Improperly used laser devices are potentially dangerous.  Effects can range from mild skin burns to irreversible injury to the skin and eye.  The biological damage caused by lasers is produced through thermal, acoustical and photochemical processes.

Thermal effects are caused by a rise in temperature following absorption of laser energy.  The severity of the damage is dependent upon several factors, including exposure duration, wavelength of the beam, energy of the beam, and the area and type of tissue exposed to the beam.

Acoustical effects result from a mechanical shockwave, propogated through tissue, ultimately damaging the tissue.  This happens when the laser beam causes localized vaporization of tissue, causing the shockwave analogous to ripples in water from throwing a rock into a pond.

Beam exposure may also cause photochemical effects when photons interact with tissue cells.  A change in cell chemistry may result in damage or change to tissue.  Photochemical effects depend greatly on wavelength.  Table 1 summarizes the probable biological effects of exposure of eyes and skin to different wavelengths.

  Summary of Laser Biological Effects

 

Photobiological Spectral Domain

Eye

Skin

Ultraviolet C

(200 nm - 280 nm)

Photokeratitis

Erythema (sunburn)

Skin Cancer

Accelerated skin aging

Ultraviolet B

(280 nm - 315 nm)

Photokeratitis

Increased pigmentation

Ultraviolet A

(315 nm - 400 nm)

Photochemical cataract

Pigment darkening

Skin burn

Visible

(400 nm - 780 nm)

Photochemical and thermal retinal injury

Pigment darkening

Photosenstive reactions

Skin burn

Infrared A

(780 nm - 1400 nm)

Cataract and retinal burn

Skin burn

Infrared B

(1.4mm - 3.0 mm)

Corneal burn, aqueous flare, cataract

Skin burn

Infrared C

(3.0 mm - 1000 mm)

Corneal burn only

Skin burn

Types of Beam Exposures 

Exposure to the laser beam is not limited to direct beam exposure.  Particularly for high powered lasers, exposure to beam reflections may be just as damaging as exposure to the primary beam.

Intrabeam exposure means that the eye or skin is exposed directly to all or part of the laser beam.  The eye or skin is exposed to the full irradiance or radiant exposure possible.

Specular reflections from mirror surfaces can be nearly as harmful as exposure to the direct beam, particularly if the surface is flat.  Curved mirror-like surfaces will widen the beam such that while the exposed eye or skin does not absorb the full impact of the beam, there is a larger area for possible exposure.

A diffuse surface is a surface that will reflect the laser beam in many directions.  Mirror-like surfaces that are not completely flat, such as jewelry or metal tools, may cause diffuse reflections of the beam.  These reflections do not carry the full power or energy of the primary beam, but may still be harmful, particularly for high powered lasers.  Diffuse reflections from Class 4 lasers are capable of initiating fires.

Whether a surface is a diffuse reflector or a specular reflector will depend upon the wavelength of the beam.  A surface that would be a diffuse reflector for a visible laser may be a specular reflector for an infrared laser beam.            

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