The biological effects of non-ionizing laser radiation include the action of visible, ultraviolet (UV), or infrared radiation upon tissues. Generally, lasers in the UV region induce photochemical reactions; lasers in the infrared region induce thermal effects.
Damage can occur when a laser beam encounters tissue, depending on the combined characteristics of both the incident laser beam and the properties of the tissue involved.
Key factors are:
- Laser wavelength, power density, and pulse duration
- Tissue propensity to reflect, transmit, or selectively absorb the laser radiation
Laser effects on the eye
The unprotected human eye is extremely sensitive to laser radiation and can be permanently damaged from direct or reflected beams.
Due to tissue characteristics, the area of the eye damaged by laser energy is dependent upon the wavelength of the incident laser beam. The retina, cornea, and lens are the areas most commonly damaged.
Retina: Laser light in the visible to near infrared spectrum can cause damage to the retina. These wavelengths are also know as the "retinal hazard region."
- Visible and near infrared (400 – 1400 nanometer or nm) laser light pose a critical hazard on the retina. Infrared A is transmitted by the cornea to the lens of the eye which narrowly focuses it on the retina, concentrating the radiant exposure of the laser by up to 100,000 times. Since the tissue structures of the retina are unable to undergo any repair, leisions caused by the focusing of visible or near-infrared light on the retina may be permanent. The most critical area of the retina is the central portion, the macula, and the fovea.
Cornea and lens: Laser light in the ultraviolet or far infrared spectrum can cause damage to the cornea or the lens.
- Ultraviolet (180 nm to 400 nm): Photochemical damage is caused by the absorption of UV light by selective sensitive portions of cells of the cornea. Many proteins and other molecules (DNA, RNA) absorb UV light and are "denatured" by the radiation. Excessive exposure to UV light can cause photophobia, redness of the eye, tearing, discharge, stromal haze, etc. These adverse effects are usually delayed for several hours but will occur within 24 hours. The lens principally absorbs UVA (315-400 nm). The lens is particularly sensitive to the 300 nm wavelength. XeCl eximer lasers operating at 308 nm can cause cataract with an acute exposure.
- Far infrared (1400 nm to 1 mm; CO2 lasers, 10600 nm): Thermal damage is caused by the heating of the tears and tissue water of the cornea by the infrared light. Excessive exposure to infrared radiation results in a loss of transparency of the cornea or surface irregularities.
Laser effects on the skin
Thermal (burn) injury is the most common cause of laser induced skin damage. Thermal damage is generally associated with lasers operating at exposure times greater than 10 microseconds and in the wavelength region from the near ultraviolet to the far infrared.
- The principal thermal effects of laser exposure depend on:
- The absorption and scattering coefficients of the tissues at the laser wavelength
- Irradiance or radiant exposure of the laser beam
- Duration of the exposure and pulse repetition characteristics, where applicable
- Extent of the local vascular flow
- Size of the area irradiated
- Tissue damage may also be caused by thermally induced acoustic waves following exposures to sub-microsecond laser exposures. Repetitively pulsed or scanning lasers involve a thermal process wherein the effects of the pulses are additive.
- Laser Accident Emergency Procedures
- Laser Hazards, OSHA Technical Manual, Section III, Chapter 6, US Department of Labor, Occupational Safety & Health Administration
- Health Sciences Laser Safety Program: Advanced Laser Safety Course, Section 2: Biological Effects, Virtual Hospital, University of Iowa