Funny timing, I just went to a tanning salon for the first time yesterday. I asked for the weakest bed (level 1), which has the most UVB (for vitamin D production). They were shocked that I wanted to use level 1, apparently no one uses it. They also suggested starting at 5 mins instead of the 1-2 minutes I wanted to do. The machine itself has a notice saying not to go over 3 mins for the first week.

I was following the protocol from this paper, which started people at 2 mins and used low wattage UVB-heavy bulbs.

Sunbeds with UVB radiation can produce physiological levels of serum 25-Hydroxyvitamin D in healthy volunteers

https://pmc.ncbi.nlm.nih.gov/articles/PMC5821157/

Unfortunately the Science Advances paper being discussed is epidemiological and doesn't distinguish between the type of bulb, length of time, and other parameters used while tanning. However it is safe to say that the average tanner cares more about getting dark than anything else.

I think there would actually be a market for vitamin D centered "healthy tanning" where only low wattage, high-UVB bulbs are used particularly in cloudy areas or where the winter is long. I'm that guessing the operating costs for that kind of business would be cheaper than your average tanning salon, too.