The eye, due to its anatomical position and its specific organic function, is continuously exposed to the stress caused by light.
Sunlight Sunlight is not white, but consists of a range of light radiations that form the so-called solar spectrum. The spectrum includes visible radiationperceived by the human eye, and non-visible radiation, respectively to the right (infrared rays) and to the left of the spectrum itself (ultraviolet-UV rays).
I UV radiation make up a total of about 8% of solar radiation, and are divided into:
- UV-A rays (320-400 nm) (75%): determine tanning and some photosensitive reactions.
- UV-B radiation (290-320 nm) (19%): they cause sunburn and are implicated in some forms of skin cancer.
- UV-C rays (200-290 nm) (approx. 6%) are by far the most dangerous and are absorbed by the ozone layer.
The blue light
The harmful effect of theradiant energy depends on its wavelength and, therefore, the energy content of the radiation. The shorter, near-ultraviolet wavelengths are much more harmful; in fact, the high-energy region of the visible spectrum (400 to 500 nm) is considerably more dangerous than the low-energy zone (500 to 700 nm). Since the transition occurs at the boundary between the colours green and blue, the phenomenon is commonly called blue light damage.
The eye and sunlight damage L'eyeDue to its anatomical position and specific organic function, it is continuously exposed to the stress caused by light. The ocular tissues absorb light radiation differently, depending on their wavelength.
The amount of radiation absorbed by the eye also varies, in relation to age.
In fact, the crystalline lens, which is the main protective factor against low-wavelength radiation, becomes less transparent as it ages and, therefore, more effective at absorbing radiation. In children, the amount of harmful light reaching the retina is greater: under the age of 10, more than 75% of ultraviolet radiation (UV-A and UV-B) is transmitted to the retina, and by the age of 25 this percentage is still 10%. The situation changes completely from the fifth decade of life onwards, when, as a result of the ageing process of the crystalline lens, the natural lens of the eye is able to absorb more potentially harmful radiation.
La cornea acts as the first filter for light radiation and is, therefore, the first structure to be affected by excessive exposure to sunlight. This increases the risk of certain corneo-conjunctival diseases.
The crystalline undergoes a physiological ageing process that can be accelerated and accentuated by UV accumulation, culminating in the development of cataracts due to denaturation and agglutination of constituent proteins.
La retina is naturally endowed with a certain resilience in the event of light damage, but if the intensity of radiation increases, the damage may become irreversible. The "point of no return"is given by the destruction of the cell bodies of the photoreceptors, cones and rods, which, as nerve cells, are unable to regenerate. Retinal diseases for which UV radiation and blue light have been shown to be involved in the induction or progression of retinal damage are: l'cystoid macular oedema, the solar retinopathy, i ocular melanomas and, above all, the age-related macular degenerationwhich is among the leading causes of blindness in Western countries.
More in detail the photic maculopathies are distinguished in three forms:
from sunlight (acute or chronic)
from non-sunlight
iatrogenic
Acute photic sun diseases, also known as solar retinopathy can have numerous causes: observation of a solar eclipse, religious ritual, collective suggestion phenomenon, intake of hallucinogens, pseudo-scientific methods to invigorate vision (Bates method), self-harm, occasional or professional.
From a clinical point of view theextent and severity of retinal damage depend on photobiological factors (such as intensity of incident radiation, climatic and geophysical conditions, exposure time) and individual factors (presence of refractive defects, presence or absence of natural crystalline lens, intake of photosensitising drugs).
Le different clinical variants depend on the mode of sunlight fixation (whether continuous or intermittent) and the type of (inadequate) screen used for observation.
[caption id="attachment_1131" align="aligncenter" width="400"] Solar retinopathy due to observation of an eclipse through an unimpressed photographic film (filter inadequate to protect the eye). Intraretinal macular oedema caused by solar radiation is visible in the white box.[/caption]
The first symptoms, apart from the sensation of glare and discomfort in the light, are the perception of afterimages and pink vision. After a few hours, a central zone of non-vision appears (central scotoma) that can last for weeks, months or forever, with severe and permanent visual impairment (for more details see IT-ARVO - Abstract "Solar retinopathy: a multimodal analysis").
The eye's natural defences
When the light is too intense, the eye uses natural protection systems:
the eyelid blink
changes in pupil diameter (miosis and mydriasis)
the dark colour of the iris, since the more intense pigmentation of the iris connective layer reduces the amount of light penetrating into the eye
the screen function of the crystalline lens
the eye pigments of the retina (lutein, zeaxanthin and melanin), which filter light, protecting nerve cells from phototoxic damage. In particular, lutein is able to filter blue light and thus protect the most delicate parts of the retina from the harmful effects of light radiation. Our body does not synthesise these pigments directly, but takes them in through our diet. When dietary intake is insufficient, it is possible to support their acquisition by taking food supplements.
The child's eye
The perfect transparency of the cornea and lens in children's eyes allows solar radiation to reach the retina more easily.
For this and many other reasons, the child's eye is even more vulnerable to sunlight than the adult's, and must be protected.
In particular, studies conducted on ocular malignant melanomas It was found that strong sunlight in the early years of life is an important risk factor for the occurrence of this type of eye cancer.
It is also possible that the Age-Related Macular Degeneration  may be the expression, in adulthood, of damage produced in early life due to excessive exposure to the UV-B. For prevent In children, the damage that can be caused to their retina and their eye in general by harmful light radiation is important:
- to wear a cap with a visor and sunglasses with special lenses that absorb potentially harmful radiation
- ensure that the child's diet provides a sufficient supply of protective substances, such as the lutein, the vitamin C, the vitamin E and omega-3 polyunsaturated fatty acids.
The hole in the ozone layer
[caption id="attachment_1137" align="alignleft" width="250"] Map of the ozone layer. NASA survey April 2014[/caption].
The amount of ultraviolet radiation reaching the earth depends on a combination of factors that include environmental conditions, the season and especially the thickness of the ozone layer.
Ozone is a 'greenhouse' gas that makes up the ozone layer, located in the upper part of the earth's atmosphere and capable of absorbing about 95% of UV-B rays, the totality of UV-C rays and of letting through, in addition to visible light, infrared and radio waves, a large part of UV-A rays, which allow us to tan (but also cause major damage to the epidermis!).
In 1974, Sherry Rowland first observed that the ozone layer had a marked thinning over the polar areas and, in 1985, it was discovered that this thinning, commonly referred to as the 'ozone hole', was increasing year by year.
The depletion of the ozone layer has as a direct consequence an increase in the amount of potentially harmful radiation reaching the Earth, which is among the causes of
important changes in the terrestrial and aquatic ecosystem and a documented increase in risk factors for the incidence of certain neoplasms, damage to the ocular system and reduction of the body's immune defences.
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