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Friday, 24 April 2015

Sunlight and viruses

While working on my April 23 blog on weather, mood and paid, I also noticed an interesting remark on viruses: Many people swear they contract the common cold when the weather changes. Although it’s not entirely clear why, experts believe it’s because rapid temperature swings weaken your immune system. The cold virus also transmits better in cold air. (source)

The relationship between sunlight and viruses appears to be an interesting one. Especially, this intriguing remark: "Further research is required to evaluate whether sunlight can significantly affect the resistance to common viral infections and vaccines." There are two known exceptions. "These are herpes simplex, in which sunlight exposure can cause reactivation, and certain papillomavirus types in which sunlight exposure can lead to the development of nonmelanoma skin cancer." (source)

According to the Aerobiological Engineering Dept. at Penn State University, the ultraviolet component of sunlight is the main reason microbes die in the outdoor air. The die-off rate in the outdoors varies from one pathogen to another, but can be anywhere from a few seconds to a few minutes for a 90-99% kill of viruses or contagious bacteria. (link 1link 2, link 3, link 4, link 5)

Western behaviour towards sunlight has changed enormously compared to previous centuries. For many centuries exposure to sunlight - and thus skin tanning - was considered inappropriate for wealthy people as it suggested that you were involved in open air, poorly paid, labour. White skins were considered a sign of wealth. Hence the sun umbrellas for women walking outside. Nowadays the wealthy appear to have always a tanned skin from vacations in the snow and/or the sun, no office labour and lots of outdoor sports (e.g., golf). This change in human behaviour did cause a substantial increase in skin cancer. "The 3 main types of skin cancer have become more common in the last 20 to 40 years, especially in those areas which are mostly Caucasian." (Wikipedia)

This observation also made me look for studies showing a correlation between sunlight and average life expectancy. Scientists have long known that ultraviolet (UV) light can affect the development of living organisms by suppressing molecular and cellular processes. The effects of UV radiation on the health and reproduction of aquatic animals are well known, but only a handful of studies have looked at the effects on the human life span or on survival rates of human infants. (source)

A recent study examined demographic data from more than 9,000 people born in Norway between 1676 and 1878. The researchers compared the data with historical evidence of cycles of solar radiation compiled by the U.S. National Oceanic and Atmospheric Administration (NOAA). The researchers found that people who were born during the years of high solar activity were less likely to survive to adulthood, with a high percentage of those deaths occurring before age 2. In addition, among women born in years of high solar activity, those who had lower incomes (and who likely would have spent more time outside, exposed to the sun) had lower fertility rates and fewer children who survived to age 20, compared with wealthier women. The study shows only a correlation between solar activity and life span; it does not show causation. (source)

As once scientist concluded: "There's a balance between too much and too little in sunlight. The real question is, what's that magic breakpoint?"