After atmospheric and ozone filtering, only about 3% of the total energy of sunlight at solar noon is ultraviolet, although that depends on latitude, season, altitude, and atmospheric conditions. The ultraviolet portion of outdoor sunlight is approximately 95% UVA and 5% UVB, although atmospheric, seasonal, and geographic variables change that ratio each time you step outside.
Doctor Laurence Feldmeyer, working under senior author Professor Gunther Hofbauer together with 13 co-authors, all of the University Hospital in Zurich, recently took a look at different UV wavelengths and what effect they had on vitamin D levels.
Feldmeyer L, Shojaati G, Spanaus KS, Navarini A, Theler B, Donghi D, Urosevic-Maiwald M, Glatz M, Imhof L, Barysch MJ, Dummer R, Roos M, French LE, Surber C, Hofbauer GF. Phototherapy with UVB narrowband, UVA/UVBnb, and UVA1 differentially impacts serum 25-hydroxyvitamin-D3. J Am Acad Dermatol. 2013 Jul 11.
Remember that ultraviolet light comes in three different categories, the shortest wavelength UVC (100 to 280 nm), which is very high energy and does not penetrate the atmosphere.
The final two ultraviolet wavelengths are UVB (280 and 320 nm), which both makes vitamin D and damages the DNA in our skin, and finally UVA (320 to 400 nm), which some studies indicate causes melanoma, but also may do something good in the skin that has yet to be discovered. For instance, dermatologists are using UVA phototherapy to successfully treat various forms of scleroderma (an autoimmune skin disorder) as well as rarer skin conditions. Therefore, UVA light clearly does something in the skin but we don’t know what it is
More on UVC
As an aside, a study from 70 years ago indicated that UVC is highly effective in both producing vitamin D and in treating rachitic rats.
Bunker JWM, Harris RS, Mosher LM: Relative efficiency of active wave-lengths of ultraviolet in activation of 7-dehydrocholesterol. J Am Chem Soc 1940, 62(3):508-511.
In 1982, Professor Michael Holick’s lab confirmed that UVC makes vitamin D. Per photon, UVC was more effective than UVB in making vitamin D in skin oils.
Feldmeyer et al (in an open trial without randomization) used three different wavelengths of ultraviolet light [UVA alone, narrowband UVB alone (311 nm), and a combination of UVA and UVB] in 116 dermatological outpatients with various skin disorders, giving each group phototherapy two to three times per week for up to 12 weeks. Numbers in the various groups were UVA (n = 38), UVA/UVB (n =30), or UVB (n = 48).
They found that the UVA light group had a reduction in 25(OH)D levels (22 to 19.0 ng/mL (P=.001). However, narrowband UVB significantly increased 25 (OH)D levels from 22 to 40 ng/mL, as did combined UVA and narrowband UVB (24 to 50 ng/mL) phototherapy.
I found it interesting that combination UVA/UVB produced more vitamin D [peak 25(OH)D of 50 ng/ml] than did UVB alone (peak 40 ng/ml), although that difference was not significant. Before therapy with UVB alone there were 8 insufficient (<20 ng/mL) and 5 deficient (<10 ng/mL) patients, and only 2 showed an insufficiency after therapy (no deficiency). In the UVA/UVB group, there were 5 insufficient and 4 deficient patients before therapy and only 1 insufficient patient after therapy (no deficiency), respectively. So even with up to 12 weeks of UV therapy, some patients did not obtain a 25(OH)D above 30 ng/ml.
Blood pressure did not change in any of the 3 groups after treatment but all had normal blood pressures to begin with. They also observed no relationship between body weight and vitamin D serum change, confirming the results of previous studies.
Another interesting finding was that Dermatology Life Quality Index (DLQI) scores (a questionnaire aimed at detecting quality of life) improved in all the patients collectively (P<.001), and in each of the 3 phototherapy groups considered separately. However, there was no difference in improvement in DLQI scores between the 3 phototherapy groups. DLQI score did not correlate with the change in 25(OH)D, although the number of completed DLQI questionnaires may have limited the observed lack of correlation to serum vitamin D changes.
So UV light, including UVA, was what apparently improved quality of life, not improvements in 25(OH)D.