Olfactory in what way? I think toxins could definitely get in by breathing it in, e.g. like mold, which is a generally underestimated, but in my opinion significant source of complex chronic conditions. So for sure those are possibilities too.

I suspect alzheimers (which is a subset of dementia), is a cluster of diseases similar to cancer. I think a large subset is due to candida, but I wouldn't be surprised if mold and other environmental toxins cause other subsets.