause molecules, particularly water molecules, to vibrate and rotate, resulting in a temperature increase.
The depth of penetration depends highly on the radiation’s wavelength. Near-infrared (IR-A) radiation, having the shortest wavelengths, penetrates the deepest, reaching the dermis and subcutaneous layers up to 5 millimeters. Mid-infrared (IR-B) is absorbed in the upper layers, while Far-infrared (IR-C) is almost entirely absorbed at the skin’s surface (epidermis).
This means IR-A causes deeper heating, while IR-C primarily heats the surface. The body uses thermoregulatory mechanisms, such as increased blood flow and sweating, to dissipate absorbed heat. Harm arises only when the intensity or duration of exposure overwhelms the body’s ability to cool itself, leading to localized overheating.
Specific Risks of Acute and Chronic Exposure
The most immediate danger from high-intensity infrared sources is acute thermal injury, particularly skin burns. Exposure to strong IR, such as from industrial furnaces or high-power heat lamps, can cause a rapid temperature rise that exceeds the skin’s tolerance. These effects are time-dependent; even moderate sources can cause damage if exposure is prolonged without sufficient cooling.
The eyes are particularly vulnerable because the aversion response triggered by visible light may not activate for invisible IR. Near-infrared (IR-A) is the most concerning wavelength for the eyes, as it can pass through the cornea and lens to reach the retina. Acute, high-intensity exposure can cause retinal damage, similar to a flash burn.
A chronic risk, especially in occupational settings like glassblowing or metalworking, is the development of “glassblower’s cataract.” This condition results from long-term, repeated exposure to intense heat and IR radiation, causing opacification of the eye’s lens. Prolonged exposure to intense heat, such as in saunas, can also lead to systemic effects like dehydration or heat exhaustion if the body’s core temperature rises too high without adequate fluid replenishment.
Protective Measures and Safety Standards
Minimizing the risk of infrared-related harm centers on controlling the intensity, distance, and duration of exposure. In high-intensity occupational settings, personal protective equipment (PPE) is the primary defense. Specialized eye protection, such as goggles or visors with IR-filtering properties, is mandatory to protect the lens and retina from thermal damage.
Engineering controls, such as total enclosure of the source or the use of protective screens, are the most effective way to prevent exposure. If enclosure is not possible, administrative controls include limiting worker time near the source or increasing distance. Since intensity drops sharply with distance, even a small increase in separation provides a significant reduction in exposure.
Occupational exposure limits, established by organizations like the American Conference of Governmental Industrial Hygienists (ACGIH), prevent thermal injury to the skin and eyes. These standards specify maximum permissible irradiance levels designed to ensure tissue temperature does not exceed the injury threshold. For the general public using common sources like infrared saunas, following manufacturer guidelines and ensuring proper hydration are the most practical protective measures.