Document EVERYTHING before you leave is what the old timers have told me.
I had not heard about the dust/smoke link, but I have been told Sleep Apnea = automatic 50% disability (M2 didn't you post about that once?). 50% disability is a magic number for several post military considerations including tax free retirement payments, veterans preference for follow-on hiring, and many states have programs for disabled vets such as free or reduced tuition for your kids.
One common misconception is the calculation of disability....M2/Huggy/Rainman being the oldest can certainly share their knowledge, but as I understand it if your initial disability rating comes in at 30%, and you subsequently develop another condition worth 20%, the follow on disability rating is not additive, instead it is a function of the remaining value. The VA will review each disabling condition that you claimed was caused by your military service. The highest disability rating will be applied to the most severe disability. The next rating will be assigned for the next less severve disability. This will continue until each disability is rated from zero to 100%. The VA uses a table that determines the maximum rating for all of the disabilities that were rated. The total disability is not additive e.g. If you were rated at 40% for disability #1, 20% for #2, and 10% for number 3, you would not have an overall disability of 70%. The table 1st considers the 40% disability and then the 20% to get a disability rating value. From that value, it considers the 10% which results in your total VA disability rating. Go to www.va.gov and search on "disability rating table"