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Re: Telescope danger to sight. Was: Venus transit ...
From: Jim Thompson
Date: 2004 May 14, 07:58 -0300
From: Jim Thompson
Date: 2004 May 14, 07:58 -0300
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 -----------------------------------------