I am positive of the effects of this issue. Having had a 319 cabin full of smoke a few years ago. Packs off smoke cleared. Packs were run for Four hours on ground to try and clear contamination before we ferried aircraft home. On arrival ALL crew felt light headed and nauseous, although no smoke was visible during flight. Just the dirty socks smell.
Since then I have been diagnosed with intermittent peripheral neuropathy, with no apparent cause, but likely chemical poisoning. Still flying, but a true believer that this is a valid issue, being blanked by the industry due to the possible legal ramifications.
In reference to the above about seals, it is indeed oil that leaks through when engines not running or during power decreases. It’s evident to investigating engineers there is oil in the bleed ducts that the ACM temps de-compose into the poisonous compounds/smoke. Sad but true, and airlines are hardly likely to be so honest during any investigations.