A study funded by the Swiss National Science Foundation (SNF) shows that Switzerland’s weak winter sunlight doesn’t provide a level of vitamin D anywhere close to the minimum recommended by the World Health Organisation (WHO).

While sun exposure increases the risk of skin cancer it also produces vitamin D, a substance essential for bone health that may also play a role in preventing respiratory infections, autoimmune diseases and certain types of cancer.

Between the end of autumn and the beginning of spring, average sunlight exposure in Switzerland comes nowhere close to providing the daily dose of 0.024 milligrams (24µg or 960 IU) of vitamin D recommended by the WHO.

Researchers measured the intensity of solar UV irradiation in Switzerland across a year and used the resulting data to create a computer simulation. The simulation estimated the impact of sunlight on the production of vitamin D as well as the risks of sunburn.

In summer, it takes a person wearing a T-shirt (22% of skin uncovered) 10 to 15 minutes to synthesize the recommended daily dose of vitamin D – Vitamin D3 is produced photochemically when UV light reacts with chemicals in the skin.

In winter, the situation is different because less skin is exposed – around 8%, representing the face and hands – and the sun’s rays are weaker. Getting the recommended daily dose of vitamin D in winter would require 6.5 hours of sun exposure a day, a level of exposure few achieve.

This research fits with the seasonal deficiency of vitamin D observed in the Swiss population. One study estimates 40% of Switzerland’s population is deficient.

But many questions remain, such as the appropriate dose – recommendations vary significantly – and the importance of food supplements. “For the moment, our chief recommendation is to avoid solariums in winter,” says David Vernez, who led the research, “The risk of skin cancer far outweighs any possible benefits.”

More on this:
SNF press release (in French) â€“ Take a 5 minute French test now

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