Well the fact is serenity of an environment imparts on health generally and induces harmony in mind n body functions. I think a key word here is serenity, which varies with our states of mind, emotion n environment per time.
Nature is everywhere, in the city, countryside, river side, desert, etc. One's state of mind plays a role in harmony with ones environment. Serenity begins from within.
Yet some environments are cleaner and healthier than others. Yet may not impact a disturbed mind.
Quite a study with many variables.