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Monday, September 04, 2006

What Sunglasses Protect the Best?

Introduction

To address the question "What type of sunglasses are most effective in the prevention of ocular exposure to ultraviolet radiation?" it is necessary to examine available varieties of sunglasses and the purpose of each. The question of protecting the eyes from ultraviolet radiation (UV) and other hazards will be examined.
UV Radiation
Ultraviolet radiation is a major component of sunlight and artificial light sources (such as tanning beds or welding torches) which cannot be felt or seen but is associated with damage to the surface of the eye. UV radiation is divided into UV-A and UV-B. UV-A is responsible for skin tanning and browning whereas UV-B is responsible for burning, blistering, and cancer. A common example of UV damage to the eye is "snow blindness" or photokeratitis where the cornea of the eye is "burned" by UV radiation. Despite the painful nature of this condition, it is not serious and usually heals within a few days. More recent studies indicate long term UV exposure may eventually damage the retina of the eye. Unlike the cornea, a damaged retina cannot be replaced, making the consequences far more severe. This damage is photochemical in nature rather than produced by heat; therefore, damage is not felt or seen. Because of this, close attention to the UV Index can provide a guide to how much exposure your eyes are likely to receive.
The UV Index
The UV Index was developed by the National Weather Services (NWS) and the Environmental Protection Agency (EPA) to describe the level of UV exposure on a 0-10 scale in order to prevent overexposure.


0-2………………Minimal Exposure
3-4………………Low
5-6……………….Moderate
7-9……………… High
10+………………Very High

The UV Index is a general guideline for overall UV exposure during the day. Simply because the UV Index is low for a given day does not mean protection should be abandoned, rather, extra protection should be added (such as a hat) on high index days.
Ocular Damage
Although the eye generally prevents most light waves (including UV) from reaching the retina by either reflecting or absorbing the waves in the tissues in front of the retina, long-term exposure is associated with increased risk for cataracts and macular degeneration. Conclusive evidence for a causal relationship between UV exposure and cataracts, macular degeneration, or other diseases of the eye is not established (American Academy of Ophthalmology, 1997), however, support for the use eye protection is nearly universal. Most professionals agree the potential for harm exists…even if not yet proven…and prevention measures to reduce the potential risk are inexpensive and readily available.
If the need to prevent UV exposure is accepted, then "what type of sunglasses are most effective?" becomes an important issue; not only to minimize the level of UV exposure, but to ensure the use of sunglasses does not inadvertently lead to greater damage. Many persons falsely assume the darker the sunglasses, the greater the protection. This assumption is potentially more harmful than no protection. The darker tint of the sunglasses causes the pupil to dilate, leaving the lens of the eye and retina exposed to more UV light than if no sunglasses were worn. For maximum protection, it is essential to understand what sunglasses are available and what protection each offers.
UV Protection
Sunglasses are able to protect against UV exposure through a chemical agent (such as CR-39) added to the lens which absorbs UV radiation. This chemical is waterproof, invisible, and odorless once applied. It is possible to have UV protection (and is recommended) on regular clear lenses as well as sunglasses. UV protection should not be confused with tint, coloring, polarization, or other characteristics of sunglasses.
Types of sunglasses
Sunglasses are highly variable in both price and protection. Consumers often believe high priced sunglasses are more protective than less expensive brands but this is not necessarily true. According to a study by the National Institute for Occupational Safety and Health (NIOSH) on average those sunglasses affording maximum UV protection cost only $1.50 more than those with the least protection. (Lasden, 1988). The Sunglass Association of America has endorsed a labeling plan designed to provide consumers with information regarding the UV protection of sunglasses. There are three general categories: special purpose lenses that block 99% of UV rays, general purpose lenses that block 95%, and cosmetic lenses that "hinder" 70% of rays. It should be noted however, that manufacturers are not required to abide by these labels and may use similar but not equal terms. For example, sunglasses labeled "UV absorbing" may absorb most UV-B but little/less UV-A whereas "UV absorption up to 400nm" is the same thing as 100% UV absorption.
Sunglass Terminology
To offer assistance in purchasing sunglasses, here is a list of common terms.
· "Blocks 99% (or 100%) of ultraviolet rays" (Excellent)
· "Blocks 99-100% of all UV light" (Excellent)
· "UV absorption up to 400 nm" (Excellent)
· "Blocks harmful UV" This does not specify how much: Caution!
· "Blocks 90% of infrared rays" Infrared is not the same as UV. Infrared is associated with heat production.
· "Blue-Blocking" The role of blue light in eye disease is still controversial but amber lenses are popular among outdoorsmen.
· "Polarized" Once again, this does nothing toward UV protection. Polarized lenses cut the amount of glare and may be combined with UV protection in some glasses.
· "Mirror-coated" Thin layers of a metallic coating are applied to an ordinary lens. This reduces the amount of visible light entering the eyes but not necessarily UV radiation.
· "Wraparound" These glasses are shaped to keep light from entering around the sides of ordinary sunglasses. By providing protection from all angles, these reduce the risk of UV exposure if combined with UV lenses.
· "Gradient" Gradient lenses are shaded from dark to lighter but afford no extra protection.
· "Photochromatic" These lenses automatically darken or lighten according to the environment and may or may not have UV protection.
· "Impact resistant" All sunglasses must meet impact standards set by the Federal Food and Drug Administration (FDA) but no lens is unbreakable.
· "Scratch resistant" This is often useful especially with plastic lenses with may scratch easily but does not offer greater UV protection.
High Risk Individuals
Some persons are at greater risk for potential UV related eye damage. These include persons with retinal eye diseases such as macular degeneration or retinitis pigmentosa; persons who have had cataract surgery especially those without UV absorbent implants; contact lens wearers; persons who are "sun sensitive" or taking photosensitizing drugs. Additionally, persons exposed to high index levels on a regular basis should take extra precautions; This may include children, construction workers, etcetera. To reduce the potential risk for UV damage, quality sunglasses should be worn during all exposure.
Recommended Features
The American Optometric Association, recommends purchasing sunglasses which:
· Block at least 99% of UV-A and UV-B radiation
· Screen out 75-90% of visible light
· Have the American Optometric Associations Seal of Acceptance
· A green, gray, or brown lens tint
· Have protective shatterproof lenses for children or active adults
· Provide wraparound protection

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