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Exposure to sunlight may be a factor in lowering blood pressure.

Studies have shown that in people with mild hypertension, blood pressure tends to be lower in the summer than in the winter, and that both average blood pressure and the incidence of cardiovascular disease tend to increase with distance from the equator.

Researchers exposed 24 healthy volunteers to ultraviolet A radiation for 30 minutes with an intensity comparable to noon on a sunny day in Southern Europe. The exposure caused a small but significant drop in blood pressure compared with when the same people were exposed to sham UVA radiation (the same amount of heat and light, but no UVA exposure).

Blood tests showed that UVA exposure caused a release of nitric oxide from the skin into the bloodstream. Nitric oxide causes arteries to dilate, and this, the scientists believe, accounts for the effect. The study appears in The Journal of Investigative Dermatology.

The senior author, Dr. Richard B. Weller, a dermatologist at the University of Edinburgh in Scotland, said that the modest effect of sunlight is significant on a population level, where small reductions in average blood pressure can have vast public health benefits.

But for an individual with high blood pressure sunbathing is not the solution. “Getting sunlight is not enough if your blood pressure is high. And if you have high blood pressure,” Dr. Weller said, “you need to get it controlled.”