Ocular Risks Of UV

Ultraviolet radiation in sunlight is one of the main sources of exposure to radiation encountered by humans. Exposure levels vary widely with the time of day, season, geographic location and meteorological conditions. Outdoor UVR levels at mid-latitudes (less than 40 degrees) can exceed the threshold for damage to the skin and the eyes in as little as 10 minutes at mid-day during spring and summer.

Although the sun is the main source of exposure to UVR for the general population, some artificial sources such as welding arcs, germicidal lamps, and tanning lamps also produce UVR. UVR from artificial sources can be more intense than solar UVR and it usually has a different spectral content. Artificial sources can add significant UVR exposure in some occupational and recreational settings. An individual’s daily UVR dose consists of the sum of all the UVR exposures, natural or artificial, he or she experiences in the course of a day.

Sunburn is the skin’s visible reaction to acute overexposure to UVR. A painful eye irritation known as "welder’s flash" or "snow-blindness" is the acute effect of UVR on the eye. Long term exposure to ultraviolet radiation has been linked to some types of cataract and to other eye, skin, and systemic conditions.

Ultraviolet radiation (UVR), sometimes also called ultraviolet light, is invisible electromagnetic radiation of the same nature as visible light, but having shorter wavelengths and higher energies.

Sunlight is the most prevalent source of UVR. Ultraviolet radiation from sunlight is divided into three wavebands, UV-A, UV-B and UV-C (in order of decreasing wavelength). The lower the wavelength, the greater the biologic activity that results from overexposure. Only UV-A and UV-B reach the Earth; UV-C is absorbed by the ozone layer.

UV-A: 315 - 400 nm "Black light"
UV-B: 280 - 315 nm "Erythemal UV"
UV-C: 100 - 280 nm "Germicidal UV"

Ultraviolet (UV) radiation from the sun
UV band	Wavelengths (nm)	Absorption	Ocular hazard
vacuum	100 - 200	Exists only in a vacuum.	none
C	200 - 290	Completely absorbed in the upper atmosphere	none
B	290 - 320	Shorter l absorbed more by cornea / longer by lens. Small amounts may reach the retina.*	conjunctiva cornea lens retina
A	320 - 380	~ 2% reaches the retina	retina
* Radiation with wavelengths as low as 305 nm (in UV-B) may reach the retina in small amounts. Though the amounts are small, UV damage is cumulative, and over time it can cause retinal injury.

Due to their different wavelengths and energies, each of these bands has distinct effects on living tissue. The highest energy band, UV-C, can damage DNA and other molecules and is often used as a germicidal agent. UV-C is rapidly attenuated in air and, therefore, it is not found in ground-level solar radiation. Exposure to UV-C, however, can take place close to sources such as welding arcs or germicidal lamps. UV-B is the most effective UV band in causing tanning and sunburn (erythema) and it can affect the immune system. Although UV-A is the least energetic UV band, and much less effective than UV-B in causing erythema and tanning, it can cause these effects at levels present out-of-doors. UV-A penetrates deeper in the skin due to its longer wavelength and plays a role in skin photoaging. UV-A can also affect the immune system.

The various tissue layers of the eye absorb the wavebands of UVR to different extents. The longer the wavelength, the deeper the penetration of ocular tissue. Of the radiation from sunlight, UV-B has the greatest impact on the eye. Overexposure can result in photokeratitis or, in rare cases, solar maculopathy. Studies show that chronic exposure to UV-B can also cause cortical cataracts. UV-A is considered a greater threat to produce skin cancer than damage the eye.

While the ozone layer filters UV-C from the sun, a welder's arc can generate toxic dosages of this potent waveband. Since UV-C is the shortest waveband of the ultraviolet spectrum, the corneal epithelium will absorb it. A welder's flash burn therefore will cause a punctate epitheliopathy.

UVR can cause both acute and chronic ocular toxicity. Among the potential acute effects:

· Photokeratitis - Photokeratitis is an acute superficial "burn" of the corneal surface, resulting from short-term exposure to high-intensity UV radiation. It may follow exposure to a welding arc ("welder's flash" or "arc-eye") or as a result of exposure to UV radiation reflected from snow ("snow-blindness"). This condition, which usually subsides by 48 hours post exposure, is caused by UV-B and UV-C radiation. The term photoconjunctivitis applies to the conjunctiva when these tissues are similarly overexposed.

· Solar retinopathy/maculopathy - Solar retinopathy/maculopathy is an acute retinal burn, generally (but not exclusively) caused by prolonged, direct viewing of the sun when it is near the zenith. Retinal injury can be caused by a photochemical action of light in the blue range or by a thermal action of visible light and near infrared radiation. Cases of solar retinitis, including substantial loss of visual function, are often reported following unprotected observation of solar eclipses. If you were to stare directly at the sun, within about 20 seconds the UVR dosage would burn your macula. This damage is irreversible.

· Photic maculopathy - In the past, patients undergoing prolonged intraocular surgery in some cases developed photic maculopathy from the surgical lamp. Eventually it was discovered that short-wavelength visible light was responsible. Today's surgical lamps not only filter ultraviolet radiation but shorter-wavelength "blue light" as well.

· Cortical cataract - Epidemiological and clinical evidence shows a link between UVR overexposure and cortical cataracts. The mechanism may be lens epithelial cell death rather than a disruption of equatorial cells, as had been thought. Early and complete protection from environmental UVR may help forestall the formation of cortical cataracts.

· Pinguecula, pterygium and other keratopathies - Circumstantial evidence suggests that UVR overexposure may lead to these anterior segment degenerations. Firm causative data are lacking, but studies show compelling correlations.

· ARMD - Much has been written about the risks for age-related macular degeneration. Researchers have identified such risk factors as smoking, gender, family history and refractive status. Evidence also suggests that UVR and blue light may be causative agents as well.

For now, the best recommendation is the same as that for other chronic UVR-related toxic ocular conditions: complete protection. Lenses that filter UVR and also selectively absorb shorter-wavelength blue light may offer protection against ARMD.

Because UVR dosage is cumulative, protection should begin as early as possible and be as complete as possible. For those at greater risk-young patients, outdoor workers, skiers, etc.-the message is all the more urgent.

· Nonprescription sunglasses and prescription glasses - The goal ideally is to block all UVR from reaching the eye and as much as possible from hitting the adnexa. Prescription spectacle lenses can meet this criterion easily. Patients can obtain extra protection by wearing lenses with various coats or tints. Lenses whose absorbing properties vary depending on the incident light generally provide good UVR protection.

Animal research suggests that the culprit waveband in ARMD is the short-wavelength portion of the visible spectrum, or blue light. For this reason, those at risk might stave off the inevitable consequences by wearing spectacle lenses that selectively absorb in the short-wavelength portion of the visible spectrum. Such lenses are orange or red-orange.

· Hats - Patients can get further protection from overhead rays by wearing wide-brimmed hats and caps, and using other means of shading. One study found that billed caps and wide-brimmed hats can block 50 percent of overhead rays. This same study discovered that poorly fitted sunglasses may admit an additional 25 percent of overhead rays.

· Contact lenses - UVR-absorbing contact lenses can lend an extra measure of protection. Rigid contact lenses will absorb all the UVR that hits the lens. Yet, because of their smaller diameter, rigid lenses will not protect the entire cornea.

Larger, soft contact lenses with a UVR inhibitor will provide more complete protection to the cornea. Although these lenses will not absorb 100 percent of incident UVR, they can offer important supplementary protection.

Example drug	Use
Sulfonamides	chemotherapy, anti-infective
Tetracycline	anti-bacterial
Sulfonylureas	diabetes
Chlorothiazides	diuretic, hypertension
Phenothiazides	tranquilizers
Chloridazepoxide	tranquilizer
Furocumarins	vitiligo, psoriasis
oral contraceptives

The relative danger from UV radiation is greater today than in previous generations due to reduced protection from the ozone layer, change in clothing (no hats, short sleeves, etc.), more outdoor recreational activities and greater longevity. In order to reduce the risk of UV injury, avoid outdoor activity between 10:00 a.m. and 2:00 p.m. This cuts exposure to 50%.

*Leo P. Semes, O.D., Review of Optometry: Here Comes the Sun: Are Your Patients Protected?

*Pitts DG. Threat of ultraviolet radiation to the eye-how to protect against it. J Am Optom Assoc 1981;52(12):949-57.

*Taylor HR. Ultraviolet radiation and the eye: an epidemiologic study. Trans Am Ophthalmol Soc 1989;87:802-53.