I've seen them as well, and I too don't have a link. although one beautiful references is the WHO/IARC study from 1998 which I believe was referenced in this thread earlier.
And your hypothesis of exposure to just about anything is right on target.
Dug up an interesting little piece of information this evening while researching bubonic plague (two cases have been confirmed here not far from us). Seems some folks have a gene that protects them from plague, which is why only HALF of Europe was decimated by the Black Death a few centuries back. A recent study (will be able to provide links but not tonight) indicates that may (typical wiggle word, I know) be the SAME gene that protects some smokers from those horrible "smoking-related" diseases.
Speaking of poetic justice...