(Note: Emphasis in all quotes are ours)
Ocular Phototoxicity. Roberts JE. J Photochem Photobiol B 2001 Nov 15;64(2-3):136-43
" We have known for some time that exposure
to intense artificial light and sunlight either causes or exacerbates
age-related ocular diseases. We now know many of the reasons for these
effects"
"Because the eye is constantly subjected to ambient radiation, each
portion of the eye contains very efficient defense systems. ...
Unfortunately, most of these protective enzymes decrease beginning at 40
years of age."
"The leading cause of irreversible blindness in the increasingly aged
population is macular degeneration...Visible and/or blue light are
particularly damaging to the aging retina because it has lost its
anti-oxidant protection... At the same time these protective agents are
becoming depleted, the aging retina also accumulates fluorescent phototoxic
chromophores. Visible light activates these chromophores and produces
reactive oxygen species (ROS). The production of ROS in aged RPE (retinal
pigment epithelium) cells leads to apoptosis and cell death. One of the
functions of the RPE is to transport nutrients to the photoreceptor cells.
With the death of the RPE cells the photoreceptor cells are no longer
nourished and they die off. The result is as loss of vision."
"The photoreceptors of the retina present a puzzling phenomenon:
they can be injured or even destroyed by light, the very element they are
designed to detect."
"An intriguing question is whether certain wavelengths of the
visible spectrum may preferentially induce apoptosis in the retina. Linked
to this is the inquiry about the chromophore(s) and the death pathways
mediating light-induced apoptosis. We obtained a striking all-or-none
response when albino rats were exposed to either monochromatic blue light of
403 nm (3.1 mW/cm2) or monochromatic green light of 550 nm (8.7 mW/cm2)... No
apoptosis and no other light-induced lesions could be found in green
light-exposed eyes, whereas massive apoptotic cell death occurred after
illumination with blue light"
" The retina represents a paradox, in that, while light and oxygen
are essential for vision, these conditions also favour the formation of
reactive oxygen species leading to photochemical damage to the retina. Such
light damage seems to be multi-factorial and is dependent on the
photoreactivity of a variety of chromophores endogenous to the
retina."
"Light is essential for vision but the trade off is the generation of
potentially damaging reactive oxygen species within the eye."
"It is tempting to suggest that
the inter-individual variability in
the onset and severity of disease dependent on both light exposure and the
concentration of retinal chromophores."
"The high-energy segment of the visible region (400-500 nm) is enormously more hazardous than the low energy portion (from 500-700 nm). Because the transition occurs at the border between the perceived colors of green and blue, the phenomenon is known as blue light hazard".
"Blue-light injury can result from viewing either extremely bright light for a
short time or less bright light for a longer time."
"Excessive light exposure in the elderly may be particularly risky, since the
biological repair processes at the cellular level are generally considered to be less
effective as one ages."
Full Abstract"Evidence has accumulated that excessive light exposure may promote age-related and inherited retinal degeneration, in which photoreceptor death by apoptosis leads to loss of vision."
"In all animals, retinal light damage was the most severe when intense light exposure began during the dark period. However, this severe damage was significantly reduced by pretreatment with the antioxidant.... Our data support the notion of a circadian rhythm of light damage susceptibility that peaks in the dark period and yet can be modulated by the exogenous administration of an antioxidant."
"The retina can be damaged by light even when levels of energy are well below the threshold for thermal damage, and the experimental damage of the retinal pigment epithelium (RPE) may be induced more easily by blue light than by longer wavelengths of visible light."
"The effectiveness of light in inducing photodamage to the retina increases
with decreasing wavelength from 500 to 400 nm."
"It is now well established that photoretinopathy is a cumulative process, and
chronic light damage may be one of the factors contributing to the development of
age-related macular degeneration (AMRD)."
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