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Protecting Patients from Ultraviolet Radiation

Karl Citek, OD, PhD, FAAO

College of Optometry, Pacific University

2043 College Way

Forest Grove, OR 97116

 

Contents

 

Introduction

Paraphrasing Dorothy in the Wizard of Oz, "Melanomas, pterygia, conjunctivitis, keratitis, cataracts, and macular degenerations, oh my!" But instead of scary imaginary lions, and tigers, and bears that lived in the Kingdom of Oz, these are just some of the scary (and very real) eye conditions that can be caused by exposure to ultraviolet (UV) radiation.

This course will describe the occurrence of UV in natural and artificial lighting environments, the effects of UV on biological tissues in and around the eye, the use of materials and products that reduce skin and ocular exposure to UV, and how to treat patients who have been exposed to UV.

For the interested reader, an extensive overview of electromagnetic radiation is also included as an appendix, which may be consulted regarding definitions of concepts, terms, and units.

 

Radiation

The Spectrum

The electromagnetic spectrum (EMS) comprises radiant energy that extends from short-wavelength, high-energy gamma rays to long-wavelength, low-energy radio waves. (See Figure 1)

 

Figure 1. The electromagnetic spectrum.

 

The zones of the EMS are based on their source, function, or effect. There is significant overlap between the zones, depending on the intensity of the energy and/or exposure duration. In general:

 

Figure 2. Release of a photon from a radioactive atom.

 

Damage From Actinic Radiation

The adjective, “actinic,” is a general term, defined as pertaining to radiation capable of producing a chemical change. While the term is often used to describe the action of UV, it is not specific or unique to any particular wavelength or group of wavelengths (bandwidth). Any effect of radiation may be beneficial, harmless, or catastrophic for biological tissues, depending on whether the action has long-term consequences. Long-term phenomena are non-reversible and cumulative in nature. For example, the denaturation of protein fibers that occurs over time in the crystalline lens cannot be repaired by the eye itself, leading to the formation of a cataract.

Draper’s law states that energy must be absorbed in order to have an effect on the medium through which it travels. Figure 3 shows the relationship between wavelength, as measured in nanometers (nm), and energy, as measured in units of electron-volts (eV).

 

Figure 3. Photon energy (eV) with respect to wavelength (nm). C=O, carbon-oxygen double bond; C=C, carbon-carbon double bond; C-H, carbon-hydrogen bond; C-N, carbon-nitrogen bond. Blue line, boundary between UV and VIS (approx. 400 nm); purple line, wavelength of excimer laser, 193.3 nm.

 

Photons with particular minimum energies are capable of breaking various bonds between the carbon, nitrogen, and oxygen atoms that are ubiquitous in biological tissues. Structural changes of molecules can occur with energy as low as 3.1 eV, which is equivalent to a wavelength of 400 nm and visible as the color violet.

Long-term (chronic) exposure to such long-wavelength UV, combined with photochemical reactions caused by short-wavelength VIS up to 500 nm (blue-green), at environmental levels has been characterized as the “Blue Light Hazard.” This can contribute to retinal damage, such as age-related macular degeneration (AMD).

Effect of Radiation on the Eyes

While much of the radiant energy that strikes our bodies and eyes is harmlessly reflected or transmitted, even a low percentage of absorption can produce significant effects. Consider the last time you had your picture taken by someone using a camera with a flash. Even though very little of the incident light from the flash was absorbed by the photoreceptors in your eyes, the light was bright enough to produce an afterimage that probably lasted for several minutes. The “red eye” in the resulting photo was the reflection of the flash from your retina. Any remaining light, of course, passed harmlessly through the retina to be absorbed, without consequence, by the retinal pigment epithelium and the choroid.

X-rays

If the exposure had been made using x-rays, you would have seen nothing, but photographic film behind you would have an image of your bones and any other dense masses in your body. Soft tissues (e.g., skin, muscle, internal organs) readily transmit x-rays, while bones, teeth, and other dense structures reflect x-rays. However, small amounts of x-rays are absorbed by all biological tissues, producing cellular changes that ultimately can result in cancer following prolonged and/or repeated exposure. Cataract is the most common effect on the eyes caused by prolonged or repeated exposure to x-rays. That is why the x-ray technician leaves the area even when the x-ray source is not facing her.

UV

A harmless ocular effect of radiation occurs, for example, when the crystalline lens is irradiated with a low-intensity “black light.” The lens will fluoresce and emit a greenish glow. (See Figure 4) The fluorescence disappears as soon as the UV source is removed, and the UV will have no deleterious effects if there was only a short period of exposure.

 

Figure 4. Fluorescence of the crystalline caused by UV-A irradiation (“black light”).

 

Vision

A beneficial effect of radiation occurs when “normal intensity” light is captured by photoreceptors, resulting in a reversible photochemical change of the photopigments in rods or cones, ultimately producing vision. However, directly viewing a high-intensity source causes a non-reversible photochemical change if the exposure period is long enough. For example, sustained fixation for several minutes of the sun at midday eventually results in photoreceptor death, known as solar retinopathy. Note that viewing the sun directly at sunrise or sunset does not produce the same catastrophic result, since much of the sun’s radiation is scattered and filtered by the atmosphere.

Laser Pointers

Handheld laser pointers produce retinal effects that also depend on exposure duration and intensity. Laser pointers typically are available in green (532 nm) and red (630 to 670 nm). Because the monochromatic coherent beam is quite intense, even for low-power battery-operated pointers, a long-lasting afterimage results following direct exposure of a few seconds. Permanent retinal damage can result after several minutes of sustained viewing. An argon green laser, operating at 514 nm (shorter wavelength and higher energy than handheld pointers) and at much higher intensities (thus requiring much shorter exposure duration), is used for some retinal photocoagulation procedures. Retinal photocoagulation also can be performed with a high-intensity krypton red laser (647 nm).

Ophthalmic Equipment

Certain diagnostic techniques, such as binocular indirect ophthalmoscopy and slit lamp biomicroscopy with a high plus lens, can induce long-lasting afterimages if the light is directed at one location on the patient's retina for an extended period. For example, the recommended maximum permissible viewing time for a biomicroscope set on high illumination with a 78D lens is 36 sec; with a Superfield NC lens it is 57 sec; with a Super 66 lens it is 32 sec; and with a 90D lens it is 52 sec.

Such procedures, as well as overall exposure to light, should also be kept to a minimum for patients with retinitis pigmentosa, because the condition can be exacerbated by exposure to light.

 

The UV Spectrum

Types of UV Radiation

UV radiation may be divided into four zones, based on the observed effects, as described in the US sun eyewear standard (ANSI Z80.3-2001) and elsewhere. Typical transitions for the zones are at wavelengths of 290 nm (UV-C to UV-B), 315 nm (UV-B to UV-A), and 380 nm (UV-A to VIS), as shown in Table 1.

 

Table 1. UV zones and approximate bandwidths. Black, commonly accepted range; gray, extended range and overlap with neighboring zone.

 

There are no sharp cut-offs between any of these zones. As a matter of fact, the International Commission on Illumination (CIE) and the Australia/New Zealand sun eyewear standard (AS/NZ 1067: 2003) extend UV-A up to 400 nm. In addition, the CIE, sun eyewear standards of the European Community (EN1836: 1997) and Australia/New Zealand (AS/NZ 1067: 2003), and the occupational eyewear standard in the U.S. (ANSI Z87.1-2003) extend UV-B down to 280 nm.

In general, sun eyewear standards do not address protecting the wearer from UV-C or shorter wavelengths, since these rarely occur in the natural environment. However, ocular (and skin) protection from these short wavelengths is considered in various occupational safety standards.

Some authorities combine far and extreme UV as a single zone, or they ignore extreme UV altogether other than when considering specific industrial environments. Also, some texts extend UV-B up to 320 nm and others extend extreme UV down to 10 nm, where there is significant overlap with x-rays.

Effects on Biological Tissues

The descriptive names are not limited to the particular zones. For example, extended exposure durations to wavelengths up to 340 nm can cause skin reddening (erythema). Similarly, wavelengths below 280 nm are used in fast-acting water disinfection systems. However, exposing water to wavelengths at 320 and 400 nm at environmental levels for 1.5 to 3 hours, respectively, can kill 99.9% of microorganisms such as E. coli and streptococcus.

The effects of UV are understood when one considers that wavelengths below 400 nm are capable both of creating ions and radicals, and of breaking molecular bonds. Consequently, changes can occur to cell structures and even to intracellular components. Breaking bonds of protein molecules causes denaturation, producing opacification (i.e., cataract) in the crystalline lens. (See Figure 5)

 

Figure 5. Cataract.(http://www.mrcophth.com)

 

Breaking bonds of collagen molecules, as in the skin, causes cinching of the fibers, producing sagging, leatheriness, and possible wrinkling. Alteration of DNA sequences within cells is the mechanism that can trigger irregular or abnormal cell replication. (See Figure 6)

 

Figure 6. DNA mutation with UV irradiation. (www.answers.com)

 

If the DNA cannot repair itself, the cell can become cancerous. Excessive UV exposure also can suppress the immune system, thus making an individual more susceptible to infectious diseases. Likewise, UV exposure can trigger an outbreak of a dormant condition, such as herpes simplex virus.

 

Penetration of UV into Tissue

The penetration and absorption of UV varies by wavelength, depending on the tissue upon which it is incident. The longer the wavelength, the deeper the radiation will penetrate through the skin or the structures of the eye. (See Figures 7 and 8)

 

Figure 7. UV penetrance of skin.

 

Figure 8. UV penetrance of eye.

 

Tears, cornea, aqueous, and vitreous can potentially transmit wavelengths as short as 290 nm. The crystalline lens of a child transmits wavelengths as short as 310 nm, while that of an older adult only transmits wavelengths of 375 nm and above. Thus, the need for ocular UV protection for children and aphakic patients is even more critical than for adults.