A wrist-watch which measures UVA plus UVB - should NOT be used to increase vitamin D - June 2014

http://sunfriend.com

Overview Skin and vitamin D has the following chart for UV when standing

Chart edited to show when shadow length = your height (sun angle = 45 degrees)

Sun Friend would indicate that you are getting a lot of UV, but only 1/4 of it is the UVB needed to make Vitamin D

Until a new version of Sun Friend is available which includes compensation for sun angle (= location & time of year & time of day)
Sun Friend should only be used to estimate vitamin D at local noon +- about 2 hours in the summer

Note: I spoke July 2015 with developers about this limitation.

Another way of seeing the huge change in UVA to UVB ratio during the day

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Worse!!

People wearing the UVA+UVB watch would learn to keep out of the hottest portion of the sun (which actually has the vitamin D) because their watch would (falsely) tell them that they would get lots of vitamin D is the late afternoon - when it is not as hot.

See also Vitamin D Life

• Getting a tan does not mean that you are getting vitamin D
• Vitamin D protects DNA against UV skin damage – 5 studies 2012-2013
• All items in category UV and Vitamin D 334 items
• UVA decreased serum levels of vitamin D - RCT Oct 2012
• Another vitamin D meter fails to distinguish UVB from UVA – Violet – Dec 2014
• UVA decreased vitamin D levels (again) – July 2013
  Perhaps the high UVA vs UVB in the late afternoon would actually DECREASES the vitamin D levels

Thus the use of this watch might result in decreased levels of vitamin D