Over the wavelength range that different lasers operate, the skin is a strong absorber, thereby protecting all the organs of the body except for the eyes.
There are three principal types of tissue damage mechanisms for laser radiation: thermal, photo chemical and thermo-mechanical. The predominant injury mechanism is determined principally by the laser wavelength, the duration and the irradiance of tissue exposure
Occular Hazards
The biological effects on the eyes at different wavelengths are:
| | Wavelength range | Primary tissue at risk of damage |
| Ultraviolet | 180 nm - 400 nm | Cornea or lens (302-315 nm) |
| Visible Near Infra-red | 400 nm - 1.4 µm | Retina |
| Medium & Far Infra-red | 1.4 µm - 1 mm | Cornea or lens (1.4 – 3.0 µm) |
Retinal eye damage from laser radiation can occur at very low power levels due to the focusing effect of the cornea and lens of the eye and the coherence of laser radiation.
The Maximum Permissible Exposure (MPE) represents a safe level of exposure to laser radiation; which varies with wavelength range and duration of exposure.
Damage to the eye can arise from different mechanisms depending on the duration of exposure to the light. In particular, for very short exposures (< 10-6 s), damage arises from thermo-acoustic transients. Simple heating effects predominate in the range 100ms - 10s and for exposures longer than 100s photo-chemical effects can predominate for wavelengths less than 600 nm. These different processes make the determination of safe radiation levels for pulsed lasers a complicated task that requires a detailed study of the MPE tables.
Skin Hazards
The biological effects of skin exposure to laser radiation are simpler to assess than those for the eye. The injury is surface burning which may occur following acute exposure to cw laser beam. This can occur in a fraction of a second for a beam of powers 0.5W or more; or over a prolonged period for photo-chemical burns from ultraviolet laser beams. Unlike retinal eye burns, skin burns may easily be treated. They are sterile and generally heal quickly.
In the visible and near infrared wavelength range, the MPE for > 10s skin exposure is 1000Wm-2, similar to the solar constant of 1400Wm-2. In the UV the MPE for skin exposure is identical to that for eye exposure.
Laser damage mechanisms for skin tissue are similar to those for the eye in terms of the effect of wavelength and exposure duration.
