2. Bioeffects

2.1 Tissue Damage There are three mechanisms by which tissue can be damaged by laser light: thermal, acoustic, and photochemical.

• Thermal:
  Thermal effects are the major cause of tissue damage by lasers. Energy from the laser is absorbed by the tissue in the form of heat, which can cause localized, intense heating of sensitive tissues. The amount of thermal damage that can be caused to tissue varies depending on the thermal sensitivity of the type of tissue. Thermal effects can range from erythema (reddening of the skin) to burning of the tissue. Factors that affect thermal damage to tissue are:
  • amount of tissue affected
  • wavelength of light
  • energy of the beam
  • the length of time that the tissue is irradiated
• Acoustic:
  Laser beams are capable of causing a localized vaporization of tissue (actually, the water contained within the tissue), which in turn can create a mechanical shockwave to be propagated through the tissue. Shockwaves can cause tearing of tissue.
• Photochemical:
  Laser light can also cause changes to the chemistry of cells, which can result in changes to tissue.

2.2 Wavelengths of Concern

The portion of the electromagnetic (EM) spectrum concerned with lasers is called the optical portion of the spectrum, which consists of the infrared (IR). ultraviolet (UV) and visible portions of the EM spectrum:

• Infrared (700 nm - 1 mm)
  • Far-IR (IR-B and IR-C) (1400 nm - 1mm)
  • Near-IR (IR-A) (700-1400 nm)
• Ultraviolet (180-400 nm)
  • Far-UV (UV-B and UV-C) (180-315 nm)
  • Near-UV (UV-A) (315-400 nm)
• Visible (400-700 nm)

2.3 Effects on Eye Tissue

The parts of the eye which are of concern in a discussion of laser hazards to the eye are the cornea, lens, pupil/iris, and retina:

• Cornea:
  The cornea is the transparent layer of tissue covering the surface of the eye. The cells on the surface of the cornea have a lifetime of only about 48 hours, therefore cell turnover is quite fast. Injury to cells on the surface of the cornea is generally repaired quickly, but injury to deeper layers of the cornea can result in permanent change to the cornea.
• Lens:
  The lens of the eye focuses light to form images in the eye. Damage to the lens can cause the destructive interference of light within the lens, resulting in a "milky" area or cataract.
• Pupil/iris:
  The pupil of the eye is the aperture through which light is directed into the eye. The iris is a layer of muscle tissue which can contract and expand around the circumference of the pupil in order to determine pupil size.
• Retina:
  The retina is made up of layers of nerve cells and is used for reception of the light in the eye. Damage to cells in the retina can result in loss of vision.

Sensitivity to Specific Wave Lengths

• Visible and IR-A: Visible and IR-A wavelengths of light are transmitted through the cornea and lens of the eye, and are absorbed mostly by the retina. The visible and IR-A portions of the spectrum (400-1400 nm) are often referred to as the "Retinal Hazard Region" because these wavelengths of light can damage the retina. The amount of hazard to the retina from viewing of a laser beam in the Retinal Hazard Region increases with increased pupil size and increased duration of the laser beam.
• UV-A: UV-A wavelengths of light are mostly absorbed in the lens of the eye and can cause photochemical damage to the lens.
• UV-B, UV-C, IR-B, IR-C: UV-B, UV-C, IR-B and IR-C rays are absorbed by the cornea of the eye. Exposure to these wavelengths can result in conjunctivitis, "milky" cornea, and inflammation.

2.4 Effects on Skin

The layers of the skin which are of concern in a discussion of laser hazards to the skin are the stratum corneum, epidermis, and the dermis.

• Stratum corneum:
  The stratum corneum, or horny layer, is the outermost layer and consists of dead epithelial cells. The stratum corneum is capable of filtering some ultraviolet and far-IR wavelengths of light and prevents the light from penetrating to the deeper layers of the skin.
• Epidermis:
  The epidermis layer lies beneath the stratum corneum and is the outermost living layer of the skin.
• Dermis:
  The dermis mostly consists of connective tissue and lies beneath the epidermis.

Sensitivity to Specific Wave Lengths

• IR-A: IR-A wavelengths of light are absorbed by the dermis and can cause deep heating of skin.
• UV-B, UV-C: UV-B and UV-C, often collectively referred to as "actinic UV," can cause erythema and blistering as they are absorbed in the epidermis. UV-B is a component of sunlight that is thought to have carcinogenic effects on the skin.