Question

1. With the aid of a suitable diagram, explain the biological hazards of a laser and the necessary safety precautions when using the laser.

Answer 1

Laser radiation of sufficient intensity and wavelenghth, exposure time will lead to irreversable damage to eye and skin. The most common cause of laser hazard on tissue are of thermal in nature. Thermal effects occurs when the laser radiation is absorbed by the skin and the intensity of the damage it can cause depends upon the rise in temperature it leads to in an organism and the duration of this heating process. Depending on this temperature elevation different damage can occur, they are Hyperthermia, Coagulation and Ablation. In hyperthermia, the temperature rise of 41°C for about ten minutes leads to cellular death. In coagulation, a tissue temperature between 50°C and 100°C for about 1s, leads to  dessication, whitening and retraction of tissues due to protein and collagen denaturation. Tissues will afterwards be eliminated (detersion processes) and the wound will scar. In ablation, the temperatures is higher than 100°C. Due to this the cell and its elements evaporate within a relatively short time.

As mentioned earlier laser radiation of particular intensity for particular period of time leads to damage to eyes. When a high-power laser-beam travels through the eye, its power gets focused on a smaller spot, localized on the retina. This power concentrated on a small diameter spot creates irreversible damages to the eye. However, the power is in itself not the only danger for the eye. Indeed, some factors are as relevant as the power concerning the potential damages : wavelength, exposure duration and continuous/pulsed nature of the exposition. As eye is a complex organ constituted of many different biological and optical elements of different refractive indexes. So, while propagating through the eye, the light ray encounters mediums with different optical index and transparencies. Depending on the medium and on the wavelength of the beam, the effect will be very different. The refractive index of cornea is1.377. The most absorbed wavelengths of cornea are located in the far infrared domain (800 to 2400 nm) and in the ultraviolet domain (less than 300 – 400 nm). These wavelengths will thus trigger the most severe damages on cornea. The intensity of injuries will have different aspects depending on the frequency of the absorbed light. Weak ultraviolet rays UV-B, UV-C triggers conjunctivitis, epithelium photokeratisis and latencies. These lesions come with red blotches and lacrimation, and are not irreversible. They disappear after a maximum of 48 hours because of the natural recovery process of the eye.

Strong ultraviolet UV-B, UV-C mostly cause damages on the Bowman membrane and on the cornea's stroma. The Bowman layer never recovers from any lesion. The thickness of the Cornea is mostly due to the stroma. It is constituted of collagen fibers with a precise diameter (35 nm) and precise spacing (59 nm). These fibers are grouped in layers parallels to the surface of the cornea. These UV rays provoke neo-vascularization of the cornea, characterized by the appearance of blood capillaries. This process can lead to a worsening of the damages and finally to an oedema associated with the production of lactic acid. The accumulation of this acid is responsible for a milky aspect of the cornea leading to a loss of transparency. These lesions are irreversible, and the cornea is lost. One can proceed with surgery, but it leads to an opaque scar. In order to get the eye functional again, the only solution is to transplant a new cornea. Weakly energetic infrared rays slightly burn the epithelium and can create astigmatism. The lesions lead to an opacity having the same diameter as the beam. When the energy delivered by the beam is higher than a certain level (typically 30J/cm²), infrared rays can damage the stroma in the same manner as UV rays do (loss of transparency of the cornea). At such powers, this type of radiation is absorbed and converted into heat, thus creating a hole in the cornea and leading to a flow of aqueous humor. These damages are irreversible and require surgery, generally leading to ether an opaque scar, or a transplantation.

Iris is constituted of colored pigments which is responsible for the eye color. The pupil is located at its center. Laser radiation does not create irreversible lesions, but only pigmented areas. They appear after a laser impact, leading afterwards to an oedema and to the apparition of a miosis. However, lesions of this kind gradually disappear within 2 to 3 weeks. But in the case of repeated impacts, the pigments may migrate towards the anterior chamber, and the iris may atrophy or even tear. At high powers, the Iris loses its color on the impact site, and in the worst case gets paralyzed and finally necrosed. Similarly it can cause damage to crystalline lens and retina.

The imapct of laser radiation on skin is comparitevly lower on skin than on the eye. The power is generally not concentrated (in the absence of any optical element), and the pain perception is quicker. The skin mostly undergoes thermal damages, as the epidermis cannot stand thermal powers higher than a few 0.1 W/cm² continuously or a few W/cm² during short pulses (peak power). The thermal effect not only depends on the beam power, but also on the skin pigmentation type. The skin pigmentation protection efficiency depends on its coloration. For example, a 5 to 10 Joules pulse has no effect on a (reflective) white skin, while it burns a pigmented skin – this stresses the role of melanin and haemoglobin in the radiation absorption process. Visible and near infrared (<1.4 µm) radiations are mostly reflected by the skin, while the other frequencies are mostly absorbed. The latter frequencies will thus be responsible for most of the skin injuries. The different layers on skin have different resistances to radiation. The thick hyperkeratotic layers are resistant while the thinner layers in the dermis, and closer to the surface it is more sensitive. The observed lesions will therefore depend on the radiation frequency. UV rays, depending on their type (UV-A or UV-B), penetrate at different depths inside skin layers. UV-B rays are absorbed in the external layer - the epidermis – and are responsible for red blotches similar to sun burns. UV-A rays penetrate more deeply than the previous ones, and are responsible for many skin diseases. Both types are responsible for skin affections including ageing, erythema (red blotches), pigmentation increase, light-sensitization or even cancer. Visible and infrared radiations act on deeper layers through thermal effects. These radiation types can then induce vessels dilatation and red blotches, leading to skin burns from the surface to deeper layers.

The necessary safety precaution while using a laser are as follows:

• Before turning on the laser pointer, always be sure that it is pointed away from yourself and others.
• Never look directly into a laser pointer.
• Never direct a laser pointer at another person.
• Follow the same rules for direct reflections of laser light from reflective surfaces.
• Purchase laser safety eyewear appropriate for your laser.
• Ensure that your laser area has appropriate signs in and around it to prevent unwary intruders from being injured by entering unprotected.
• Procure and/or install laser safety equipment as is necessary for your laser. These may include permanent or temporary barriers or enclosures, laser safety glasses, laser safety apparel, laser signs, a special intrusion-proof power system, the permanent designation of a room or set of rooms, special locks, special paint, and the hiring of a Laser Safety Officer (LSO).
• Be knowledgeable of the laser system you are installing and what precautions are necessary for it. If you are not sure what you need to do, consult a laser professional such as a certified Laser Safety Officer (LSO).
• Never cut corners where laser safety is concerned.
• Educate all people who will be working in or near a laser area.