That would seem to be a likely answer, but the data does not bear it out, at least according to Prager.
Asbestosis is offered as another possible reason, but asbestosis does not bring those rare cancers. Restricted pulmonary efficiency, but not rare cancers.
And even more interesting, it seems that the DELTA Group and USGS samples are very very low in asbestos readings. That is, afterwards there was not very much asbestos present there.