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How UV rays damage young eyes

Idol Eyes Australia blog – All about sunglasses and eyes

I originally intended for this blog to be an easy read for young mums and dads explaining the need to protect young eyes from harmful UV radiation damage. Hence the simplicity of the blog and the terms used so far. Recently I have been receiving emails asking for more information regarding how UV rays actually damages young eyes. Luckily there has been a lot of scientific study on this topic.

The main point remains true, the only way to protect the eyes from harmful UV radiation is by wearing good quality sunglasses from birth.


How UV rays damage young eyes.

The human eye is exquisitely sensitive to light (i.e., visible radiant energy), and when the eye is dark-adapted, the retina can detect only a few photons of blue-green light. It is therefore not at all surprising that ocular tissues are also more vulnerable to ultraviolet (UV) and light damage than the skin. For this reason, humans have evolved with certain anatomical, physiological, and behavioral traits that protect this critical organ from the intense overhead solar ultraviolet radiation (UVR) when we are outdoors during daylight. For example, the UV exposure threshold dose for photo-keratitis (“welder’s flash” or “snow blindness”) – if measured as falling on a horizontal ground surface, would be reached in less than 10 minutes around midday in the summer sun.

There are three critical ocular structures that could be affected by UV exposure: the cornea, the lens, and the retina. UV damage to these structures causes eye diseases such as cataracts, pterygium, actinic & droplet keratites, pinguecula and macular degeneration.

Diagram 1 shows that the cornea transmits radiant energy only at 295nm and above. Therefore all UV below 295nm is absorbed by the cornea. 92% of UV radiation at 300nm and 37% of UV from 340nm to 360nm is also absorbed by the cornea. (All this absorbed UV in the cornea can cause pterygium, actinic & droplet keratites plus pinguecula). The aqueous humor then absorbs some UV before reaching the lens. The crystalline lens absorbs almost all incident energy wavelengths of 300nm to nearly 400nm. (All this absorbed UV in the lens causes cataracts plus hardening and yellowing of the lens). But in youth, a very small amount of UV-A reaches the retina, note the lens becomes more absorbing with age. Thus, there are intraocular filters that effectively filter different parts of the UV spectrum and allow only in the order of 1% or less to actually reach the retina, except in children under 9 years. Nevertheless, this small fraction of energy (if photo-toxic) could still be of concern. Finally, oblique rays entering the eye from the temporal side, can actually reach the equatorial (germinative) area of the lens.

UV absorption within an eye
Diagram 1. UV absorption within an eye

Diagram 2 clearly displays how 2-5%* of UV rays at 320nm reach the retina of children under 9 years, while none of these more dangerous UV rays reach the retina of older age groups. *Note the large variation of 2-5% UV rays at 320nm reaching the retina, this is due to the large variation of age in this group. Roughly babies will have 5% UV rays reaching the retina while a 9 year old will receive 2%. From 10 years of age this 1-2% of UV rays above 340nm still reaching the retina will diminish to 1% by age 30 and less than 1% by 60-70 years.

UV + visible light absorption within an eye of children below 9 years
Diagram 2. UV + visible light absorption within an eye of children below 9 years

Diagram 3 indicates that from age 10 until young adulthood vision should be good. Very little UV is reaching the retina but still around 48% of UV rays from 320nm to 400nm are entering the lens of the eye. These UV rays reaching the lens of the eye causes cataracts, hardening and yellowing of the lens making focusing a problem leading to prescription eyewear at around 40 to 45 years.

UV + visible light absorption within an eye of 10 years to young adult
Diagram 3. UV + visible light absorption within an eye of 10 years to young adult

In Diagram 4 (age 60-70) there are clear signs of UV damage and yellowing of the cornea, with the lens now restricting the amount of visible light reaching the retina. Visible light at 400nm has gone from 15% down to 1%, and visible light at 460nm has gone down from 65% down to 40%. This makes seeing in the dark harder, resulting in the need for really bright lights to read.

UV + visible light absorption within an eye of 60 -70 years adult
Diagram 4. UV + visible light absorption within an eye of 60 -70 years adult

How UV energy is absorbed – the magic of the chromophore.

For optical radiation to have an effect on matter the radiation needs to be absorbed, i.e.the radiant energy needs to be transferred to the material in which the effect is to occur. Two main mechanisms can be distinguished through which the absorbed radiant energy can take effect:
a) Heat: radiant energy is converted into molecular motion (kinetic energy) such as vibration, rotation and translation. Thus the temperature is increased (photo-thermal effect). Here, the radiant energy (measured in Joules, J) absorbed per unit time (s) in a certain volume determines the rise in temperature, i.e. the absorbed radiant power (J/s = Watt, W) per unit volume (m3) or the (specific) absorption rate (W/m3) is the determining factor (next to how fast the absorbing volume is cooled by heat exchange with its environment).
b) Photo-chemistry: radiant energy can cause excitation of atoms or molecules by moving the outermost (valence) electrons to higher orbital energy levels. This energy can subsequently be utilized in (photo-) chemical reactions, yielding “photo-products”. The radiation needs to be within a certain wavelength range (the “absorption band”) for the excitation to take place as the radiant energy is absorbed in discrete quanta, “photons”, which must match the energy required for the excitation. The (part of the) molecule that absorbs the radiation is dubbed the chromophore.

Chromophores and their absorption bands. (adapted from Jagger 1967)
Chromophores and their absorption bands. (adapted from Jagger 1967)

Of the three types of optical radiation, UV radiation is photo-chemically most active (the photons carry the highest energy), and it is absorbed by certain common chromophores in organic molecules (e.g. C=O, C=S and aromatic rings; the latter are abundantly present in DNA (Figure 1)). Clearly, light is also photo-chemically active in the eye: visual perception starts with the photo-isomerisation of opsin proteins (in G-protein coupled receptors which trigger the neural signalling).

Protecting the eyes from UV rays is easy and does work.

The majority of UV damage to eyes is caused by an accumulation of small amounts of damage over many years. Just by wearing a good quality, close fitting pair of sunglasses with 100% UV protection when outdoors, you can eliminate your chance of  getting UV related eye diseases. And yes, this does work, I personally know of people in their seventies who have protected their eyes with sunglasses every-time they are outdoors, and who’s eyes show no signs of UV related ageing, retaining 20/20 vision so they don’t even require prescription eyewear to see.

References:

  1. European Commission: SCENIHR
  2. Sliney 2002
  3. Jagger 1967

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