There is credible evidence that America's serial killer epidemic was directly caused by environmental pollution, especially heavy metals like lead and cadmium. Bringing back air pollution will bring back serial killers.
Serial killers? Boring. If someone comes up with credible evidence that environmental pollution causes people to be trans, then we'll see some actual environmental protection. ... Sad, dude.
... Wait, is there? This is a fairly common hypothesis for _violence in general_ (it seems to rather neatly explain the big fall in violent crime in most of the developed world in the 90s/noughties, though it's a bit of a just-so story and the timing only really works if you squint at it), but the serial killer thing (in terms of very large numbers of them) seems to be a particularly American phenomenon; if it was heavy-metal linked you'd expect clusters in other heavily industrialised countries.