There have been many indications that UVA without UVB decreases vitamin D level in the blood
The Random Controlled trial on this page confirms it for tanning beds

All items in the category UV and Vitamin D 334 items

UVA through window glass decreases vitamin D
tanning beds bone and vitamin D – 2009.pdf file
Study of Vitamin D3 from UV lamps – Aug 2010.pdf file
Hypothesis uva cause melanoma – 2009.pdf file
US truckers die 16 years earlier – partially due to vitamin D deficiency behind glass a lot every day, just getting UVA
Overview UV and vitamin D
Hypothesis: UVA through window glass decreases vitamin D - 2009
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Changes in serum 25-hydroxyvitamin D and cholecalciferol after one whole-body exposure in a commercial tanning bed: a randomized study.

Jacob H Langdahl, Louise Lind Schierbeck, Ulrich Christian Bang and Jens-Erik Beck Jensen Endocrine () (2012) Mar 6

We wanted to evaluate the cutaneous synthesis of 25OHD and cholecalciferol after one whole-body exposure to ultraviolet radiation type B (UVB) in a randomized setup. Healthy volunteers were randomized to one whole-body exposure in a commercial tanning bed with UVB emission (UVB/UVA ratio 1.8-2.0%) or an identical placebo tanning bed without UVB.

The output in the 280-320 nm range was 450 µW/cm(2). Blood samples were analyzed for 25OHD and cholecalciferol at baseline and during 7 days after treatment.
We included 20 volunteers, 11 to UVB and 9 to placebo treatment.

During the first 6 h, no significant differences in 25OHD between the groups were found. (?)

At the end of the study, we found a mean increase of 25OHD in the UVB group of 4.5 nmol/l (SD 7 nmol/l) compared to
a decline of -1.2 nmol/l (SD 7 nmol/l) in the placebo group (p = 0.1).

A linear mixed model yielded an increase of 25OHD in the UVB group of 1.0 nmol/l per 24 h (p < 0.01).
For cholecalciferol, we found a near significant increase of 1 pmol/l per hour in the UVB group compared to the placebo group during the first 6 h (p = 0.052).
One tanning bed session had significant, but modest impact on the level of 25OHD during 7 days after exposure to UVB.

PMID 22391940
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UV transmission by standard window glass

No UVB (Vitamin D) gets thru, but most UVA (Tan) gets thru

See also Vitamin D Life

• UVA decreased vitamin D levels (again) – July 2013