I'm also a Vietnam-era Vet; I did Advanced Training for 7 months at Ft. Devens, which became an EPA Superfund Cleanup site in '89 for things like drinking water containing Benzene, TCE, PFOS and PFAS. I also worked two years in an electronics shop, where TCE was always in the air (we had a large degreasing vat of it). It took decades for me to develop Leukemia and Hyperthyroidism - but I did. Is there any good data on how long it took for symptoms to develop before the VA will flatly deny benefits - even if I do have IMO letters?
To my knowledge and belief, there isn't anything as precise as what you're asking for. However, it's my experience that if you can prove exposure to carcinogenic chemicals then service connection will follow even though decades may have passed.
This is evident in the way agent orange presumptive cancer conditions are managed. If a 75 or 80 year old boots on the ground Vietnam veteran gets prostate cancer tomorrow, the service connection is automatic although that much time has passed.
The same would hold true if you establish service connection for the chemicals you were exposed to and your current disease with a well written IMO or nexus letter. I think that most agree that the ill effects of chemical exposures can take decades to show signs and symptoms of harm.
You may benefit by researching cases similar to yours that have been denied and appealed and then won or lost at the BVA. Click here and then use the search function you'll find to look for key words like <benzene TCE hyperthyroidism> and learn what the court thought of these adjudicated claims and appeals. These cases do not set precedent so you can't cite one and then you're done...you have to duplicate every step of the winning appeal but many vets prevail in similar cases.
Trichlorethylene (TCE) was used by the 55 gallon drum for all kinds of applications...I remember the stuff all too well. TCE was the only thing that would remove the Cosmoline gunk that our weapons were packed in for shipment and storage. With just that thought, I can still smell Cosmoline. I was 91D20 at the 98th General Hospital in Germany and we used TCE to clean all sorts of things. Our huge steam pressure autoclaves would need flushing with the stuff to remove hard water scale.
I think we vented that into the open air straight out through the ceiling.
Now I wish I hadn't remembered that.
Good luck sir.