They are probably referring to some kind of large-scale environmental contamination, but as far as I'm aware it's quite difficult to make any of these theories

They are probably referring to some kind of large-scale environmental contamination, but as far as I'm aware it's quite difficult to make any of these theories fit with the timeline of international historical trends. Typically the differences in rates and magnitudes are apparent across borders and thus would more easily be explained with cultural factors rather than an environmental contaminant that is (largely) agnostic to cultural effects.