https://www.reddit.com/r/interestingasfuck/comments/8iflwh/c...
Because UVB scatters more than UVA (Rayleigh scattering), shade increases the UVB:UVA ratio from 1:1 to about 0.52:0.35 under a shade umbrella, or 0.53:0.37 under a tree. See Table 1 (paywalled, sorry) https://doi.org/10.1016/j.jsbmb.2005.04.039
of course, most that go to a tanning booth care more about looks than health
UVB shouldnt just be legal, it should be mandatory.