I have read a lot about so called causitive agents over the years and it could as simple as drinking liquids too hot or too cold, using mouthwash containing alcohol, second hand exposure to cigarette smoke or any smoke for that matter, fireplaces, kerosene heaters, etc. Soot is suspected as a causitive agent, especially from diesel engines. Paint and ceiling samples can be sent to labs for analysis if you suspect that. The immune system (and genetics) obviously plays a large role in it. Basically the theory is that anything that acts as an irritant can cause a mutation. That's why, for that small group without a definitive reason, the cause can be so elusive. If we really understood all of the reasons for cancer we would have had a cure by now.