I've seen a few posts about the fluoride study in the Lancet. I did a little research - pulling up the actual paper (and holding my nose when I saw it was being shared via Elsevier) and then looking up the meta-study of Fluoride that it's based on - a paper by Anna L. Choi, Guifan Sun, Ying Zhang, and Philippe Grandjean (the latter being one of the authors of the Lancet paper), published in "Environmental Health Perspectives".
That paper is a meta-analysis of 27 other papers, most of which are studies of areas in China where there are high levels of fluoride due to burning high-fluoride coal. The control areas tend to be wood-burning.
1. They're exposed to at least as much airborne fluorine, including airborne hydrogen fluoride, as in the water.
2. There are also higher levels of other toxic chemicals in the study area (one study mentions arsenic specifically).
3. The fluorine levels even in the water are excessive, and in some papers it's noted the control areas have similar levels to western areas that fluoridate the water.
So the applicability of most of these studies, and thus the meta-study, to dental fluoridation is doubtful at best. The meta-analysis explicitly didn't consider these issues (the Choi/Sun... paper lists the criteria they used), so I certainly wouldn't consider it to have isolated the effect of fluorine from all the other factors involved.