Rami Arieli: "The Laser Adventure" Laser Safety Appendix, page 4

Interaction of laser radiation with the biological tissue

The two main points that need to be repeated:

Laser Radiation is Electromagnetic Radiation, and need not be confused with ionizing radio-active radiation. Only very short wavelength electromagnetic radiation such as x-rays or g rays have ionizing effects.
Electromagnetic radiation can interact with a biological tissue in three possible processes:

Reflection, transmission and absorption. The relative amount of each process depends on the properties of the laser radiation and the biological tissue. Reflection and transmission have negligible effects on the biological tissue, and the main process causing damage to the biological tissue is absorption.

Laser radiation absorbed in a biological tissue can cause different types of effects:

Thermal effects - The laser radiation cause the molecules of the biological tissue to vibrate, and heat is created inside the tissue. The damage to the tissue ranges from albumin denaturation, to burning (evaporating) the tissue.
Thermo-Acoustic Effects - Especially when the laser radiation comes in high power very short pulses (less than a millisecond). This process is based on the instantaneous high power density, which causes very high local temperature. As a result, the liquid within the tissue evaporates to gas. Because of the increase in volume of the gas compared to liquid phase, the cell envelope is torn in a blast.
Photochemical Effects - When the laser radiation wavelength is suited to excite reaction between organic molecules.

In the last few years, some evidence was discovered of irreversible changes in biological tissue after prolong exposure to low level laser radiation at wavelengths that were considered to have no effects at all.