The ambiguity of the cause is a crucial thematic element:
Understanding the specific type of cancer helps explain "how" he got it. how did walter white get lung cancer
: Skyler White explicitly mentions Walt's history of working in application labs 20 years prior. She recalls him complaining about inadequate ventilation hoods while handling dangerous chemicals, suggesting he may have inhaled industrial carcinogens over time. The ambiguity of the cause is a crucial