Depletion in ozone has led to an increase in UVB radiation reaching the earths surface, with many negative consequences for the eye for example pterygium, cataract, age related macular degeneration and exfoliation syndrome. Due to organisations taking control over carbon emission which can affect the ozone layer it is thought the ozone depletion should become low, and therefore reduce the amount of UVB radiation which reaches the earth's surface, however this is difficult to evaluate. In the meantime it is very important for understanding the need for the correct eye protection from these harmful rays to avoid the risks of eye diseases which can be triggered due to the high UV radiation.
Ultraviolet radiation (UVR) is electromagnetic radiation divided into 3 different categories based on their wavelength range: UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm). The shorter wavelength corresponds to a higher frequency, the higher the frequency the greater energy it has to break bonds betweens cells, UVC is the most damaging UVR however it is little concern to human health nearly all of UVC is prevented in reaching the earth's surface by the stratosphere (Osborne, 2007). The eyes are exposed to UVR daily throughout our lives (Bergmanson and Soderberg, 1995). The main source of UVR is from the sun. In addition to UVR from the direct sunlight the environment includes UVR which is scattered off the molecules of the atmosphere and reflection from clouds, ground and other surfaces. When optical radiation is incident on a surface, it is transmitted absorbed or reflected, if the surface is highly polished almost all the incident radiant light rays are reflected in accordance with the Law of Reflection: Angle of incidence equals the angle of reflection (Cullen, 2011).
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The utmost importance of understanding the need for protection from harmful UVR is because of the increasing amount of UVB reaching the earth's surface which is a result of the decrease in the stratospheric ozone levels (Kerr and McElroy, 1993). Ozone molecules in the stratosphere protect humans and ecosystems on earth from the harmful effects of UVR, primarily UVB (Cullen, 2011). Ozone is made in the stratosphere in which UVC coverts oxygen into ozone molecules, this is done by UVC photons which exhibit high energy and they first split some of the oxygen molecules into free individual oxygen atoms, which then interact with some of the oxygen molecules forming ozone. The ozone molecules are very efficient at absorbing UVB radiation, and therefore split again into free individual oxygen molecules again. Heating in the air is caused by this ability of the ozone molecules for strongly absorbing UVB radiation. Most of the ozone is present at a height of 20-30 km however some ozone extends to ground level. Radiation is stronger in the layers higher up in the stratosphere and therefore the higher layers are heated up more compared to the lower layers. This results in the higher layer to have a lower density compared to the lower layers and therefore resulting in little vertical movement of air. The problem of ozone depletion occurs due to the emission of artificially generated substances which are broken down by UVC if they reach the upper stratosphere where they combine with the free oxygen molecules preventing the formation of ozone. Due to the already very little ozone in the atmosphere this depletion in ozone can cause serious consequences. (Bjorn and McKenzie, 2008).
The future of the depletion of ozone is uncertain due to an increase in the amount of greenhouse gasses which may not only increase the earth's surface temperatures but also decrease the amount of ozone (Bjorn and McKenzie, 2008). However due to the Montreal protocol the vast amount of depleting ozone is beginning to settle down and decline more slowly and as a consequence an increase in stratospheric ozone may be expected which will therefore reduce the levels of UVB exposure on the earth's surface (Maione et al., 2013). A recent analysis of the total ozone data from multiple satellite instruments from years between 1979-2009 have seen an increase in ozone levels over Antarctica (Salby et al., 2011).
Solar elevation angle highly determines the environmental exposure to UVB radiation. The length of the pathway through the ozone layer of the upper stratosphere is smallest when the sun is high in the sky and resulting in the most UVB ground surface exposure. A simple and universal rule known by many is called the 'shadow rule' in which states people must consider protective clothing whenever a person's shadow is shorter than their height (Sliney, 2002). However unstated this rule of thumb is more focused towards skin protection as a study found this rule not to follow eye protection, using UVR censored dummy heads they measured ocular UVR exposure as a function of the time of day in September and November. They found ocular protection is needed between the times of 8-10AM and 2-4PM due to the angle of the sun in relation to the eyes in those times of day (Sasaki et al., 2011). In the study dummy heads were used and in relation to real human being heads will differ in size and shape as all individuals have different facial features, giving them different kinds of coverage some more than other due to deep set eyes for example.
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Figure 1Very little harmful UVR reaches the retina. Unlike UVR visible light passes through all the layers of the eye hitting the photoreceptors on the retina and forms an image in the brain. UVR however is mainly absorbed by the anterior structures of the eye; most of the UVR are absorbed by the cornea, followed by the aqueous and the lens (Figure 1 showing the relative absorption of visible and UV radiation within ocular structures (Sliney, 2002)) (Sliney, 2002). As well as the intraocular protective structures the facial features and characteristics of human responses also protect the retina. The position of the upper eyelid and brow ridge creates a shadow effect over the front surface of the eyes, from below the cheeks and the nasal side of the bridge of the nose creates a shadow. A physical tendency to screw up eyes and covering direct sunlight with hands is a normal human characteristic in protecting oneself from the harmful rays of sunlight, as well as the normal physiological response of pupils constricting in bright light (Sasaki et al., 2011).
The superficial structures of the eye: the cornea and conjunctiva are particularly vulnerable to UVR. Many studies have taken place showing the chronic effects UV may have on the eye; however the majority of these studies have focused their attention on pterygium and cataract. Therefore, more focus is necessary on the diseases which have some uncertainty with the associations with UVR exposure such as age-related macular degeneration, pingecula, ocular melanoma and exfoliation syndrome.
Pterygium is a triangular shaped fibrovascular subepithelial ingrowth of degenerative bulbar conjunctival tissue over the limbus and onto the cornea. Initially it appears as a small, grey corneal opacity near the nasal limbal region. The conjunctiva grows over the corneal opacity and moves towards the cornea in a triangular manner (J., 2003). The development of pterygium appears to have a significant association with UVR exposure, a study (Sekelj et al., 2007) took place to evaluate the possibility of UVR as an associated factor of primary and pterygium recurrence. The study involved 58 subjects who had a conjunctival autograft transplantation which was a form of surgical treatment for pterygium. The subjects were divided into 2 groups: group 1 had a history of working outdoors in the sun for a minimum of 5 hours, group 2 had a history of working indoors.1 month after surgery had taken place the subjects returned to workplace and pterygium recurrence rate was measured. The results showed 65.52% of workers with primary and recurrent pterygium were from group 1 and had greater UVR exposure and 34.48% were from group 2 and had little UVR exposure. The recurrence rate in group 1 was 27.03% and group 2 was 10%, the study therefore concluded there is a significantly higher incidence of pterygium recurrence in individuals who are exposed to sunlight for a longer period of time. This study may show UVR has some sort of interaction with the eye enforcing pterygium formation however there are many other aspects which were not taken into account which may show unreliable results, with this specific study for example it was stated group 2 had no UVR exposure this would obviously not be the case as the majority of people are outdoors for some reason if not for work purposes, which indicates they would also have UVR exposure. This may therefore suggest in group 2 the 34.48% of subjects who had pterygium could be due to the unknown UVR exposure or again due to another external factor. Other factors for example such as working environment were not taken into account as it is known factors such as allergens, noxious chemicals and irritants may also contribute to pterygium formation (Mackenzie et al., 1992) (Taylor, 1989), group 1 included farmers and construction workers who are evidently exposed to such factors on their daily working basis. However, the study has shown chronic exposure to UVR to have a rate of 65.62% increase in pterygium formation which compliments others studies which also show a rate of reported pterygium between 40-70%. (Yan et al., 2006) also found a positive correlation between UVR exposure and the formation of pterygium. He compared 95 subjects with pterygium with individuals matched exactly the same as the subjects in terms of clothes, race, gender the only difference was the amount of UVR exposure.
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The prevalence of pterygium in various countries have been measured and they all show similar results suggesting the increase of sunlight to have an increase in the rate of pterygium in the population. A study (Yoon et al., 2011) found the occurrence of pterygium increases with sunlight exposure, age and occurs more in males compared to females. Similarly (McCarty et al., 2000) found a greater prevalence in males compared to females and also found country residents had a rate of pterygium 5 times greater than the urban residents. This could suggest the country areas which are less developed will have more open spaced land compared to the urban areas which are more developed and therefore may have many taller buildings which could block out more UVR. Also urban residents are more likely to have non-agricultural jobs which would suggest they work mostly indoors and would therefore have less direct sunlight compared to the country residents. Greater prevalence in males compared to females may be due to cultural reasons in which males are taking part in more outdoor work than females who prefer to work indoors. In the Tibetan culture in which involves females working outdoors found women had a greater risk than males in having pterygium (Lu et al., 2007). A recent study taken place in Iran (Rezvan et al., 2012) similarly found a greater prevalence of pterygium formation in males compared to females which would suggest the culture and how males are more likely to take part in outdoor work. They found outdoor workers had a greater prevalence than indoor workers again suggesting the greater sunlight exposure to be the cause of this, however this could be due to external factors such as dust. A negative correlation was found of pterygium and education levels, pterygium prevalence decreased with an increase in education levels (illiterate to college), this may be due to an increase in education the individual may have a better understanding of the need for eye care and sun protection and therefore wears protective eyewear/clothing in sunlight or it may be due to the fact that a greater education would suggest a better job which would be indoors rather than working outdoors. Supporting these results similar findings were also confirmed in other studies (Khoo et al., 1998).
Epidemiological studies have suggested an association between chronic exposure to sunlight with an increased geographical prevalence within an area of latitude 37 north and south of the equator (Lue and Chen, 2009). This is supported in the study (Rezvan et al., 2012) which states the very high pterygium prevalence's in Brazil, Peru and South of China. This would support UVR has an effect on the prevalence of pterygium as stronger sunlight in areas near the equator and low latitude suggests the greater UVR reaching those geographical areas. Up to 20% of incident rays of UV are scattered and reflected from the surface of the sea, this supports the finding of the greater prevalence of pterygium in fishermen and sailors who are exposed to the greater amount of UV (Diffey, 2002).
All the studies mentioned had in common the fact that the prevalence of pterygium formation increased with age, which supports the theory of chronic exposure of UVR causes pterygium. As people age, cells become more susceptible to get damaged and cellular repair mechanisms and control of cellular proliferation become less effective, therefore pterygium may develop in a process similar to neoplastic proliferation (Kwok and Coroneo, 1994) (Tan et al., 1997). It has been suggested scattered light may expose the basal stem cells at the limbal region to increased amounts of UVR, leading to mutations in tumour suppressor genes such as p53 and the alterations in the expression of various growth factors such as VEGFA (Detorakis and Spandidos, 2009). Another theory is UVB irradiation may cause the release of pro-inflammatory cytokines into tears bathing the mucosal surface, with result in chronic inflammation and fibrovascular proliferation leading to pterygium formation (Norval et al., 2011).
The crystalline lens is an avascular, transparent biconvex shaped structure enclosed in a capsule. Any opacity in the lens capsule is a cataract. There are 3 main types of cataract: Posterior subcapsular in which a cataract lies in front of the posterior capsule and obtains a plaque-like appearance, nuclear cataract in which changes in the lens nucleus causing yellowing of the lens as the cataract progresses in severity it begins to appear brown and cortical cataract in which the opacities begin between lens fibres and appear as radial spoke like opacities. Cataract's begin and reduce contract sensitivity, as they progress they may obstruct vision depending on their location within the lens (J., 2003).
Many epidemiological studies have concluded UVR exposure is an important risk factor for cataract. A epidemiological study (McCarty et al., 1999) measured the prevalence for cataract in Australia of the population aged 40 years and over, they found a significant risk factor for posterior subcapsular cataract and also individuals with cortical cataract who had a greater mean of UVB exposure compared to the people who did not have cortical cataract. However they also found many other aspects of human health which could contribute to the formation of cataract these include: medication, diabetes, long duration of gout, hypertension and smoking.
A study (Wong et al., 1993) conducted assessments of subject's eye health after recruiting them from 15 main fishing towns in total they collected results from 685 subjects aged between 55-74 years old. The results from the study showed a higher prevalence of nuclear and cortical cataracts with the subjects with a greater sun exposure. However, little information was collected about the subject's background and health/medication information which are all factors which may also affect cataract formation. The method recruiting subjects for the purpose of the study does not reflect the entire fishing community as it was advertised as posters, people would personally come to the test centre to measure the health of the eyes, surely people with problems would be more inclined to have their eyes tested rather than people who believe their eyes are in good health.
Similarly, the epidemiological study (Delcourt et al., 2000) undertaken in France found similar findings in which subjects who spend a greater time outdoors had a greater prevalence for cortical cataract compared to subjects who spent more time indoors. It was found that females had a greater prevalence for cataract unlike pterygium formation, this is thought to be due to the different hormone levels in females compared to males (Cumming and Mitchell, 1997). Similarly to pterygium they found subjects with a higher level of education had a 40% lower risk of developing cataract, this could be due to similar reason to pterygium formation which is a greater level of education may suggest those people have a greater understanding about the need for sun protection or have better jobs and therefore spend more of their time working indoors i.e. offices compared to working outdoors. Similar (Health, 1991) found activities related to greater sunlight exposure were associated with an increase in prevalence of cortical or mixed cataract (mixed cataract involves cortical cataract in 90% of cases), as well as other factors such as poor education and use of cortisone.
(Andley et al., 2011) investigated whether UV blocking contact lenses protect against UVB radiation-induced damage to the lens in which human epithelial cell line was exposed to UVR for 2 minutes with or without being covered by a UV blocking contact lens. They found significant protection of the human epithelial cell line from the UV blocking contact lens. However, only simple cells were used and therefore different effects may occur with a whole lens taking into consideration lens depth. Also, the cell line was exposed to UVB for a very short period of time and therefore it does not show the long term protecting effect of the UV blocking contact lens.
The common factor in the studies mentioned above are the prevalence of cataract formation increased with age, which supports the theory of chronic exposure of UV causes cataract. It is important to remember epidemiological studies cannot control all external factors such as the subject's diet, genes, and social behaviours. As it is known there are many other factors which are known to be a risk factor for cataract formation: smoking, medication (McCarty et al., 1999), age, myopia and diabetes (Duan et al., 2013).
Age-related macular degeneration (AMD) mostly becomes apparent after 50 years of age and is a disease of the macular area (J., 2003) it is more common in the dry form but the more severe form is Wet AMD and is the most common loss of vision in developed countries (Norval et al., 2011). There are currently no effective treatment strategies for most patients with AMD and therefore attention is more focused on efforts to prevent or stop the progression of ADM via their diet or by quitting smoking (Sin et al., 2013).
Many studies have associated the link between chronic UVR exposure and AMD formation. AMD is significantly more common in higher UVR exposure environments or in populations which have a greater UVR exposure for example, an epidemiological study (Vojnikovic et al., 2007) which was conducted in Croatian Island, having one of the highest solar radiation in Europe, found an incidence of AMD of 18% in the fishermen and agricultural group and only 2.5% in the urban population group. The results show how UVR exposure has a significant risk of AMD prevalence however the group sizes may not represent the whole of the population because from group 1 of the fisherman and agricultural workers present were 1300 subjects and group 2 of the urban workers were 71 . However, a study conducted to examine the link between sunlight and AMD (Fletcher et al., 2008) involved 4753 subjects from different areas of Europe, underwent fundus photography and interviewed about lifetime sunlight exposure found people with greater lifetime exposure of sunlight had a greater prevalence of AMD. Similarly (Jia et al., 2011) also found a greater prevalence in people with greater sunlight exposure compared to people with little exposure to sunlight.
Alternatively (Tomany et al., 2004) examined subjects in a period of 5 or 10 years, also found a relationship between environmental exposure to sunlight and the prevalence of AMD however they also found the subjects who wore protective factors such as sunglasses and hats also found AMD in those subjects. As well as many studies which show a link between sunlight exposure and prevalence of AMD, there are also many studies which show the opposite. (Hirvela et al., 1996) examined 560 subjects and found the only prevalence of AMD was increasing age, and found no associations between, working outside, myopia, glaucoma, diabetes and smoking. (McCarty et al., 2001) found no significant difference between lifetime exposure and AMD, they found age and smoking for long period of their life had an association with AMD. Similarly, (Delcourt et al., 2001) found no association between lifetime sunlight exposure and prevalence of AMD as well as (Anonymous and Age Related Eye Dis Study Res, 2005) who only found associated factors were smoking and high BMI. Other risk factors found for AMD are cholesterol, diabetes (Tan et al., 2007) and hyperopia (You et al., 2012).
Exfoliation syndrome (ES) is the growth of extracellular matrix material which can appear on the lens, zonules, cilary body, iris, trabceular meshwork and conjunctiva, it is formed by abnormal basements membrane of aging epithelial cells (J., 2003). Exfoliation syndrome has studies which support the association between ES and greater UVR exposure.
(Vojnikovic et al., 2007) conducted a study to measure the association between UVR exposure and AMD prevalence however; they also found exfoliation syndrome was seen in 28% of one of the groups which had greater sunlight exposure as they took part in agricultural or fishing occupations and 0% in the urban population which was group 2. A epidemiological study (Stein et al., 2011) conducted of 626901 subjects from a period of 6 years from 47 states of America assessed the risk of ES and geographical latitude, they found an increased risk of ES in the northern tier residence (above 42 N) and a lower risk in the southern tier (below 37 N), they also found every additional sunny day increased the risk of ES by 1.5%, however, external factors such as diet and lifestyle were not taken into consideration. A more recent study confirms this, (Kang et al., 2012) found individuals who lived further south in the U.S. are less likely to develop ES compared those living in the northern states of America.
Hospital based study in Pakistan (Rao et al., 2006) conducted of 1860 patients, found an ES prevalence of 6.45%. However, the patients were recruited from hospital which represents less of the population of Pakistan as hospitals are expensive and not everyone has the opportunity for hospital treatment. In other parts of the world the prevalence of ES in order of increasing latitudes (2013) : Sri Lanka (7.5 N) 1.1% (Rudkin et al., 2008), Nigeria (7.62) 2.7% (Olawoye et al., 2012), South India (22 N) 3.8% prevalence of ES (Arvind et al., 2003), Jordan 9.1% (31.9 N) (Al-Bdour et al., 2008) the prevalence of ES increased along with increasing latitude, however, these studies do not take into account external factors such as diet, lifestyle or genetics.
Sunglasses are made to national standards to protect the individual from harmful UVR while maintaining the optical quality and not sacrificing transmission of visible light. There are different national standards for sunglasses American standards (ANSI Z80.3), Australian standards which are combined with the New Zealand standards (AS/NZS 1067) and the European standard (EN1836). The European standards and the Australian standards are very similar and only differ in their definition of UVA; European defines UVA as 315-380 nm and Australian standards defines UVA as 315-400 nm. The standards also differ in the amount UV transmission European standards states UVB transmission should be no more than 5% and the American standards state UVB transmission should be no more than 1% (Almutawa et al., 2013). Unlike the European and Australian standards in which sunglasses must be built to the standards, the American standards are not mandatory and they are not required to create or label their sunglasses to the requirements (Wang et al., 2010) this could potentially cause more harm, as a pair of sunglasses could provide shade with little UV protection as the lack of light will dilate the pupil enabling more UV radiation to enter through the pupil, whereas the consumers would purchase the product believing they are protecting their eyes and due to little understanding they are causing more damage. This also suggests either the lack of importance or understanding of the need for correct eye protection in America.
A study compared the difference in UVB transmission for various types of sunglasses (Rosenthal et al., 1988). They found sunglasses provided greater protection from UVB compared to un-tinted prescription eyewear however this may be due to the uncontrolled factor of different eye sizes. They also found a displacement of sunglasses by 6 mm from the forehead the majority of the eyewear allowed 20% of incident UV radiation to reach the eyes.
Figure 2The perfect pair of sunglasses as well has providing UVR protection should also provide excellent coverage, more ocular damage occurs from UVR from scattered and reflected light from the periphery (Figure 2- light rays reaching the eye around a pair of sunglasses from different directions (Sliney, 2002)), hence it is very important for sunglasses to be closely wrapped around the eyes, however, most sunglasses are used as fashion purposes and do not fulfil such requirements, this can either be seen as the lack of knowledge of UVR people who purchase such frames. Sunglasses fitted with not enough coverage allow more UVR to reach the surface of the eye from around the frame. Also, the pupil which will be covered by the lens will have a larger pupil and will therefore enable more light to enter the pupil from the periphery of the frame (Segre et al., 1981) similar finding were reported in (Rosenthal et al., 1986). (Sakamoto et al., 1999) found the eye size and back surface reflection to be important factors in protecting the eyes from UV rays.
(Lu et al., 2007) found sunglasses/crystal spectacles protected the subjects from prevalence of pterygium, the crystal spectacles are uniquely worn by Tibetans, and they similarly protect the individual from UV radiation and are composed of crystallised stone.
There are many studies which show an association between greater UVR exposure and prevalence for eye diseases, and many of the studies show an increase in eye protection and coverage provides adequate protection from such diseases. However, all the studies which took place have recorded their subjects lifetime sunlight exposure subjectively via a questionnaire, this can cause unreliable results because an individual may estimate their exposure to be greater or lower than it actually is, however, apart from a questionnaire it is very difficult to measure life time sunlight exposure. Sunlight exposure differs daily due to rain or cloud coverage therefore it is again difficult to measure the amount of sunlight an individual is exposed to. Other risk factors for the diseases are also difficult to measure for example, diet, medication, genetics and lifestyle and other external factors which could affect the study's results. It is important to be aware and make a greater awareness of UV protection to enable people to make the correct choices for UV protection for example purchasing the correct sunglasses with the correct marking on them stating they follow the standards and provide the correct UV protection and wearing hats and general UV protection. This will promote greater safety from sunlight resulting in less prevalence of eye diseases.
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