Let the Sunshine in

By C. CLAIBORNE RAY

Published: April 15, 2008

Correction Appended

Q. Am I still getting vitamin D when I’m outside on a gray, cloudy day?

Victoria Roberts

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A. Just as it is possible to get a sunburn on an overcast day, it is possible to get the ultraviolet-B radiation needed for the skin to synthesize vitamin D even when skies are cloudy and gray. Efficiency varies, however, with the season, the altitude, skin color, the length of skin exposure and the percentage exposed, and air pollution.

Complete cloud cover halves the energy of ultraviolet rays, and shade reduces it by 60 percent, according to the National Institutes of Health.

The wavelengths of ultraviolet light needed to make vitamin D are from about 290 to 320 nanometers, within the ultraviolet-B range, which is also associated with sunburn and skin damage.

At the earth’s northern latitudes for much of the year, and at the midlatitudes in winter, the sun does not stay far enough above the horizon (45 degrees) for the angle of the sun’s rays to guarantee an efficient ultraviolet-B bath. Northerners may have to rely on the vitamin D synthesized in the summer or on foods and supplements.

Readers may submit questions by mail to Question, Science Times, The New York Times, 620 Eighth Avenue, New York, N.Y. 10018, or by e-mail to question@nytimes.com.

This article has been revised to reflect the following correction:

Correction: April 23, 2008
An answer in the Q&A column on April 15, about people getting vitamin D from exposure to the sun on a cloudy day, and a column on May 17, 2005, about getting the vitamin from exposure to sunlight through a window, incorrectly attributed a position to the National Institutes of Health about sun exposure and vitamin D. The columns also referred to an outdated fact sheet posted on the N.I.H. Web site. The recommendation for 10 to 15 minutes of exposure twice a week is the view of an author cited by the institutes; the N.I.H. itself makes no recommendation on sun exposure. An updated fact sheet on the topic can be found at http://dietary-supplements.info.nih.gov/factsheets.