Introduction:

A laser is a device that emits electromagnetic radiation via a process of optical amplification based on the stimulated emission of photons. Laser radiation is characterized by an extremely high degree of coherence, monochromaticity, brightness, and directionality. The wavelength of laser light is extremely pure when compared to other sources of light and all of the photons that make up the laser beam have a fixed phase relationship with respect to one another. Because its properties, laser have made possible a countless number of scientific, commercial, industrial, and medical applications ^[1,2]. When laser beam directed, reflected, or focused upon an object, laser light will be partially absorbed, raising the temperature of the surface and/or the interior of the object, potentially causing an alteration or deformation of the material. These properties which have been applied to laser surgery and materials processing can also cause tissue damage. Damage can result from both thermal and photochemical effects ^[3,4]. In addition to the direct hazards to the eye and skin from the laser beam itself, the non-beam hazards, in some cases, can be life threatening, e.g. electrocution, fire, and asphyxiation ^[4]. So it is of great importance to be familiar with the possible hazards of different class of lasers, and the most important the safety procedures for students and early careers who could be involved in laser operation and usage, to provide a safe working environment.

Laser in Medicine:

Laser based instruments are widely used in medical applications. They are used in treatment of cancer, removal of tumors of vocal cords, brain surgery, plastic surgery, gynecology and oncology. Laser therapy causes less bleeding and damage to normal tissue than standard surgical tools do, and there is a lower risk of infection ^[5,6]. Surgical removal of tissue with a laser is a physical process similar to industrial laser drilling. Carbon-dioxide lasers operating at 10.6 micrometers can burn away tissue as the infrared beams are strongly absorbed by the water that makes up the bulk of living cells. A laser beam cauterizes the cuts, stopping bleeding in blood-rich tissues such as gums. Similarly, laser wavelengths near one micrometer (Neodymium-YAG Laser) can penetrate the eye, welding a detached retina back into place, or cutting internal membranes that often grow cloudy after cataract surgery. Less-intense laser pulses can destroy abnormal blood vessels that spread across the retina in patients suffering from diabetes, delaying the blindness often associated with the disease. Ophthalmologists surgically correct visual defects by removing tissue from the cornea, reshaping the transparent outer layer of the eye with intense ultraviolet pulses from Excimer Lasers ^[6,7,8]. So getting the right amount of the right wavelength of laser energy to the right tissue to damage or destroy only that tissue, and nothing else.

Laser Hazards:

However, if laser beam hit untargeted tissue, it can be hazardous and can cause damage to that tissue, particularly for the eye (sometimes also for the skin), mostly because they can have high optical intensities even after propagation over relatively long distances. Even when the intensity at the entrance of the eye is moderate, laser radiation can be focused by the eye’s lens to a small spot on the retina, where it can cause serious permanent damage within fractions of a second – even when the power level is only of the order of a few milliwatts. Laser damage of the eye is not always immediately noticed: it is possible e.g. to burn peripheral regions of the retina, causing blind spots which may be noticed only years later (Fig. 1) ^[3,7].

Fig. 1.

(a) Simple schematic of the eye, (b) Retinal Injury ^[9].

On the other hand, lasers can harm the skin via photochemical or thermal burns. Depending on the wavelength, the beam may penetrate both the epidermis and the dermis. The epidermis is the outermost living layer of skin. Far and Mid-ultraviolet (the actinic UV) are absorbed by the epidermis. A sunburn (reddening and blistering) may result from short-term exposure to the beam. UV exposure is also associated with an increased risk of developing skin cancer and premature aging (wrinkles, etc) of the skin. Laser effects on tissue depend on the power density of the incident beam, absorption of tissues at the incident wavelength (Fig. 2), time beam is held on tissue, and the effects of blood circulation and heat conduction in the affected area ^[10].

Fig. 2.

Different wavelengths of light penetrate the skin ^[10].

The correct control of laser parameters mentioned above will lead to a successful treatment of the target area, otherwise will lead to skin damage if it uncalculated well. An example is a current work of our group on project related skin temperature evaluation during laser therapy, using long-pulse Nd-YAG laser for portwine stain treatment (Fig. 3). A monitoring of skin temperature during therapy was studied to prevent the raise of skin temperature during therapy to unwanted level which can cause skin damage, a cooling system was used on skin to reduce the thermal and pain effect during therapy. Skin temperature without and with cooling system are shown in Fig. 4. (a) and (b) respectively.

Fig. 3.

Lesion laser therapy.

Fig. 4.

Skin temperature during laser therapy, (a) without cooling, (b) with air cooling.

In addition to the direct hazards to the eye and skin from the laser beam itself, it is also important to address other hazards associated with the use of lasers. These non-beam hazards, in some cases, can be life threatening, e.g. electrocution, fire, and asphyxiation ^[4].

Laser Classifications:

Because of the wide ranges possible for the wavelength, energy content and pulse characteristics of laser beams, the hazards arising from their use varies widely. It is impossible to regard lasers as a single group to which common safety limits can apply. A system of laser classification is used to indicate the level of laser beam hazard and maximum Accessible Emission Levels (AELs) have been determined for each class of laser^[11]. The previous classification system, which was based on five classes (1, 2, 3A, 3B & 4), has been replaced with a new system of seven classes (1, 1M, 2, 2M, 3R, 3B & 4) and these are described in Table (1). And their level of hazards as shown in Fig. 5.

Table 1.

Classification of lasers ^[11,12].

Fig. 5.

Laser classes hazard level ^[13].

Safety Procedures:

So from what mentioned previously, it is essential to design, use and implement a safe laser working environment to minimize the risk of laser accidents, especially environment involving students, patients and early careers (i.e. teaching laboratories and clinical application). There are two principle international laser safety standards:

Both standards share a common classification system for lasers based on their power output, wavelength and pulse duration. They are designed to provide a safe laser working environment and provide measures control including administrative, engineering and procedural controls (which especially should be applied for class 3B and class 4 lasers) and can be summarized as follows ^[4,16,17]:

Fig. 6.

Different examples of laser warning signs ^[17].

Fig. 7.

Laser protective eyewear.