Biological Effects of Lasers--Guide to Laser Safety Management

INTRODUCTION

The chief concern over laser use has always been the possibility of eye injury.

While skin presents a larger target, it’s the possibility of injury to the eyes that drives laser safety, funding, controls, and application. The effect of laser radiation varies with the wavelength and the part of the eye it interacts with. In addition, biological effects from direct exposure and diffuse reflection exposure differ. This section explains the anatomy of the eye and skin and issues associated with biological effects.

THE EYE

The major danger of laser radiation is from beams entering the eye. The eye is the organ most sensitive to light. A laser beam with low divergence entering the eye can be focused down to an area 10 to 20 µm in diameter.

A 40-mW laser is capable of producing enough energy (when focused) to instantly burn through paper. The energy density (measure of energy per unit of area) of the laser beam increases as the spot size decreases. This means the energy of a laser beam can be intensified up to 100,000 times by the focusing action of the eye for visible and near-infrared (NIR) wavelengths. If the irradiance entering the eye is 1 mW/cm2, the irradiance at the retina will be 100 W/cm^2. Thus, even a low-power laser in the milliwatt range can cause a burn if focused directly onto the retina.

Light from an object (such as a tree) enters the eye first through the clear cornea and then through the pupil, the circular aperture (opening) in the iris.

Next, the light is converged by the lens to a nodal point immediately behind the lens; at that point, the image becomes inverted. The light progresses through the gelatinous vitreous humor and, ideally, back to a clear focus on the retina, the central area of which is the macula. In the retina, light impulses are changed into electrical signals and then sent along the optic nerve and back to the occipital (posterior) lobe of the brain, which interprets these electrical signals as visual images.

Damage to the eye is dependent upon the wavelength of the beam. In order to understand the possible health effects, it’s important to understand the functions of the major parts of the human eye, as shown in Figure 1.

===2.1 PARTS OF THE HUMAN EYE

===2.1.1 The Cornea

The cornea is the transparent layer of tissue covering the eye. Damage to the outer cornea may be uncomfortable (like a gritty feeling) or painful but will usually heal quickly. Damage to deeper layers of the cornea may cause permanent injury.

===2.1.2 The Lens

The lens focuses light to form images onto the retina. Over time, the lens becomes less pliable, making it more difficult to focus on near objects. With age, the lens also becomes cloudy and eventually opacifies. This is known as a cataract. Every lens develops cataracts eventually.

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Above: Fig. 1 Human eye components.

ciliary body pigment epithelium sclera choroid lamina cribosa optic nerve sheath macula lutea retina optic axis visual axis disc fovea lens cornea iris conjunctiva rectus tendon zonule fibers ora serrata aqueous humor Section human eye

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===2.1.3 The Retina

The part of the eye that provides the most acute vision is the fovea centralis (also called the macula lutea). This is a relatively small area of the retina (3 to 4%) that provides the most detailed and acute vision as well as color perception. This explains why eyes move when you read; the image has to be focused on the fovea for detailed perception. The rest of the retina perceives light and movement. If a laser burn occurs on the fovea, most fine (reading and working) vision may be lost. If a laser burn occurs in the peripheral vision, it may produce little or no effect on vision.

Four kinds of light-sensitive receptors are found in the retina: rods and three types of cones. Each cone is tuned to absorb light from a portion of the spectrum of visible light, long-wavelength light (red), middle-wavelength light (green), and short-wavelength light (blue). Each type of receptor has its own special pigment for absorbing light. Each consists of a transmembrane protein called opsin, which is coupled to the prosthetic group retinal. Retinal is a derivative of vitamin A and is used by all four types of receptors.

===2.2 BLINK AND AVERSION RESPONSE

Fortunately the eye has a self-defense mechanism: the blink and aversion response. Aversion response is the closing of the eyelid or movement of the head to avoid exposure to bright light. The aversion response is commonly assumed to occur within 0.25 sec and is only applicable to visible laser wavelengths. This response may defend the eye from damage where low-power lasers are involved but cannot help where high-power lasers are involved. With high-power lasers, the damage can occur in less than a quarter of a second.

A study by Reidenbach et al.* states that even for the larger spot size on the retina, the frequency of the blink reflex has been shown to be less than 35%, and the same is true for the maximum of the pupil size, that is, for low ambient light conditions. In this study 503 volunteers were irradiated in the lab and 690 in 4 different field trials with laser radiation. Out of these only 15.5% and 18.26%, respectively, showed a blink reflex. The respective numbers as a function of wavelength are 15.7% (670 nm), 17.2% (635 nm), and 22.4% (532 nm) for long-, middle-, and short-wavelength light. An increase from 4.2% to 28.1% in blink reflex frequency was achieved when the ambient illuminance was decreased from 1700 l× to 1× l using an LED as a large extended stimulating optical source instead of a collimated laser beam.

A further dependency was found concerning the irradiated area on the retina.

Increasing the retinal spot from 6.4 to 9.4 mm2 to 33.7 to 46.8 mm2 resulted in an increase of the blink reflex percentage from 20 to 33.3%.

* Reidenback, H.D., Hofmann, J., Dollinger, K., and Seckler, M. A Critical Consideration of the Blink Reflex as a Means for Laser Safety Regulations. International Laser Safety Conference, 2005.

===2.3 WHERE LASER RADIATION GOES

===2.3.1 Ultraviolet-B and Ultraviolet-C (100 to 315 nm)

The surface of the cornea absorbs all ultraviolet (UV) ranges of these wavelengths, which produce a photokeratitis (welders flash) from a photochemical process, which causes a denaturation of proteins in the cornea. This is a temporary condition because the corneal tissues regenerate very quickly, less than 24 hours.

===2.3.2 Ultraviolet-A (315 to 400 nm)

The cornea, lens, and aqueous humor let in UV radiation of these wavelengths, and the principal absorber is the lens. Photochemical processes denature proteins in the lens, resulting in the formation of cataracts.

===2.3.3 Visible Light and Infrared-A (400 to 1400 nm)

The cornea, lens, and vitreous fluid are transparent to wavelengths. Damage to the retinal tissue occurs by absorption of light, and its conversion to heat occurs by the melanin granules in the pigmented epithelium or by photochemical action to the photoreceptor. The focusing effects of the cornea and lens increase the irradiance on the retina by up to 100,000 times. For visible light of 400 to 700 nm, the aversion reflex, which takes 0.25 sec, may reduce exposure by causing the subject to turn away from a bright light source. However, this won’t occur if the intensity of the laser is great enough to produce damage in less than 0.25 sec or when light of 700 to 1400 nm (NIR) is used, as the human eye is insensitive to these wavelengths.

===2.3.4 Infrared-B and Infrared-C (1400 to 1.0 × 10 + 6 nm) Corneal tissue will absorb light with a wavelength longer than 1400 nm. Damage to the cornea results from the absorption of energy by tears and tissue fluids, causing a temperature increase and subsequent denaturation of protein in the corneal surface.

Above: Fig. ===2 Transmission and damage mechanism by wavelength.

Frequency Wavelength Photochemical Effects Thermal Effects UV A 320-400 nm Visible 400-700 nm IR A 700-1400 nm UV C 100-280 nm UV B 280-320 nm IR B 1400-3000 nm IR C 3000-1000000 nm Cornea Lens Retina Cornea; Cornea

===3 SIGNS OF EYE EXPOSURE

Symptoms of a laser burn in the eye include a headache shortly after exposure, excessive watering of the eyes, and sudden appearance of floaters. Floaters are swirling distortions that occur randomly in normal vision, usually after a blink or when eyes have been closed for a couple of seconds. Floaters are caused by dead cells that detach from the retina and choroid and float in the vitreous humor.

Ophthalmologists often dismiss minor laser injuries as floaters because of it’s difficult to detect minor retinal injuries. Minor corneal burns cause a gritty feeling, like sand in the eye.

Exposure to a visible laser beam can be detected by a bright color flash of the emitted wavelength and an after-image of its complementary color (e.g., a green 532-nm laser light would produce a green flash followed by a red after image). When the retina is affected, there may be difficulty in detecting blue or green colors secondary to cone damage, and pigmentation of the retina may be detected.

Exposure to the Q-switched Nd:YAG laser beam (1064 nm) is especially hazardous and may initially go undetected because the beam is invisible and the retina lacks pain sensory nerves. Photoacoustic retinal damage may be associated with an audible pop at the time of exposure. Visual disorientation caused by retinal damage may not be apparent until considerable thermal damage has occurred.

Exposure to the invisible carbon dioxide laser beam (10,600 nm) can be detected by a burning pain at the site of exposure on the cornea or sclera. Table 2.1 summarizes the hazards caused by various light wavelengths.

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TABLE ===1

Hazards Caused by Various Light Wavelengths Wavelength Range Effect on Eye Effect on Skin Ultraviolet C 200-280 nm Photokeratitis Erthema (sunburn)

Skin cancer Accelerated skin aging Ultraviolet B 280-315 nm -- Photokeratitis Increased pigmentation Ultraviolet A 315-400 nm Photochemical cataract Pigment darkening Skin burn Visible 400-700 nm Photochemical Thermal retinal injury Pigment darkening Skin burn Near-infrared 700-1,400 nm Cataract and retinal burn Skin burn Mid-infrared 1,400-3,000 nm, Corneal burn, Aqueous flare, cataract, Skin burn Far-infrared 3,000-100,000 nm Corneal burn Skin burn

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===4 DAMAGE MECHANISMS

===4.1 ELECTROMECHANICAL AND ACOUSTIC DAMAGE

This type of damage requires beams of extremely high-power density (10^9 to 10^12 W/cm^2) in extremely short pulses (nanoseconds) to delivery fluences of about 100 J/cm^2 and very high electric fields (10^6 to 10^7 V/cm), comparable to the average atomic or intermolecular electric field. Such a pulse induces dielectric breakdown in tissue, resulting in a microplasma or ionized volume with a large number of electrons. A localized mechanical rupture of tissue is caused by the shock wave associated with the plasma expansion. Laser pulses of less than 10 µsec can induce a shock wave in the retinal tissue that causes tissue rupture. This damage is permanent, as with a retinal burn. Acoustic damage is more destructive to the retina than a thermal burn. Acoustic damage usually affects a greater area of the retina, and the threshold energy for this effect is substantially lower. The ANSI MPE values are reduced for short laser pulses to protect against this effect.

===4.2 PHOTOABLATION

Photoablation is the photo-dissociation, or direct breaking, of intramolecular bonds in biopolymers, caused by absorption of incident photons and subsequent release of biological material. Molecules of collagen, For example, may dissociate by absorption of single photons in the 5- to 7-eV energy range. Excimer lasers at several UV wavelengths (ArF, 193 nm/6.4 eV; KrF, 248 nm/5 eV; XeCl, 308 nm/4 eV) with nanosecond pulses focused on tissue at power densities of about 108 W/cm^2 can produce this photoablative effect. UV radiation is extremely strongly absorbed by biomolecules, and thus absorption depths are small, of the order of a few micrometers.

===4.3 THERMAL DAMAGE

Thermal damage is caused by the conversion of laser energy into heat. With the laser's ability to focus on points a few micrometers or millimeters in diameter, high power densities can be spatially confined to heat target tissues. Depth of penetration into the tissue varies with wavelength of the incident radiation, deter mining the amount of tissue removal and bleeding control.

The photothermal process occurs first with the absorption of photon energy, producing a vibrational excited state in molecules, and then in elastic scattering with neighboring molecules, increasing their kinetic energy and creating a rise in temperature. Under normal conditions the kinetic energy per molecule (kT) is about 0.025 eV. Heating effects are largely controlled by molecular target absorption such as free water, hemoproteins, melanin, and other macromolecules such as nucleic acids.

===4.4 PHOTOCHEMICAL DAMAGE

Light below 400 nm does not focus on the retina. The light can be laser output, UV from the pump light, or blue light from a target interaction. The effect is cumulative over a period of days. The ANSI standard is designed to account only for exposure to laser light. If UV light from a pump light or blue light from a target interaction is emitted, additional precautions must be taken.

===5 LASER RADIATION EFFECTS ON SKIN

Laser radiation injury to the skin is normally considered less serious than injury to the eye, since functional loss of the eye is more debilitating than damage to the skin, although the injury thresholds for both skin and eyes are comparable (except in the retinal hazard region (400 to 1400 nm)). In the far-IR and far-UV regions of the spectrum, where optical radiation is not focused on the retina, skin injury thresholds are about the same as corneal injury thresholds. Obviously, the possibility of skin exposure is greater than that of eye exposure because of the skin's greater surface area.

The layers of the skin that are of concern in a discussion of laser hazards are the epidermis and the dermis. The epidermis layer lies beneath the stratum corneum and is the outermost living layer of the skin. The dermis mostly consists of connective tissue and lies beneath the epidermis. Figure 2.3 shows the three layers of the skin.

Above: Fig. ===3 Labeled cross section of human skin. ANATOMY OF THE SKIN Nerve Endings Sebaceous Gland Hair Follicle Sweat Gland Blood Vessel Epidermis Dermis Subcutaneous Muscle Bone

===5.1 EPIDERMIS

The epidermis is the outer layer of skin. The thickness of the epidermis varies in different types of skin. It’s the thinnest on the eyelids, at .05 mm, and the thickest on the palms and soles of the feet, at 1.5 mm. The epidermis is made up of five layers. From bottom to top the layers are stratum basale, stratum spinosum, stratum granulosum, stratum licidum, and stratum corneum. The bottom layer, the stratum basale, has cells that are shaped like columns. In this layer the cells divide and push already formed cells into higher layers. As the cells move into the higher layers, they flatten and eventually die. The top layer of the epidermis, the stratum corneum, is made of dead, flat skin cells that shed about every 2 weeks. There are three types of specialized cells in the epidermis.

The melanocyte produces pigment (melanin), the Langerhans cell is the frontline defense of the immune system in the skin, and the Merkel cell's function is not clearly known.

===5.2 DERMIS

The dermis also varies in thickness depending on the location of the skin. It’s 0.3 mm on the eyelid and 3.0 mm on the back. The dermis is composed of three types of tissue that are present throughout, not in layers. The types of tissue are collagen, elastic tissue, and reticular fibers.

The two layers of the dermis are the papillary and reticular layers. The upper, papillary, layer contains a thin arrangement of collagen fibers. The lower, reticular, layer is thicker and made of thick collagen fibers that are arranged parallel to the surface of the skin.

The dermis contains many specialized cells and structures. The hair follicles are situated here with the erector pili muscle that attaches to each follicle. Sebaceous (oil) glands and apocrine (scent) glands are associated with the follicle.

This layer also contains eccrine (sweat) glands, but they are not associated with hair follicles. Blood vessels and nerves course through this layer. The nerves transmit sensations of pain, itch, and temperature. There are also specialized nerve cells called Meissner's and Vater-Pacini corpuscles that transmit the sensations of touch and pressure.

===5.3 SUBCUTANEOUS TISSUE

The subcutaneous tissue is a layer of fat and connective tissue that houses larger blood vessels and nerves. This layer is important in the regulation of temperature of the skin and the body. The size of this layer varies throughout the body and from person to person.

Figure ===4 shows the depth of penetration into the skin for different wavelengths of laser radiation. There is quite a variation in depth of penetration over the range of wavelengths, with the maximum occurring around 700 to 1200 nm. Injury thresh olds resulting from exposure of less than 10 seconds to the skin from far-IR and far UV radiation are superficial and may involve changes to the outer dead layer of the skin. A temporary skin injury may be painful if sufficiently severe, but it will eventually heal, often without any sign of injury. Burns to larger areas of the skin are more serious, as they may lead to serious loss of body fluids. Hazardous exposure of large areas of the skin is unlikely to be encountered in normal laser work.

A sensation of warmth resulting from the absorption of laser energy normally provides adequate warning to prevent thermal injury to the skin from almost all lasers except for some high-power far-IR lasers. Any irradiance of 0.1 W/cm2 produces a sensation of warmth at diameters larger than 1 cm. One-tenth of this level can be readily sensed if a large portion of the body is exposed. Long-term exposure to UV lasers causes long-term delayed effects such as accelerated skin aging and skin cancer.

Above: Fig. ===4 Skin transmissions by wavelength.

To the skin, UV-A (0.315 to 0.400 µm) can cause hyperpigmentation and erythema. UV-B and UV-C, often collectively referred to as "actinic UV," can cause erythema and blistering, as they are absorbed in the epidermis. UV-B is a component of sunlight that is thought to have carcinogenic effects on the skin.

Exposure in the UV-B range is most injurious to skin. In addition to thermal injury caused by ultraviolet energy, there is the possibility of radiation carcinogenesis from UV-B (0.280 to 0.315 mm) either directly on DNA or from effects on potential carcinogenic intracellular viruses.

Exposure in the shorter UV-C (0.200 to 0.280 µm) and the longer UV-A ranges seems less harmful to human skin. The shorter wavelengths are absorbed in the outer dead layers of the epidermis (stratum corneum), and the longer wavelengths have an initial pigment-darkening effect followed by erythema if there is exposure to excessive levels. IR-A wavelengths of light are absorbed by the dermis and can cause deep heating of skin tissue.

===6 TISSUE OPTICS

An introduction to tissue optics will serve the reader well. Optical properties of skin reflect the structure and chemical composition of the skin. When a beam of light reaches the skin surface, part of it’s specularly reflected by the surface, while the rest is refracted and transmitted into the skin.

The light transmitted into the skin is scattered and absorbed by the skin. After multiple scattering, some of the transmitted light will reemerge through the air-stratum interface into the air. This reemergence is called diffuse reflection.

The amount of diffuse reflection is determined by the scattering and absorption properties of the skin tissue. The stronger the absorption, the less the diffuse reflection; the stronger the scattering, the larger the diffuse reflection. Following absorption of a photon by the skin, the electrically excited absorbing molecule may rapidly return to a more stable energy state by re-emission of a photon with lower energy, that is, fluorescence emission.

Figures 5 through 11 show examples of biological damage. There are several major contributors to the absorption spectrum. In the UV, absorption increases with shorter wavelength because of protein, DNA, and other molecules.

In the infrared, the absorption increases with longer wavelengths because of tissue water content. In the red to NIR absorption is minimal.

Whole blood is a strong absorber in the red to NIR regime, but because the volume fraction of blood is a few percent in tissues, the average absorption coefficient that affects light transport is moderate. However, when photons strike a blood vessel they encounter the full strong absorption of whole blood. Hence, local absorption properties govern light-tissue interactions, and average absorption properties govern light transport.

Melanosomes are also strong absorbers, but their volume fraction in the epidermis may be quite low, perhaps several percent. Again, the local interaction of light with the melanosomes is strong, but the melanosome contribution to the average absorption coefficient may modestly affect light transport.

Above: Fig. 5 Blood in vitreous.

Above: Fig. 6 Pooled blood.

Above: Fig. 7 Retinal injuries.

Above: Fig. 8 Retinal injuries. Human retina; fovea; optic nerve

Above: Fig. 9 Severe fovea and macula damage from NIR exposure.

Above: Fig. 10 Retinal injuries.

Above: Fig. 11 Corneal burn.

===6.1 SCATTERING CELLULAR STRUCTURES

Photons are most strongly scattered by structures whose size matches the photon wavelength. Scattering of light by structures on the same size scale as the photon wavelength is described by the Mie theory. Scattering of light by structures much smaller than the photon wavelength is called Rayleigh scattering. Following are some structures that scatter light.

===6.1.1 Mitochondria

Mitochondria are intracellular organelles about 1 µm in length (variable) that are composed of many folded internal lipid membranes. The basic lipid bilayer membrane is about 9 nm in width. The refractive index mismatch between the lipid and the surrounding aqueous medium causes strong scattering of light.

Folding of lipid membranes presents larger lipid structures that affect longer wavelengths of light. The density of lipid-water interfaces within the mitochon dria make them especially strong scatterers of light.

===6.1.2 Collagen Fibers, Fibrils, and Fibril Periodicity

Collagen fibers (about 2 to 3 µm in diameter) are composed of bundles of smaller collagen fibrils about 0.3 µm in diameter (variable). Rayleigh scattering from collagen fibers dominates scattering in the infrared wavelength range. On the ultrastructural level, fibrils are composed of entwined tropocollagen molecules.

The fibrils present a banded pattern of striations with 70-nm periodicity resulting from the staggered alignment of the tropocollagen molecules, which each have an electron-dense head group that appears dark in electron micrographs. The periodic fluctuations in refractive index on this ultrastructural level appear to contribute a Rayleigh scattering component that dominates the visible and UV wavelength ranges.