Sun-bathing and influenza

Putting on your sunglasses and lying on soft sandy beaches, taking 20 minutes of your time while enjoying the sun light, you flip once to get a uniform redness on the skin. A favourite hobby of my girlfriend apart from watching TV series and being industrious in the lab. She believes that the sun light she gets actually prevents her from falling ill. On the contrary, I would rather avoid the sun light and use the sun-block as I believe the harmful UV rays from the sun damage your skin and may also lead to skin cancer.
Apparently, both of our believes were true. In fact, she was right as the sun does prevent her from falling sick and this had already been proven scientifically. In 1981, Dr. Robert Edgar Hope-Simpson proposed that the cause of influenza epidemic is associated with the seasonal fluctuation of vitamin D especially during the winter and rainy seasons in tropical countries. These are the time when people tend to stay indoor to avoid from freezing their ass and getting wet. The reason for his proposal was simple as he discovered that influenza epidemics in Great Britain in the 17th and 18th century occurred in widely separated communities, before modern transportation could explain its wide spread of the disease. According to the chart, the number of influenza cases varies according to the latitude and specific months of the year.

Why is the sun significant? The answer is plain old vitamin D. Humans have always made vitamin D, when their naked skin meets with the ultraviolet B radiation from the sun. The liver then converts the vitamin D into 25-hydroxyvitamin D and then cells all over the body converts this into steroid hormone which increase the level of your immune system and other important functions. Apart from increasing our immunity, vitamin D increases antimicrobial peptides that destroys the influenza virus while simultaneously modulates macrophages (white blood cells within tissues) preventing them from releasing excessive inflammatory cells called chemokines and cytokines into infected lung tissue.

Instead of falling sick due to Vitamin D deficiency, why don't we grab our shades, put on our bikinis or shorts, lie on the couch and expose our skin under the sun. 15 minutes for 2 to 3 times a week is the easiest prescription. You may need more if you have darker skin due to regional pigmentation, and a little less if you live near the equator. But according to WHO, it is best to limit exposure of the sun's UV rays that are strongest between 10am to 4pm. A pair of sunglasses is essential as it provides 99 to 100 percent UVA and UVB protection before you start damaging your eyes which may lead to cataract. In addition, it is of interest to know that sometimes vitamin D is not made in your skin even though you see the sun. At latitude 52 deg North (particularly London), between October to March, no vitamin D is made in the skin because atmospheric ozone easily filters out UVB radiation.

The best way of getting sufficient sun for the necessary production of vitamin D is to follow the UV index. They are categorized from zero upward, and as the number goes higher the risk of damaging your eyes and skin increases. Below is a rough guideline from the WHO and you can even check for daily UV index of your area from wunderground.com. Do apply sunscreen if you do feel the sun rays are burning. A broad-spectrum sunscreen of SPF 15+ is sufficient and you can re-apply every two hours, but do apply 15-30 minutes before you start burning your lovely skin. The number 15+ does not refer to the UV index, it is called the sun protection factor. So, wearing a sunscreen with SPF 15+ means your skin will not burn until it has been exposed to 15-20 times the amount of solar energy that would normally cause it to burn. But this also depends on the time of the day you spend under the sun. Also take note that SPF 20 in America is the same as SPF 10 in Europe.

If you don't prefer sunbathing, there are other sources of vitamin D within your reach. For example, cod liver oil, and fishes like sardines, herring and salmon are sources of vitamin D that are within our reach for consumption. Other source of vitamin D includes erythemal doses of UV radiation which contains UVA and UVB radiation are typically administered to athletes.

Well, I guess it's time for me to get some sun now. I hope this article may help you a little.

References:

1. J.J Cannell, E. Giovannucci et al. "Epidemic influenza and vitamin D," Epidemiol Infect, 134, pp. 1129-1140, 2006.
2. Nina Shen Rastogi, "Is sunbathing good for you?" Slate, http://www.slate.com/id/2187566/
3. Dr. Dan Rutherford, "Sunbathing," Netdoctor, http://www.netdoctor.co.uk/health_advice/facts/sunbathing.htm