The nature of laser beam damage and the threshold levels at which each type of injury may occur depends on several parameters. These include wavelength of light, energy of the beam, divergence and exposure duration. For pulsed lasers, parameters also include the pulse length, pulse repetition frequency and pulse train characteristics. The ANSI Z136.1 standard establishes Maximum Permissible Exposure (MPE) limits for laser radiation. MPE’s need to be determined for each specific laser so that a Nominal Hazard Zone (NHZ) can be established. The Nominal Hazard Zone is the area around a laser in which the applicable MPE is exceeded. The EHS can assist in determining MPEs and NHZs. When an MPE is exceeded, damage can occur to the skin, retina, lens, cornea, and conjunctival tissue surrounding the eye (See Fig. 1). For lasers over 500 mW, the beam can ignite flammable materials and initiate a fire.
Thermal burn, acoustic damage, and photochemical damage to the retina may occur from laser light in the near ultraviolet (UV), visible and near infrared (IR) regions (below 400 nm – 1400 nm). Damage occurs as the laser light enters the eye and is focused on the retina. Normal focusing of the eye amplifies the irradiance by approximately 100,000; thus, a beam of 1 mW/cm2 results in an exposure of 100 W/cm2 to the retina. Energy from the laser beam is absorbed by tissue in the form of heat, which can cause localized intense heating of sensitive tissues. The most likely effect of excess exposure to the retina is thermal burn that destroys retinal tissue. Since retinal tissue does not regenerate, the damage is permanent, which may result in the loss of sight in the damaged area.
Laser Generated Airborne Contaminants (LGAC)
Air contaminants associated with the use of Class 3B and Class 4 lasers. LGACs result from the interaction of the laser beam with target or other materials and can include metallic fumes and dust, chemical fumes and aerosols containing biological contaminants. LGACs are generally only formed when target irradiance reaches
107 W/cm2. Local or area ventilation must be adequate to keep airborne contaminant levels below worker exposure limits.
Intrabeam viewing of the direct beam and the specularly reflected beam are most hazardous when the secondary reflector is a flat and polished surface. Secondary reflections from rough uneven surfaces produce more diffuse reflections and are usually less hazardous. Extended source viewing of normally diffuse reflections are not normally hazardous except for very high power lasers (Class 4 lasers). Extra care should be taken with IR lasers since diffuse reflectors in the visible spectrum may reflect IR radiation differently and produce greater exposures than anticipated.
The figures below represent the ways in which laser light can be viewed:
Risk of skin injury can be significant when working with high power infrared lasers or lasers which emit in the ultraviolet spectral region. Risks from infrared lasers include thermal burns and blistering or charring of the skin. Risks from UV lasers include sunburn, skin cancer, skin aging and photosensitization.