It's known that exposure to benzene, chloramphenicol, and other chemicals can lead to PNH, although the exact process may be less clear than the statistical correlation.
PNH involves a genetic mutation, and it may be that the mutation is already present but not symptomatic until the bone marrow is damaged (e.g., from chemical exposure), after which the defective cells gain a "competitive advantage" over normal cells, letting them proliferate and cause the symptoms of PNH.
Are you asking for the purposes of a V.A. claim? The
issue has come up before, with evidence from a patient's medical records and statements from their personal physicians apparently carrying as much weight as the medical research literature.