If I had to guess I'd think microplastics[1] because we actively consume them and they're found in our tissues and they didn't really exist before the 50s/60s.

But I haven't seen any research showing correlations in real human beings yet.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10340669/

PFAS are big in media right now but likely not the main cause.