I served in the Marines between 81-84. I was exposed to excessive diesel fuel and fumes. I am currently being treated for heart disease, endocrine issues, lung issues, hypertension, and dry eye syndrome. I understand that excessive exposure can lead to these type of issues. I was denied disability benefits. What do you think?
When we file a claim that alleges we have diseases brought on by a given chemical exposure, we have to prove it. If we just tell VA we are sure that's what happened to cause our health problems and we don't offer proof, we can expect a denial.
In this case you have to prove you were exposed to diesel fumes. Then you have to prove how much time you were exposed and under what circumstances. I guess we could agree that most of us are exposed to diesel fumes off and on so now you have to prove that you were exposed to what you call 'excessive' fumes and at what level of exposure those excess fumes would cause disease.
Once you've done that you have to provide scientific evidence that diesel fumes cause, contribute to or aggravate the diseases you claim. That's usually best accomplished with an IMO https://www.vawatchdog.org/imo-ime-medical-opinions-exams.html
That you have heard that such exposure can cause disease may or may not be correct. But if you want to claim it as service connected, you've got more work to do.