Risks and benefits of sun exposure

  • Currently 0.00/5

Rating: 0.0/5 (0 votes cast) login to rate

Add to Favorite Print This Page Publish on Twitter
Bookmark and Share

The ultraviolet radiation in sunlight, though a principal source of vitamin D3 compared to diet, is mutagenic. PMID 12174089. Supplementing diet with vitamin D3 supplies vitamin D without this mutagenic effect. Supplemental vitamin D thus has a very good risk-benefit ratio, relative to exposing skin to sunlight. Since sunlight exposure supplies vitamin D but is mutagenic, it presents both health benefits and health risks.[1] Sunlight is the major source of vitamin D-producing Ultraviolet B radiation, which has a wide range of positive health effects, including possibly inhibiting the growth of some cancers.[2] On the other hand, long-term sunlight exposure is known to be associated with the development of skin cancer, skin aging, immune suppression and eye diseases such as cataracts.[3] Sun exposure has also been associated with the timing of melatonin synthesis and reduced risk of seasonal affective disorder.[4] A number of public health organizations state that there needs to be a balance between the risks of having too much and the risks of having too little sunlight.[5][6] There is a general consensus that sunburn should always be avoided.

However, not all physicians agree with the assertion that there is an optimal level of sun exposure, with some arguing that it is better to minimize sun exposure at all times and to obtain vitamin D from other sources.[7]

Serum levels of 25(OH) D3 are below the recommended levels for a large portion of the general adult population and in most minorities, indicating that Vitamin D deficiency is a common problem in the United States.[8]

Contents

Benefits

According to the U.S. National Institutes of Health Office of Dietary Supplements, most people in the United States can meet their vitamin D needs through exposure to sunlight, even though a large portion have serum 25(OH)D3 levels below recommendations.[1] Ultraviolet (UV) B radiation with a wavelength of 290-315 nanometers penetrates uncovered skin and converts cutaneous 7-dehydrocholesterol to previtamin D3, which in turn becomes vitamin D3.[9][10][11] Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis.[11] The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February;[12] in far northern latitudes, this reduced intensity lasts for up to 6 months. In the United States, latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year.[13]

Complete cloud cover reduces UV energy by 50%; shade (including that produced by severe pollution) reduces it by 60%.[14] UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D.[15] Sunscreens with a sun protection factor of 8 or more appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly.[16] Skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.[1]

The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested by some vitamin D researchers, for example, that approximately 5-30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis and that the moderate use of commercial tanning beds that emit 2%-6% UVB radiation also is effective.[17][11] Individuals with limited sun exposure need to include good sources of vitamin D in their diet or take a supplement.

There is some evidence that bright light exposure reduces seasonal affective disorder[18] and it is a standard treatment for certain circadian rhythm sleep disorders.

Risks

Despite the importance of the sun to vitamin D synthesis, it is prudent to limit exposure of skin to sunlight[16] and UV radiation from tanning beds.[19] According to the National Toxicology Program Report on Carcinogens from the Department of Health and Human Services, broad-spectrum UV radiation is a carcinogen thought to contribute to most of the estimated 1.5 million skin cancers and the 8,000 deaths due to metastatic melanoma that occur annually in the United States[16][20] Lifetime cumulative UV damage to skin is also largely responsible for some age-associated dryness and other cosmetic changes.

It is not known whether a desirable level of regular sun exposure exists that imposes no (or minimal) risk of skin cancer over time. The American Academy of Dermatology advises that photoprotective measures be taken, including the use of sunscreen, whenever one is exposed to the sun.[21]

Is there a safe level of sun exposure?

According to a 2007 study submitted by the University of Ottawa to the Department of Health and Human Services in Washington, D.C., there is not enough information to determine a safe level of sun exposure at this time.[12]

There is no consensus on which component of ultraviolet radiation (UVA or UVB or UVC) actually is carcinogenic,[22] and the composition of ultraviolet radiation changes throughout the day: At high noon, ultraviolet radiation reaching ground level is 95% UVA and 5% UVB, while before 10am and after 2pm this percentage changes over time to 99% UVA and 1% UVB.[23] This is caused by the reflection of UVB rays back into space due to sun angle as the earth slowly rotates on its axis. The rate of change is faster the farther the position moves away from the equator (ie more north or south).[24] 99% UVA / 1% UVB sunlight is known as "bronzing" rays, as it provides the deepest colour tanning.[25]

On average over a day, 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA. UVC is almost completely absorbed by the ozone layer and does not penetrate the atmosphere in any appreciable quantities.[26] As a result, only the combination (UVA, UVB, and UVC) known as "ultraviolet radiation" is listed as a carcinogen, the components are only "likely to become" known carcinogens. Solar radiation, also known as "sunlight" is also listed as a carcinogen because it contains ultraviolet radiation. This means also that the UV Index is a measure of total ultraviolet radiation, and not just Vitamin D-producing UVB.[27]

Sunlight is therefore the only listed carcinogen that is known to have health benefits, in the form of helping the human body to make Vitamin D. This makes sunlight unique on the list of known carcinogens.[28]

Homo sapiens evolved near the equator, and human skin colour is a visible indication of direct sun exposure due to the pigmentation level. With new evidence of Vitamin D receptors in all body tissues, experts advise having a balance between Vitamin D from sun exposure and Vitamin D from supplements. The only way to quantify adequate levels of Vitamin D is with a serum 25(OH) D3 test.[29]

Lifetime sun exposure

There are currently no recommendations on the total safe level of lifetime sun exposure.[12] According to epidemiologist Robyn Lucas at Australian National University,[30] analysis of lifespan versus disease shows that far more lives are lost to diseases caused by lack of sunlight than by those caused by too much.[31] It is inappropriate to recommend total avoidance of sunlight.[32]

Most people receive between 50% and 80% of their lifetime sun exposure before the age of 18[33][unreliable source?]. If one is fair skinned, 10 minutes of exposure to sunshine at high noon (in summer) will produce 10,000 IU of Vitamin D; darker skin requires longer exposure.[31] Note that summer peak daily UVB radiation can be 1000x higher than winter peak daily UVB radiation in temperate regions. For example, in Boston, the summer solstice sun peak altitude is 71 degrees and the corresponding UVB radiation is 73% of max (90 degree sun altitude); the winter solstice sun peak altitude is 24 degrees and the corresponding UVB radiation is 0.03% of max (90 degree sun altitude).[34][35] The current recommendations for Vitamin D supplementation (between 200 IU and 400 IU[36] are not based on sun exposure levels of Vitamin D production, but on fears of toxicity as each person's Vitamin D status depends on dozens of environmental and nutritional factors.[37] Because of this balance between internal production and external supplementation of Vitamin D, it is up to each individual to be aware of how they feel, and to consider sun exposure and Vitamin D status as part of their overall health.[38]

It has recently been discovered that vitamin D receptors are present in most if not all cells in the body. Additionally, experiments using cultured cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects much broader than a role in mineral homeostasis and bone function. This is an active area of research and a much better understanding of this area will likely be available in the near future.[39]

Sun exposure and survival from malignant melanoma

A recent study in the February 2, 2005 issue of the Journal of the National Cancer Institute looked at markers of sun exposure in more than 500 people who had recently been diagnosed with malignant melanoma. The researchers found that solar elastosis, or sun damage to the skin, was independently associated with a surprising increased survival from melanoma.[40]

Sunscreen use within the last 10 years or during childhood was not associated with worse survival from melanoma. And all measures of sun exposure (i.e., history of severe sunburn, high levels of intermittent sun exposure, solar elastosis) were associated with improved survival from melanoma. Furthermore, participants who reported high skin awareness, but not those who reported skin examinations, had better survival.[40]
If confirmed, the results of this study suggest that whereas excessive sun exposure leads to the development of melanoma, sun exposure may protect against the progression of melanoma into more fatal disease.

Seasonal variation and sun exposure

There is ample evidence that the death rate in elderly patients increases in winter months. In a recent study of seasonal mortality in terminal cancer patients in the United States, it was found that compared to those patients who died during June, July, and August, the number of deaths of patients increased an average of 20% in January, February, and March. This near-sinusoidal pattern was remarkably consistent over a five year period.[41]

Mortality in the general population declines in the late summer to early fall months. In the Mediterranean countries, the lowest average daily mortality was observed in September. The fewest deaths were in August in Sweden and North America. The fewest deaths in Japan occurred in July. In the southern hemisphere, the lowest mortality in Australia occurred in March and in February for New Zealand.[42]

In Scotland, patients who suffer cardiopulmonary arrest (heart attack) in winter have a significantly lower likelihood of surviving. People who arrested in winter were 19% less likely to survive compared to those who arrested in summer.[43] For Atrial Fibrilation (AF), significantly more hospital admissions occurred in winter compared to summer.[44]

Deaths due to variceal bleeding in France occurred with a clear annual periodicity and peaked in winter (December, January) in the overall population. The distribution of cumulative monthly deaths differed by 24%, with a peak 14% above average in December and a trough 10% below average in July.[45] In the French Three-City study, high blood pressure, defined as a systolic blood pressure of 160 mmHg or higher, or a diastolic blood pressure of 95 mmHg or higher, was detected in 33.4 per cent of participants during winter and 23.8 percent during summer.[46]

UV-B monitoring in the United States

Over the past several years, levels of ultraviolet radiation have been tracked across North America. The image below from the UV-B Monitoring and Research Program (UVMRP) at Colorado State University shows levels of UVB Radiation (reported in Vitamin D equivalents) in June of 2008.[47]

The UVMRP is a data collection and research program of the United States Department of Agriculture (USDA), headquartered at Colorado State University in Fort Collins, Colorado.[48] High energy ultraviolet solar radiation can significantly damage plants, crops, animals, and ecosystems, alone or in combination with other environmental stress factors such as temperature and moisture. To address these concerns, the UVMRP was established by the USDA in 1992 to provide cost-effective monitoring of UV-B levels over wide geographic areas of the United States.[47]


UVB Levels in Vitamin D Equivalents:[49]

File:All20080601-1206872731 con.png

List of UV-B Monitoring Sites across North America:[49]

State Town Station Opened Closed Longitude Latitude Elevation
AlaskaFairbanksPoker Flat Research Range (roof)08/26/00open147.432 W65.118 N509
ArizonaFlagstaffAbyss Site at Grand Canyon National Park03/14/96open112.184 W36.059 N2073 m
CaliforniaDavisUC Davis Climate Station06/29/94open121.777 W38.534 N18 m
CaliforniaHoltvilleUC Desert Research and Extension Center02/18/98open115.446 W32.806 N-18 m
ColoradoNunnCentral Plains Experimental Range01/30/94open104.756 W40.806 N1641 m
ColoradoSteamboat SpringsDRI Storm Peak Lab (roof)03/18/99open106.744 W40.455 N3220 m
ColoradoLamarUltra-High Energy Cosmic Rays research [Auger Project]12/17/03open102.616 W38.065 N1131 m
FloridaHomesteadBeard Research Center at Everglades National Park05/05/98open80.68 W25.39 N0 m
GeorgiaGriffinUniversity of Georgia-Griffin Campus-Bledsoe Research Farm06/07/94open84.41 W33.181 N267 m
HawaiiWaimeaNOAA Mauna Loa Observatory (roof)11/11/97open155.576 W19.536 N3409 m
IllinoisBondvilleBondville Environmental and Atmospheric Research Site08/06/93open88.372 W40.053 N213 m
IndianaWest LafayetteAgronomy Center for Research and Education04/07/01open86.992 W40.475 N216 m
LouisianaBaton RougeLSU Central Research Station02/09/96open91.167 W30.364 N6 m
MainePresque IsleNorthern Maine Regional Office10/27/95open68.039 W46.699 N155 m
MarylandQueenstownWye Research and Education Center01/30/96open76.151 W38.917 N5 m
MarylandBeltsvilleUSDA Beltsville Agricultural Research Center - South Farm05/30/99open76.95 W39.015 N64 m
MichiganPellstonUniversity of Michigan Biological Station at Douglas Lake08/02/94open84.68 W45.562 N230 m
MinnesotaGrand RapidsNorth Central Research and Outreach Center11/13/96open93.53 W47.184 N424 m
MississippiStarkvilleMississippi Agricultural and Forestry Experiment Station09/13/00open88.782 W33.469 N88 m
MontanaPoplarFort Peck Assiniboine and Sioux Tribes09/30/97open105.102 W48.308 N634 m
NebraskaMeadHigh Plains Regional Climate Center05/16/96open96.492 W41.153 N355 m
New MexicoLas CrucesJornada Experimental Range09/22/94open106.74 W32.616 N1317 m
New YorkGenevaNew York State Agricultural Experiment Station at Geneva08/05/94open77.031 W42.877 N219 m
New ZealandAlexandraNational Institute of Water and Atmospheric Research -Lauder11/20/01open169.682 E45.039 S390 m
North CarolinaRaleighNCSU Air Quality Educational Unit10/03/02open78.68 W35.728 N120 m
North DakotaFargoNDSU Microclimatic Research Station10/29/04open96.812 W46.897 N275 m
Ohio(2001)OxfordEcology Research Center10/06/9304/03/0184.724 W39.531 N285 m
OklahomaBillingsUS Department of Energy ARM / SGP / CART02/17/99open97.485 W36.604 N317 m
OntarioTorontoEnvironment Canada building (roof)02/11/98open79.468 W43.781 N210 m
SaskatchewanReginaBratt's Lake Observatory (roof)12/03/97open104.713 W50.204 N587 m
TexasPanther JunctionCastolon Site at Big Bend National Park12/28/96open103.513 W29.145 N677 m
TexasSeguinTexas Lutheran University (roof)03/16/04open97.983 W29.571 N172 m
TexasHoustonUniversity of Houston - north Moody Tower (roof)08/03/06open95.341 W29.718 N76 m
TexasEl PasoUTEP Undergraduate Learning Center (roof)04/19/06open106.506 W31.769 N1186 m
UtahLoganUtah Climate Center12/15/95open111.892 W41.667 N1369 m
VermontBurlingtonProctor Maple Research Center10/22/96open72.869 W44.528 N390 m
WashingtonPullmanPalouse Conservation Field Station07/13/94open117.192 W46.76 N805 m
WisconsinDancyLake Dubay05/20/96open89.772 W44.708 N381 m

References

  1. 1.0 1.1 1.2 "Dietary Supplement Fact Sheet: Vitamin D". Office of Dietary Supplements, National Institutes of Health. http://dietary-supplements.info.nih.gov/factsheets/vitamind.asp. 
  2. "Sun 'cuts prostate cancer risk'". BBC News. June 19, 2005. http://news.bbc.co.uk/1/hi/health/4090972.stm. Retrieved April 1, 2010. 
  3. Lucas RM, Repacholi MH, McMichael AJ (June 2006). [Expression error: Missing operand for > "Is the current public health message on UV exposure correct?"]. Bulletin of the World Health Organization 84 (6): 485–91. doi:/S0042-96862006000600018. PMID 16799733. 
  4. Mead MN (April 2008). "Benefits of sunlight: a bright spot for human health". Environmental Health Perspectives 116 (4): A160–7. PMID 18414615. PMC 2290997. http://www.ehponline.org/members/2008/116-4/focus.html. 
  5. "Risks and Benefits" (PDF). http://www.cancer.org.au//File/PolicyPublications/PSRisksBenefitsSunExposure03May07.pdf. Retrieved 2010-05-13. 
  6. "Microsoft Word - Risks and Benefits of Sun Exposure Position Statement - Fi..." (PDF). http://www.sunsmartschools.co.nz/be/Editor/Doc/PS_RisksBenefits_SunExposureSept08.pdf. Retrieved 2010-05-13. 
  7. "The Skin Cancer Foundation - The Vitamin D Dilemma | Vitamin D". Skincancer.org. http://www.skincancer.org/scf-journal-2008.html. Retrieved 2010-05-13. 
  8. Zadshir A, Tareen N, Pan D, Norris K, Martins D (2005). [Expression error: Missing operand for > "The prevalence of hypovitaminosis D among US adults: data from the NHANES III"]. Ethnicity & Disease 15 (4 Suppl 5): S5–97–101. PMID 16315387. 
  9. Hayes CE, Nashold FE, Spach KM, Pedersen LB (March 2003). [Expression error: Missing operand for > "The immunological functions of the vitamin D endocrine system"]. Cellular and Molecular Biology 49 (2): 277–300. PMID 12887108. 
  10. Holick MF (October 1994). "McCollum Award Lecture, 1994: vitamin D--new horizons for the 21st century". The American Journal of Clinical Nutrition 60 (4): 619–30. PMID 8092101. http://www.ajcn.org/cgi/pmidlookup?view=long&pmid=8092101. 
  11. 11.0 11.1 11.2 Holick, Michael F. (February 2002). [Expression error: Missing operand for > "Vitamin D: the underappreciated D-lightful hormone that is important for skeletal and cellular health"]. Current Opinion in Endocrinology & Diabetes 9 (1): 87–98. doi:10.1097/00060793-200202000-00011. 
  12. 12.0 12.1 12.2 Cranney A, Horsley T, O'Donnell S, et al. (August 2007). [Expression error: Missing operand for > "Effectiveness and safety of vitamin D in relation to bone health"]. Evidence Report/technology Assessment (158): 1–235. PMID 18088161. 
  13. Holick MF (2006). "Vitamin D". In Shike, Moshe; Shils, Maurice Edward. Modern nutrition in health and disease. Hagerstwon, MD: Lippincott Williams & Wilkins. ISBN 0-7817-4133-5. 
  14. Wharton B, Bishop N (October 2003). [Expression error: Missing operand for > "Rickets"]. Lancet 362 (9393): 1389–400. doi:10.1016/S0140-6736(03)14636-3. PMID 14585642. 
  15. Holick MF (2005). "Photobiology of vitamin D". In Feldman, David Henry; Glorieux, Francis H.. Vitamin D. Amsterdam: Elsevier Academic Press. ISBN 0-12-252687-2. 
  16. 16.0 16.1 16.2 Wolpowitz D, Gilchrest BA (February 2006). [Expression error: Missing operand for > "The vitamin D questions: how much do you need and how should you get it?"]. Journal of the American Academy of Dermatology 54 (2): 301–17. doi:10.1016/j.jaad.2005.11.1057. PMID 16443061. 
  17. Holick MF (July 2007). [Expression error: Missing operand for > "Vitamin D deficiency"]. The New England Journal of Medicine 357 (3): 266–81. doi:10.1056/NEJMra070553. PMID 17634462. 
  18. "Summer sun for winter blues". CNN. July 12, 1999. http://www.cnn.com/HEALTH/alternative/9907/12/sun.depression/. Retrieved 2010-03-08. 
  19. [Expression error: Missing operand for > "The association of use of sunbeds with cutaneous malignant melanoma and other skin cancers: A systematic review"]. International Journal of Cancer 120 (5): 1116–22. March 2007. doi:10.1002/ijc.22453. PMID 17131335. 
  20. "Ultraviolet (UV) Radiation, Broad Spectrum and UVA, UVB, and UVC - National Toxicology Program". Ntp.niehs.nih.gov. 2009-01-05. http://ntp.niehs.nih.gov/index.cfm?objectid=BD4CD88D-F1F6-975E-792094AC1CE4B062. Retrieved 2010-05-13. 
  21. American Academy of Dermatology. Position statement on vitamin D. November 1, 2008. [1]
  22. "11th Report on Carcinogens". Ntp.niehs.nih.gov. http://ntp.niehs.nih.gov/ntp/roc/toc11.html. Retrieved 2010-05-13. 
  23. "Ultraviolet (UV) Radiation". Fda.gov. http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/Tanning/ucm116425.htm. Retrieved 2010-05-13. 
  24. "Sun Protection Information Sheet, Travellers". Nathnac. http://www.nathnac.org/travel/misc/travellers_sun.htm. Retrieved 2010-05-13. 
  25. "www.bronzeagetanningsalon.com". www.bronzeagetanningsalon.com. http://www.bronzeagetanningsalon.com/systems.php. Retrieved 2010-05-13. 
  26. "UVC Radiation". Dermatology.about.com. http://dermatology.about.com/od/glossaryu/g/uvc.htm. Retrieved 2010-05-13. 
  27. "UV Index Sun Awareness Program". Hc-sc.gc.ca. 2009-09-03. http://www.hc-sc.gc.ca/hl-vs/sun-sol/uv-prog/index-eng.php. Retrieved 2010-05-13. 
  28. "11th Report on Carcinogens". Ntp.niehs.nih.gov. http://ntp.niehs.nih.gov/ntp/roc/toc11.html. Retrieved 2010-05-13. 
  29. "25-hydroxy vitamin D test: MedlinePlus Medical Encyclopedia". Nlm.nih.gov. http://www.nlm.nih.gov/medlineplus/ency/article/003569.htm. Retrieved 2010-05-13. 
  30. "ANU - National Centre for Epidemiology and Population Health- NCEPH". Nceph.anu.edu.au. 2009-12-07. http://nceph.anu.edu.au/Staff_Students/staff_pages/lucas.php. Retrieved 2010-05-13. 
  31. 31.0 31.1 "Time in the Sun: How Much Is Needed for Vitamin D? - US News and World Report". Usnews.com. 2008-06-23. http://www.usnews.com/health/family-health/articles/2008/06/23/time-in-the-sun-how-much-is-needed-for-vitamin-d.html. Retrieved 2010-05-13. 
  32. http://www.mja.com.au/public/issues/177_11_021202/luc10478_fm.pdf
  33. "Sun Safety". Kidshealth.org. http://kidshealth.org/parent/firstaid_safe/outdoor/sun_safety.html. Retrieved 2010-05-13. 
  34. "How does UVB intensity vary with solar elevation above horizon? - Yahoo! Answers". Answers.yahoo.com. 2008-01-07. http://answers.yahoo.com/question/index?qid=20080107072603AAfpfSP. Retrieved 2010-05-13. 
  35. "Sun or Moon Altitude/Azimuth Table". Aa.usno.navy.mil. 2008-09-02. http://aa.usno.navy.mil/data/docs/AltAz.php. Retrieved 2010-05-13. 
  36. "Vitamin D". Eatrightontario.ca. http://www.eatrightontario.ca/en/viewdocument.aspx?id=218. Retrieved 2010-05-13. 
  37. "Vitamin D: Vitamin Deficiency, Dependency, and Toxicity: Merck Manual Professional". Merck.com. http://www.merck.com/mmpe/sec01/ch004/ch004k.html. Retrieved 2010-05-13. 
  38. "Running on Vitamin D at Runner's World". Runnersworld.com. 2008-02-15. http://www.runnersworld.com/article/0,7120,s6-242-301--13364-0,00.html. Retrieved 2010-05-13. 
  39. "Vitamin D (Cholecalciferol, Calcitriol)". Vivo.colostate.edu. 2010-04-24. http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/otherendo/vitamind.html. Retrieved 2010-05-13. 
  40. 40.0 40.1 McCoy, Krisha. "Study Suggests History of Sun Exposure May Actually Increase Melanoma Survival Rates". Mbmc.org. http://www.mbmc.org/healthgate/GetHGContent.aspx?token=9c315661-83b7-472d-a7ab-bc8582171f86&chunkiid=90212. Retrieved 2010-05-13. 
  41. "Seasonal mortality in terminally ill cancer patients.". ASCO. http://www.asco.org/ASCOv2/Meetings/Abstracts?&vmview=abst_detail_view&confID=65&abstractID=32137. Retrieved 2010-05-13. 
  42. "Seasonality of mortality: the September phenomenon in Mediterranean countries - Falagas et al. 181 (8): 484 - Canadian Medical Association Journal". Cmaj.ca. doi:10.1503/cmaj.090694. http://www.cmaj.ca/cgi/content/full/181/8/484. Retrieved 2010-05-13. 
  43. J P Pella, J Sirelb, A K Marsdenc, S M Cobbeb (1999-07-30). "Seasonal variations in out of hospital cardiopulmonary arrest - Pell et al. 82 (6): 680 - Heart". Heart.bmj.com. http://heart.bmj.com/content/82/6/680.abstract. Retrieved 2010-05-13. 
  44. "Seasonal variation in morbidity and mortality related to atrial fibrillation". BiomedExperts. http://www.biomedexperts.com/Abstract.bme/15458696/Seasonal_variation_in_morbidity_and_mortality_related_to_atrial_fibrillation. Retrieved 2010-05-13. 
  45. The American Journal of Gastroenterology (2001-06-01). "Access : Seasonal variations in variceal bleeding mortality and hospitalization in France : The American Journal of Gastroenterology". Nature.com. http://www.nature.com/ajg/journal/v96/n6/full/ajg2001452a.html. Retrieved 2010-05-13. 
  46. "Seasonal Variation In Blood Pressure". Medicalnewstoday.com. http://www.medicalnewstoday.com/articles/135805.php. Retrieved 2010-05-13. 
  47. 47.0 47.1 "UV-B Monitoring and Research Program at Colorado State University". Uvb.nrel.colostate.edu. http://uvb.nrel.colostate.edu/UVB/index.jsf. Retrieved 2010-05-13. 
  48. "UV-B Monitoring and Research Program Monitoring Network at Colorado State University, Overview". Uvb.nrel.colostate.edu. http://uvb.nrel.colostate.edu/UVB/uvb_overview.jsf. Retrieved 2010-05-13. 
  49. 49.0 49.1 "UV-B Monitoring Project, United States Department of Agriculture; Fort Collins, Colorado". Uvb.nrel.colostate.edu. http://uvb.nrel.colostate.edu/UVB/uvb_network.jsf. Retrieved 2010-05-13. 
  • Currently 0.00/5

Rating: 0.0/5 (0 votes cast) login to rate

Add to Favorite Print This Page Publish on Twitter
Bookmark and Share
close about Number of comments per page:
Time format: relative absolute
You need JavaScript enabled for viewing comments