Biological and health effects of laser radiation

• Pages: 47-50

Appendix B – Biological and health effects of laser

radiation

1 Optical properties of tissues

Laser radiation may be either absorbed, scattered or reflected from biological tissues. In

most cases, a combination of all of these effects occurs. However, the biological effect is

caused only by absorption. From approximately 280 nm to 3.0 µm in the infrared, reflection

may exceed 10 per cent, and significant penetration will also occur, such that scattering may

play an important role in determining the final exposure to the target tissue.

1.1 Ultraviolet radiation

It is generally accepted that the absorption of ultraviolet radiation (UVR) takes place in

organic molecules. The inorganic components of tissue do not absorb at wavelengths longer

than 200 nm. The absorbed energy may give rise to photochemical reactions.

Protein molecules and uracanic acid are highly absorbing in the UVC and dominate all

absorption up to wavelengths of approximately 300 nm. At wavelengths longer than

approximately 300 nm, melanin pigment granules play an important role in the scattering and

absorption of UVR in the skin. The granules are concentrated as a shield above the cell nuclei

and protect them by the absorption of UVR and partly by radiation scattering. Scattering is

probably of greatest significance at longer wavelengths. Furthermore, there is evidence

suggesting that melanin may serve another protective role: as a free-radical scavenger.

The depth of penetration of UVR into the human body is very limited. Penetration is

somewhat greater at longer wavelengths and some penetration can take place into the dermis at

wavelengths above 300 nm in Caucasians. The thickness of the outermost, dead, horny skin

layer (stratum corneum) increases following UVR exposure, thus further attenuating later UVR

exposures (Health Council of the Netherlands, 1979).

The same general considerations hold for exposure of the eye. At wavelengths less than

290 nm, the cornea will be able to absorb incident UVR completely. However, the lens and

tissues of the anterior part of the eye may be exposed to UVR if the wavelengths are greater

than about 290 nm. The retina is normally protected by UVR absorption by the cornea and lens,

but in aphakes (individuals who have had their lens removed by cataract surgery) and in young

children, significant amounts of UVR at wavelengths above about 290 nm may reach the retina.

Because of the biological effectiveness of UVR, even smaller amounts of UVR reaching the

retina may be of concern when considering a lifetime of exposure.

1.2 Visible and infrared radiation

Light and infrared radiation of wavelengths less than 1,400 nm penetrate skin and ocular

tissues since water is relatively transparent at these wavelengths. Melanin is the principal

absorber at wavelengths less than 1 µm. The absorption by melanin and its subsequent

temperature elevation can lead to thermal injury of the iris and the retina in the eye and of the

epidermis of the skin. Since water becomes more highly absorbing with increasing wavelengths

greater than 1,400 nm (IRB and IRC), depths of penetration for infrared radiation are very

superficial (WHO, 1982).

47