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Here are some of the medical literature results that I found in my limited
search using Pubmed.  (By the way, a search on "sextant" produced only 29
references: 28 regarding prostate cancer, and 1 on dental work.  None had
anything to do with the sextants we use in navigation.  So the world's
medical literature appears to be silent on the issue of sextants and human
health.)

How light reaches the eye and its components. Sliney DH. Int J Toxicol. 2002
Nov-Dec;21(6):501-9.  United States Army Center for Health Promotion and
Preventive Medicine, Aberdeen Proving Ground, Maryland 21010-5403, USA.
David.Sliney@apg.amedd.army.mil
"The human eye is exquisitely sensitive to light (i.e., visible radiant
energy), and when dark-adapted, the retina can detect 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 UV damage
that would otherwise be certain from the intense bath of overhead solar
ultraviolet radiation (UVR) when we are outdoors during daylight. For
example, the UV exposure threshold dose for photokeratitis ("welders' 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. The cornea
transmits radiant energy only at 295 nm and above. The crystalline lens
absorbs almost all incident energy to wavelengths of nearly 400 nm. In
youth, a very small amount of UV-A reaches the retina, but the lens becomes
more absorbing with age. Thus there are intraocular filters that effectively
filter different parts of the UV spectrum and allow only of the order of 1%
or less to actually reach the retina. Nevertheless, this small fraction of
energy--if phototoxic--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."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=12537646


Phototoxicity to the retina: mechanisms of damage.  Glickman RD. Int J
Toxicol. 2002 Nov-Dec;21(6):473-90. Department of Ophthalmology, University
of Texas Health Science Center, San Antonio, Texas 78229-3900, USA.
glickman@uthscsa.edu
"Light damage to the retina occurs through three general mechanisms
involving thermal, mechanical, or photochemical effects. The particular
mechanism activated depends on the wavelength and exposure duration of the
injuring light. The transitions between the various light damage mechanism
may overlap to some extent. Energy confinement is a key concept in
understanding or predicting the type of damage mechanism produced by a given
light exposure. As light energy (either from a laser or an incoherent
source) is deposited in the retina, its penetration through, and its
absorption in, various tissue compartments is determined by its wavelength.
Strongly absorbing tissue components will tend to "concentrate" the light
energy. The effect of absorbed light energy largely depends on the rate of
energy deposition, which is correlated with the exposure duration. If the
rate of energy deposition is too low to produce an appreciable temperature
increase in the tissue, then any resulting tissue damage necessarily occurs
because of chemical (oxidative) reactions induced by absorption of energetic
photons (photochemical damage). If the rate of energy deposition is faster
than the rate of thermal diffusion (thermal confinement), then the
temperature of the exposed tissue rises. If a critical temperature is
reached (typically about 10 degrees C above basal), then thermal damage
occurs. If the light energy is deposited faster than mechanical relaxation
can occur (stress confinement), then a thermoelastic pressure wave is
produced, and tissue is disrupted by shear forces or by cavitation-nonlinear
effects. Very recent evidence suggests that ultrashort laser pulses can
produce tissue damage through nonlinear and photochemical mechanisms; the
latter because of two-photon excitation of cellular chromophores. In
addition to tissue damage caused directly by light absorption, light
toxicity can be produced by the presence of photosensitizing agents. Drugs
excited to reactive states by ultraviolet (UV) or visible light produce
damage by type I (free radical) and type II (oxygen dependent) mechanisms.
Some commonly used drugs, such as certain antibiotics, nonsteroidal
anti-inflammatory drugs (NSAIDs), and psychotherapeutic agents, as well as
some popular herbal medicines, can produce ocular phototoxicity. Specific
cellular effects and damage end points characteristic of light damage
mechanisms are described."

Eclipse retinopathy.Michaelides M, Rajendram R, Marshall J, Keightley S.
Eye. 2001 Apr;15(Pt 2):148-51.
Department of Ophthalmology, Queen Mary's Hospital, Sidcup, Kent, UK.
drmichelmichaelides@hotmail.com
"PURPOSE: Solar retinopathy is a well-recognised clinical entity of macular
damage caused by viewing the sun, induced by a photochemical process. The
term 'eclipse retinopathy' is frequently employed when the condition is
sustained as a result of viewing a solar eclipse. Considerable public
excitement had been raised in anticipation of the full solar eclipse on 11
August 1999. Whilst experience has shown that visual morbidity is likely to
be temporary, current evidence is anecdotal and restricted to isolated case
reports and series. This study was conducted to establish the true visual
morbidity associated with a solar eclipse, and whether it was temporary or
permanent. METHODS: A 3 month active case ascertainment study was carried
out from July to September 1999 to record cases presenting to
ophthalmologists with visual symptoms arising from solar viewing. Further
information about the cases was sought using a short questionnaire. A
follow-up questionnaire requesting outcome data at 6 months was also
employed. RESULTS: There were 70 reported cases of visual loss. The average
age was 29.5 +/- 12.9 years. Half the cases presented to an ophthalmologist
within 2 days of the eclipse. An abnormal macular appearance was reported in
84% of patients at presentation. There have been no reported cases of
continued visual loss or symptoms at 6 months. CONCLUSIONS: This is the
largest nationwide study of the visual effects of a solar eclipse ever
undertaken. There were no recorded cases of permanent visual loss, which
corroborates the previous evidence that visual morbidity is likely to be
temporary. It would appear probable that public health education was most
effective in reducing visual morbidity and hence keeping the consequent
burden on the NHS to a minimum."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=11339579

"The increase in solar ultraviolet radiation can have various direct and
indirect effects on human health, like the incidence of ocular damage. Data
of eye damage in residents of three suburban regions in Greece and in two
groups of monks/nuns and fishermen are examined here. The statistics
performed on these data provides new information about the plausible
association between increased levels of solar ultraviolet radiation,
air-pollution at ground level, and the development of ocular defects."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=12094527

"The retinal damage arising soon after exposure to sunlight showed many
different aspects in the OCT images of the 4 cases examined. All retinal
layers seemed to be altered, but these alterations disappeared after 1
month, and the OCT findings remained the same after 1 year."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=12569943

"After gazing at a solar eclipse the ocular surface and tear film changed.
While the aqueous layer remained pathological in many patients, the lipid
layer and the mucous layer recovered spontaneously."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=12658911

"Following the present study, the existence of an excess risk of ocular
melanoma in welders may now be considered as established. Exposure to
ultraviolet light is a likely causal agent, but a possible role of other
exposures in the welding processes should not be overlooked."
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=11545460

Case reports of solar retinopathy in Nigeria:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=15008308

Chronic sun-gazing tomography findings:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=14962431

Unusual case of solar retinopathy:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Ab
stract&list_uids=14662608

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
Outgoing mail scanned by Norton Antivirus
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