Oh, no, that's not correct. Vapor pressure deficit is the relation between humidity and temperature, and that's the basis of how plants transport water and food through their tissues as they photosynthesize, breathe, grow, etc. I personally find that, for indoors, 40% is far too low, I like it much better in the mid 60s-70s on up. But then, I grow mostly Sativa-dominant lines that like that RH. That said, the Afghani-doms seem to like it plenty good, too. Not necessarily a long time in the 90s range, but if it occasionally wanders up there I haven't found ill effect. I know others have, of course.
In order to better understand what's going on, you need to understand what vapor pressure deficit is. Look up @ttystikk 's stuff.
