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Information Sheet No. 13

Revised August 2008

Infrared Radiation

Introduction

The infrared (IR) region of the electromagnetic spectrum lies between visible radiation and microwaves.  Infrared radiation is commonly thought of as radiated heat and is generally perceived by the body as a feeling of warmth. The wavelengths, between 0.7 and 1000 micrometres (mm), can be subdivided into three regions with varying biological effects:

Region	Wavelength
IRA	0.7 - 1.4 µm
IRB	1.4 - 3 µm
IRC	3 - 1000 µm

For comparison, visible light covers wavelengths from 0.4 - 0.7 µm.

Many surfaces which reflect light are poor reflectors of IR, especially IRC, although most shiny metals are good reflectors.  Glass and quartz are opaque to IRC of wavelengths greater than 4 µm.  Air molecules absorb at specific wavelengths distributed throughout the IR spectrum.

Infrared sources and applications

Domestic heating appliances emit predominantly in the infrared, but very hot bodies, such as the sun, also radiate in the visible, and to a lesser extent, in the ultraviolet regions.  Electric arcs and many discharge tubes emit IR as well as ultraviolet, visible, microwave, and radiofrequency waves.  Most IR lasers emit a very narrow band of wavelengths.

In industry, infrared lamps are used for warming food, and curing plastics, paints and other materials.  Infrared is also given off by any very hot body, such as furnaces and molten metals.

Biological effects

Because of the reduced photon energy relative to visible wavelengths, IRA induces few photochemical reactions in the body and IRB and IRC none.  However, tissue which has absorbed IR is raised in temperature and at sufficiently high intensities may be damaged or destroyed by thermal reactions.  Injuries sustained from exposure to high powered sources which emit IR may be caused by the simultaneously produced visible or ultraviolet radiation.

(a)      Effects on eyes

The lens of the eye strongly absorbs IRA radiation.  Continual exposure over several years may lead to the formation of cataracts, and glass blowers, furnace and molten metal workers, have traditionally been at some risk.  A single, brief, intense exposure from an IRA laser, or heat conducted from the exposed pigmented iris or aqueous humour, may also produce a cataract.

IRC and most IRB is absorbed wholly by the cornea.  The exposure tolerance level is similar to that of skin and when exceeded results in a painful but short-lived (about 48 hour) thermal injury, apparently without long-term effects.  Some high-powered IRA lasers can damage the retina.
 
Sudden temperature elevations of the anterior of the eyes due to high level IR exposure are readily sensed causing pain and a blink reflex to limit further exposure.  For this reason the hazard of high level IRB and IRC exposure is often greater to the skin than the eyes.

(b)      Effects on skin

If the skin is exposed to IR radiation it is readily sensed as a feeling of warmth but at high levels it results in pain and burns.  The tolerance to moderate exposures depends on the ability of vascular flow within the irradiated region to conduct excess heat to cooler parts of the body.  If the irradiated area is large the body may suffer from thermal stress.

A considerable proportion of visible and IRA radiation is reflected, especially from fair skin, but the proportion of the IRB and IRC absorbed is greater than 90%.

Protection and recommended limits

There is significant hazard only in cases of occupational exposure to hot surfaces (>600ºC) and to IR laser radiation.

The exposure from hot surfaces may be reduced by keeping exposure times as short as possible, erecting shields of absorbing materials, covering or removing reflective metal surfaces in the vicinity, limiting the area of viewing ports, remaining at distances large compared with the dimensions of the hot surface, and wearing fire-proof clothing and goggles which transmit light (for viewing the work area) but not IR. A conventional arc welding helmet provides sufficient protection against excessive IR, visible, and ultraviolet radiations.  Most materials opaque to light are also opaque to IR radiation.  Good ventilation reduces heat stress.  The International Commission on Non-Ionizing Radiation Protection has published recommended exposure limits for IRA and IRB.  These can be found in Health physics 73(3):539-554, 1997: Guidelines on limits of exposure to broad-band incoherent optical radiation (0.38 to 3µm).

Protection from IR laser radiation is described in the joint Australian/New Zealand Standard AS/NZS 2211:1997 – Laser safety.

For further information, contact

National Radiation Laboratory
P O Box 25099
Christchurch
New Zealand

Phone:      + 64 3 366 5059
Fax:          + 64 3 366 1156               
Internet:    http://www.nrl.moh.govt.nz

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