Exposure to forests improves mood. Mood affects outcomes. It's a question of whether the exertion of going somewhere healthier is great enough to kill you (and if anyone attributes your death to the attempt, which I don't think we always did).

Also post-industrial Europe and especially London had awful, awful air quality. I'll bet if you look at the 'seasons' for going to Bath, for instance, that they overlapped with the worst weather for creating smog in London.