Safety

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© Springer Nature Switzerland AG 2020

S. Stübinger et al. (eds.)Lasers in Oral and Maxillofacial Surgerydoi.org/10.1007/978-3-030-29604-9_20

20. Laser Safety

Ferda Canbaz1   and Azhar Zam1  
(1)

Department of Biomedical Engineering, University of Basel, Basel, Switzerland
 
 
Ferda Canbaz (Corresponding author)
 
Azhar Zam

Abstract

Since their invention, lasers have been successfully employed in many applications, ranging from research—in fields such as chemistry, physics, archeology, and medicine—to industry. Lasers are both a practical tool and a potentially dangerous piece of equipment. As users have different educational and experiential backgrounds, common safety rules and regulations must be specified. Currently, safety guidance is provided by international committees (such as the International Commission on Non-Ionizing Radiation Protection, the International Electrotechnical Commission, and the American National Standard). According to the regulations, users should be trained before working with lasers in order to provide a safe environment. Safety regulations focus mainly on protection of the human eye and skin, which are the organs most vulnerable to laser exposure. To protect these organs, it is important to know maximum exposure levels and the class of laser being used. This chapter offers some insight into working with lasers, highlighting the biological aspects underlying injury risks for different parts of the human body, laser classification details, and basic rules for laser safety.

Keywords

LasersLaser safetyLaser exposureMaximum permissible emission (MPE)

20.1 Lasers

Lasers are optical oscillators that produce spatially and temporally coherent light. Laser light is directional, monochromatic, and intensely bright. The output of a laser beam spreads due to diffraction, but it does not spread as much as the beam of a flashlight. Hence, even after propagating over long distances, a laser beam can still be hazardous to the human eye and skin. With regard to color, lasers produce light with a very narrow spectrum. The narrow spectral width and the limited cross-sectional area of the beam gives some indication of how bright laser output can be, even with modest output power [1, 2]. Compared to incoherent light sources, these properties make direct viewing of lasers dangerous, even at low power levels. When it comes to laser safety, irradiance must be taken into account. Irradiance is defined as the radiant flux per unit area ($$ \raisebox{1ex}{$ d\varphi $}\!\left/ \!\raisebox{-1ex}{$ d A$}\right. $$, where is the unit flux and dA is the unit area). Lasers can produce continuous-wave or time-dependent output. In the case of pulsed lasers, it is possible to generate high peak power levels at low average output power levels. Since the peak power of a pulsed laser can be more than a million times higher than the average power, users must be more careful when using pulsed lasers. The maximum permissible exposure (MPE) of a laser depends on its operation wavelength, operation type (pulsed or continuous wave), and duration of exposure. MPE levels for skin and eyes are specified in specific laser safety standards [3, 4].

20.2 Why We Need to Avoid Laser Exposure

The eye is the part of the body most sensitive to light exposure. The human eye is formed by a cornea, pupil, iris, focusing lens, retina, and optical nerves. A detailed image of the human eye can be seen in Fig. 20.1. Figure 20.2 illustrates how the eye sees. For example, light from a candle passes through the cornea and the pupil. The image is reversed and projected to the retina. The retina then converts the image to an electrical signal. With the help of optical nerves, the electrical signal is transferred to the brain. The brain, in turn, interprets the electrical signal so as to reproduce the image.

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Fig. 20.1

A schematic representation of parts of the human eye

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Fig. 20.2

Schematic of the formation of images on the retina

Laser radiation reaches different parts of the human eye, depending on its wavelength [3, 4]. The range of light from 100 to 315 nm (Ultraviolet-B and Ultraviolet-C) is absorbed by the cornea. As a result of this absorption, a photochemical process occurs in the cornea. This process is known as photokeratitis. Corneal damage resulting from exposure to light in the range of 100–315 nm is generally temporary. Corneal regeneration is a quick process; it takes less than 24 h. From 315 to 400 nm (Ultraviolet-A), the cornea and aqueous humor let the light in, but it is absorbed by the focusing lens. This kind of absorption, in particular, leads to denaturation of the proteins in the lens, resulting in the formation of cataracts. In the light ranges 400–800 nm (visible) and 800–1400 nm (infrared-A), light can reach the retina. Retinal tissue damage occurs due to absorption, similar to the other parts. Generation of heat bases on either melanin granules in the pigmented epithelium or photochemical reaction in the photoreceptor. The aversion reflex can prevent damage in the retina only if the light intensity is lower than that which affects the retina in 0.25 s. However, in the wavelength region of 700–1400 nm, the human eye remains insensitive; thus lasers may cause damage to the retina. An additional problem with the visible and infrared-A wavelength region pertains to the focusing effect of the cornea and lens. These parts can increase retinal irradiance by a factor of 100,000 just by focusing light onto the retina, leading to possible retinal damage even with low power levels. Above 1400 nm (infrared-B and infrared-C), the cornea absorbs the light and does not let it propagate further. The resulting heat causes protein denaturation on the cornea surface.

Light exposure can be dangerous not only to the human eye but also for the skin. In terms of functionality, damage to the eye is much more significant than that to the skin. However, since the surface area of the skin is much larger than that of the human eye, the possibility of light exposure is higher. Figure 20.3a shows the layers that make up human skin. The epidermis is the outmost part of the skin and is formed of five different layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. Stratum corneum is the outer layer of the epidermis, and it is made of flat dead cells (Fig. 20.3, inset). Next comes the dermis, which includes different types of tissues such as collagen, elastic tissue, and reticular fibers. The subcutaneous tissue lies beneath the dermis layer; here, fat and connective tissues are present. As in the case of the human eye, different wavelengths penetrate the skin at different depths [3, 4]. For example, light with a wavelength of 800 nm can penetrate the subcutaneous layer of the skin (Fig. 20.3b). Damage thresholds for the human eye and skin are comparable. Depending on the penetration depth, a temperature increase occurs at different levels of the skin. For example, deep heating can be caused by exposure to light in the infrared-A region. If skin damage is isolated in a small area, it can be painful, but it will be temporary. The recovery process may be long; however, it does not impair functionality. Damage to a large area of the skin is not common in laser laboratories but could cause severe injury, due to the extensive loss of body liquid.

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Jul 22, 2021 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Safety
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