You will have to show that your health problems are causally linked to your contact with certain environmental hazards while serving in the military.
What if I was exposed to particulate matter and it has caused asthma?
You would qualify if you were in contact with particulate matter or large burn pits in Iraq, Afghanistan, and Djibouti in the Horn of Africa, for example.
It might have also been toxic soil, can I qualify for that as well?