What is light?
Light especially sunlight is one of the enigmatic forces in the universe. Light has both particle and wave-like properties. Light is measured in wavelength which are usually expressed in nanometer [nm]
Our eyes can only see a portion of the whole electromagnetic spectrum of light emitted by the sun. These portions of light from sunlight that our eyes can see are called Visual Spectrum. Visual Spectrum falls between 400 – 700 nm. Other lights like Ultraviolet [UV] and Infra-red [IR] are imperceptible to the human eye. The eye cannot perceive them just because they fall below the visual spectrum. i.e. the wavelengths fall between 280 – 400 nm.
Dangers pose by sunlight to our eyes [light toxicity]
Sunlight is definitely associated with the formation of cataracts. The invisible UV component of sunlight promotes free-radical damage in the lens. The brighter the sun, the greater the UV radiation dose. The effect of radiation on our eyes, are cumulative throughout life. They add up as we go along. That is why skin cancers can develop four or five decades after major sun exposure. Cataracts are similar symptoms of overexposure to the sunshine.
Changes in the earth’s ozone layer mean that more UV is radiation is now reaching the earth’s surface. Therefore, the need to prevent cataracts will only increase. This is why it is advisable for everyone who is exposed to reflected UV light from snow, on beaches or tanning salon to cover their eye area with UV blocking sunglasses to prevent exposure to UV radiation. Nature did not intend for humans to be exposed to so much UV light.
The eye is so susceptible to damage as a result from heat from Infra-Red in the range of 800 – 13000 nm. Major damage can occur to the lens and retina. Infra-Red in form of extended source such as blast furnace can cause the protein in the eye lens to coagulate (solidify). This coagulation of lens can cause cataract. (Glass blower cataract)
Scientific explanation on the damaging effect of sunlight on the eye
The mechanisms of damage from UV exposure are photochemical and thermal. UV radiations are of 3 types, namely: UVA [380 – 315 nm], UVB [315 – 290 nm], and UVC [290 – 200 nm]. Photochemical damage to the eye is predominantly caused by the UVC and UVB wavebands. The thermal damage is caused by the UVA. Photochemical damage is usually within the cell nucleus of the lens. Damage results when the energy of the UV matches the receptor of the cell. Since the lens inside our eye is a protector against radiations, it must then absorb all the incident radiation of UV to be an effective protector. Thus radiation [UV] absorbed by our lens caused biological tissue damage. This is how cataracts are formed in our lens.
UVC is completely absorbed by the Ozone layer in the atmosphere (called Stratosphere} whereas UVA and UVB (above 288 nm) are transmitted through the Ozone layer to reach the earth. In the earth, UVB (below 295 nm) is absorbed by the cornea. This is what caused many people corneal burns and Pterygium. UVA can penetrate the corneal quite well but it significantly absorbed by the lens inside our eye. This is what caused Cataracts. Also, a small but significant amount of UVB and UVA (above 305 nm) that could filter through the corneal and lens is absorbed by the Retina. This cause what is known as Solar Retina burns.
Changes in environment [ozone layer depletion] pose the most serious concern for ocular problems from exposure to UV radiation. Yearly loses of Ozone layer has caused an increase in the amount of UVB reaching the earth (and our eye). Beginning in the early 1980’s, the atmospheric ozone layer has been declining by about 3 % per year. According to researches conducted in 19993 by Fredrick, a 1% decline in ozone layer results in a 1.3% increase in biological activity.
The environmental loses in stratospheric ozone results in dramatic increases in UV (especially UVB below 325 nm) that get to the earth and consequently our eye. This dramatic increase further emphasizes the need for eye and skin protection. The increasing access of UV radiation especially UVB waveband has been epidemiologically associated with human corneal damage, pterygium, pinguecula, climatic droplet keratopathy, cataracts and retinal burns or lesions.
Sources of Radiations
- The Sun (Natural source: The sun emits the highest proportion)
- Electric Arcs (used by welder). This is artificial source
- Sunlamp (Artificial source)
- Furnaces (used in steelmaking and glass blowing. This is an artificial source)
Ocular [eye] damage associated with UV radiation.
Ocular damage occurs due to absorption of radiation by the anterior structures of the eye (i.e. Cornea, Conjunctiva, Iris, etc) and the posterior surface (lens and retina). The eye damage associated with UVB and UVA exposure include:
- Pterygium – An abnormal tissue growth on the eye
- Pinguecula – A yellowish discolouration of the eyeball
- Corneal damage – e.g in corneal burns
- Lens damage – e.g. as in cataract
- Retinal damage – e.g as in retina burns
- Ultimately, radiation damage can cause age related macular degeneration. Macular degeneration is one of the most common cause of blindness in Western world.
People who require ocular protection against exposure to Ultraviolet Radiation
- People with retinal disorder
- Patient who had just had cataract operation (aphake)
- People with cataract
- People with pterygium and pinguecula
- People who are prescribed photosensitizing drugs (e.g. contraceptives, cholorothiazides, antibiotics, etc.
- Workers on vocations rich in UV radiation like Arc welding, Electronic chips, graphic arts, watermen, researchers,
- People on avocation rich in UV like – Snow skiing, Sunbathing at the beach, mountain climbing, etc.
- People who spend excessive hours in sunlight: UV exposure above 8 hours a day. E.g. policemen, traffic warden, open market women, taxi drivers, etc.
- People who use sunlamps or visit solariums: Solarium sources are rich in UVA and contain UVB, and both have been associated with skin cancer.
- Children who are exposed to excessive UV in sunlight.- this is to delay the photochemical damage to the cornea, lens, retina and skin.
The primary eye care practitioner is in particular good position to advise the patient or public in general, about the risk of exposure to UV radiation and the protection regime that should be used against UV radiation exposure.
The wavelength range of the sunlight that reaches the earth lies between 288 – 2600 nm and it contains almost all the sun’s total irradiance. The hours between 10.00 am and 2.00 pm are particularly important clinically, because over 50% of the UV radiation that reaches the earth in any 24 hours period arrives within the hours of 10.00 am to 2 .00 pm. Thus a patient can limit exposure to UV radiation by simply selecting the appropriate hours for outside activities. When unavoidable, wear protective sun lenses with UV blockers.
We can limit exposure to UV radiation by prudent selection of outdoor activities [like games]. For example, the beach sand reflects about 35% of solar UV radiation: whereas the green grass in the lawn tennis reflects about 3.5 of solar UV radiation. The freshly fallen snow reflects about 85 – 95% of the incident UV radiation. Also, the snow skier is at risk of about 15% increase in UV radiation for each kilometer [3000 ft] of altitude above sea level. So skiers should always protect themselves when on the ski slope.
People on the beach may think they are being protected from sunlight by lying under an umbrella but the umbrella itself acts as a collector of UV radiation from the sunlight being reflected from the sand and concentrate it to the area underneath the umbrella. So, when on the beach, protect your eyes by wearing sunshade with UV blockers.
Wearing a hat with a 4 inches brim reduces the ocular exposure to UV radiation in sunlight by about 50%.
Antioxidants like Vitamins C, Vitamin E, Selenium and the Carotenoids help to inhibit the damaging effects of UV light on the retina. Take these supplements as regular as possible.
Light is the great love the eyes can’t live with and can’t live without. While light allows us to see, excessive exposure to wrong kinds of light can severely damage our eyes [and our skin too], overtime. We live by light, we thrive by light, and we suffer from too much or too little light. We need light to reach our retina* to produce sight, just as we need it to reach our skin to produce Vitamin D, an essential hormone.
The good news is that light allows sight and can create psychological sense of wellbeing. The bad news is that too much light, especially UV light, can damage our eyes.
With the loss pf the ozone layer, one would expect a dramatic increase in UV related eye problems. So protection lies in clinical intervention by advising our patients to wear the proper UV absorbing ophthalmic lenses or sun-lenses. UV absorbing contact lenses and Intra-ocular lenses [IOL] afford protection for the cornea, lens, and retina against UV radiation.
Finally, this discussion clearly demonstrates that UV radiation should not be allowed to reach the human eye.