Laser risks

Risks of working with lasers

The directionality feature of laser beams makes them able to produce an extremely high power density (W/cm2). This feature, together with the laser beam’s specified wavelength (or several discrete and defined wavelengths), enables the laser beam to be focused into a small spot with the use of a lens.  The power density, which is already high, can increase significantly.

The laser's ability to concentrate intense power levels into a tiny spot is the basis for laser uses in many fields of science. It also enables lasers to be used as a powerful cutting tool for metals, hard ceramic materials and even diamonds. However, the high power density raises safety concerns for the working environment where laser systems are used, since the nominal outputs of most laser systems are significantly higher than the threshold levels allowed for exposure to the eyes or skin.

Biological laser risks

Exposing eyes to a laser beam can often result in severe visual impairment, and contact with the skin can also result in a deep and painful burn. Therefore, it is important to know the potential damage that can be caused by a laser beam, and how these dangers are associated with different characteristics of the laser beam, such as wavelength, intensity, duration of exposure and more. Armed with such information, a scale of increasingly severe risks can be formulated. The specific wavelength of a given laser, together with its position on the risk scale, enables us to determine, with a fairly high degree of certainty, the protective measures needed to work safely with and around the particular laser equipment.

Momentary (or longer) exposure of any tissue to laser beams may result in actual injury caused by one or more of the following three potentially harmful mechanisms:

1. Photothermal damage: The laser beam causes vibrations in molecules in biological tissue, thus creating heat in the tissue. The damage to the tissue ranges from alterations in protein properties (albumin) to burns, and tissue vaporizing until it carbonizes.
2. Photo-acoustic damage: High power density results in high local temperatures, causing the fluids in the tissue cells to change their state and become gases, thus altering their volume and creating a mechanical shock wave that spreads to the adjacent cells and is liable to cause them to tear.
3. Photochemical damage: Certain wavelengths in the UV spectrum and blue light in it cause reactions between organic molecules, or can destroy chemical connections in molecules. The effect is long  lasting.

In practice, the dominant damage mechanism in each individual case depends on the characteristics of the laser beam and the characteristics of the tissue.

Physical risks of the laser

Fire and explosion: High-intensity lasers can cause flammable materials (fabric, paper, plastic, wood, etc.) to explode upon contact with flammable liquids and gases.

Risk of electrocution: Caused by the high input voltage for laser systems.

UV (non-ionizing) radiation risks: Radiation in this spectral field originates from flash lamps and continuous laser discharge tubes (CW), especially when using piping or mirrors to transfer UV radiation beams (such as quartz).

Risks of ionizing and non-ionizing radiation:

An excimer laser uses electronic gas discharge tubes (gas electron tubes) that operate with a high voltage between their electrodes. The voltage between the anode and cathode in the tube can be over 5 kilowatts and its operation can cause the following symptoms:

• Plasma creation (ionized gas aggregation mode)
• Emission of non-ionizing radio frequency (RF) radiation and electrical radiation (ELF)
• Emission of X-ray radiation.

Chemical risks of lasers

1. Evaporation of toxic substances: The laser beam’s contact with chemicals can cause evaporation of toxic substances into the surrounding area.
2. Chemical risks from the particular laser source:
   • Color lasers containing toxic chemicals
   • Lasers containing toxic gasses such as fluorine
   • Leakage of the laser’s cooling liquids
   • Risk of oxygen displacement when using lasers with inert gases (nitrogen, helium)

Of all the body’s organs, the skin and eyes are the only ones that could be directly exposed to harmful laser radiation. The source of damage to these organs may involve the three previously described damage mechanisms (thermal, acoustic and chemical), but the relative contribution of each of these to damage may be influenced by the anatomy and the particular tissue components of the skin or eye. In addition, when discussing the nature of the potential damage, the body’s natural defense mechanisms that help us under certain conditions should be taken into account, and could reduce the severity of the damage in the following ways:

• The painful feeling that develops at the beginning of a burn causes us to move away from the beam, thereby reducing the severity of the possible burn. The painful feeling is felt mainly on exposure to laser beams in the visible and infrared spectra with a relatively large power density.
• The eyelid reflex (eyelid closure), activated when the eye is exposed to the intense brightness of light in the visible spectrum, helps reduce the amount of possible damage from the eye’s exposure to a laser beam in this spectrum. The eyelid’s maximal response time is estimated at a quarter of a second, so it provides partial protection only against exposure that lasts longer than that time.

Determining the specific damage to the skin or eyes therefore entails considering the specific structure of the tissues in each organ separately and the location of the possible laser damage.

Damage to skin

The thickness of the outermost skin layer, the epidermis, is 0.1-0.2 mm. This layer contains a high level of water content and granules of melanin. Beneath the epidermis is the dermis layer, which can measure up to 2 mm in thickness. Consisting of collagen and elastin fibers, the dermis also has a high water content. In this layer of the skin can be found the small capillary blood vessels and blood vessels with larger diameters. The subcutaneous (below the outer skin) skin layer contains mostly fat. In practice, because of the components found in the skin tissue, laser beams can cause multiple types of skin damage: thermal, acoustic and chemical, with the nature of the damage in each case depending on the given wavelength of the laser beam, its intensity and the size of the exposure area. For thermal and acoustic damage, it should be noted again that the damage is relative to the power density of the beam (W/cm2) where it comes into contact with skin.

Damage to eyes

Regarding potential laser damage to the eyes, it is important to emphasize the following:

1. The cornea and lens together form the eye’s optical imaging alignment, with a total optical power of approximately 50 diopters. To perform their functions, the cornea and lens must have a high degree of transparency so that the image they create on the retina is sharp and clear. Physical harm to the corneal tissue and lens, or impairment of their transparency, can be considered damage.
2. Vitreous fluid gives the eyeball its round, ball-like structure. In addition, like the cornea and the lens, the eyeball must have a high degree of transparency. Impairment of vitreous fluid transparency can be considered damage.
3. The retina serves as a light-sensitive surface where the image of the observed object is created. Here the optical image is transmuted into electrical signals transmitted to the brain to process the image. The retina is located on the focal plane of the optical imaging alignment, and for normal functioning, it must remain close to the inner wall of the eyeball. Separation of the retina from the eyeball’s inner wall or damage to part of its surface can be considered damage.
4. The visual center of the retina, (the macula, or the lighter central patch) plays a crucial role in the function of vision, since only this area has the ability to distinguish between colors. It is only at the center of this area (the fovea, with a diameter of only about 0.25 mm on the imaging alignment’s optical axis) that a sharp image produced. In many cases, damage to the visual center ends in severely impaired visual ability.

Because of the high directionality of the laser beam, it appears to the eye’s optical imaging alignment as an object located at infinity (or at a great distance).  Its image is focused to a tiny spot on the retina, where the power density can be over 100,000 times higher than that of the unfocused beam.  When looking directly at a laser beam, this tiny image falls exactly in the center of the retina's vision, thus posing a high risk of severe visual impairment and even blindness.

The variety of biological risks resulting from exposure to a laser beam were presented in the previous section in qualitative terms only, without linking the laser beam’s parameters (wavelength, intensity, etc.) to the expected extent of damage. In reality, the amount of expected damage must be quantified and linked to the laser beam’s parameters. A quantitative analysis of the extent of the damage, and a determination of how to address the various levels of damage, are carried out as follows:

1. Determining the permissible exposure levels that have no risk;
2. Dividing the higher exposure levels into several categories of risk;
3. Determining the necessary safeguards to prevent exposure to the various degrees of risk.

Exposure levels that have no biological risk are those that cannot be detected using conventional diagnostic measures, such as an ophthalmoscope, immediately following exposure to the laser beam or later. Permissible exposure levels that are applicable across all populations cannot be determined because of the natural range of anatomical characteristics among people (different skin pigmentation levels, epidermal thickness, etc.), and because the damage will vary according to these properties in the exposed area. Instead, threshold levels can be set that use statistical averages. Therefore, setting the permissible threshold levels is carried out as follows:

1. Determining the Exposure Dose ED50;
2. Determining the Maximum Permissible Exposure (MPE).

The exposure value ED50 indicates the level of exposure likely to cause minimal damage in 50% of those exposed to this level. This value includes statistical consideration of the range of values for anatomical characteristics. Of course, this level of exposure is dangerous for half the total population. Therefore, to protect the majority of the population, a lower exposure level is defined:

MPE - Maximum Permissible Exposure, according to the following relationship: MPE = (ED₅₀)/10.

This value specifies the maximum permissible exposure level at which there is no risk. It is frequently assumed that limiting exposure to a tenth of the value capable of causing minimal harm to 50% of the population has a large enough security coefficient to protect the general population. However, it is important to emphasize that such a sweeping assumption cannot actually be made, and, in fact, some (admittedly a small percentage) of the population is likely to still be harmed by exposure to the value defined by MPE. This is because the MPE value is based on a statistical mean. Therefore, for any given level, however low, there may always be an individual harmed, even at values less than the prescribed threshold. That is, lowering the MPE to a hundredth of the value of ED50 (or less) does not guarantee protection for the entire population. Furthermore, setting MPE to a value equal to ED50/100 or less would make it very difficult to use laser technology without first ensuring full protection for the general population. The trade-off between risks and needs was accounted for in defining the value of MPE, in addition to the general provision warning: "Do not take unnecessary actions, even if they involve exposure to values lower than permitted."

A laser's risk level is categorized according to the laser beam’s Accessible Emission Limit (AEL), allowing a "safety package" to be adapted to the laser. Categorization is made according to the degree of potential damage that the laser beam is capable of causing, applying the following criteria:

• The laser beam’s power relative to the maximum permissible exposure (MPE);
• The direct or dispersed beam’s ability to damage the eye;
• The direct beam’s ability to damage the skin or ignite flammable materials.

Risk Level 1: (Class 1)

This risk level refers to a laser product with a radiation level that is not dangerous. There may also be a situation where a dangerous laser is located inside a casing with safety systems that prevent any penetration of a dangerous laser beam. Any failure to fully engage the safety systems can increase the laser’s risk level.

Risk Level 1M: (Class 1M)

This risk level refers to a laser product with a radiation level that is not dangerous under normal circumstances. However, such a product’s beam can be dangerous to the eyes when viewed using a focused optical system.

Risk Level 2: (Class 2)

This level includes laser products that emit visible light (400-700 nm). Their radiation levels put the eye at risk only if exposure lasts more than 0.25 seconds (the longest response time for the blink reflex).

Risk Level 2M: (Class 2M)

This level refers to a laser product that emits visible light (nm 400-700). Its radiation level is not dangerous to the eye if exposure lasts less than 0.25 seconds (the longest response time for the blink reflex); such a product’s beam can be dangerous to the eyes when viewed using a focused optical system.

Risk Level 3R: (Class 3R)

This risk level includes laser products that pose a danger to the eye when there is direct contact with the beam’s radiation. The accessible emission limit for this risk level is up to five times that of Risk Level 2 in the visible light spectrum and up to five times that of Risk Level 1 in other spectra. It is dangerous to the eyes in the invisible spectrum from 0.7mW.

Risk Level 3B: (Class 3B)

This risk level refers to a laser product from which the radiation from a direct beam or reflected by a mirror is dangerous to the eye during any exposure, but is generally not dangerous to the skin.

Risk Level 4: (Class 4)

This risk level includes laser products whose contact with the eyes and skin is dangerous, whether with a direct, reflected or diffused beam. The beam of such a product can ignite flammable materials. Also, the laser beam, when interacting with matter, can create plasma and toxic volatile materials.