[rosser]

From a quick skimming of The Fine Article, it seems to mean people living in "modern" environments — like, e.g., plumbing. If I had to guess, diet is probably among the most salient characteristics, with lessened sun exposure from spending time indoors also being material.

[chiefalchemist]

Diet, time inside/outside, exercise, environmental pollutants, stress, water supply (drinking and bathing), consumer products with (often untested) chemicals...

I'd say there are plenty of western/Western attributes that could have a cumulative (?) impact.