Composition, Fe3+/ΣFe, and crystal structure of non-asbestiform and asbestiform amphiboles from Libby, Montana, U.S.A. by M.E. Gunter, M.D. Dyar, B. Twamley, F.F. Foit Jr., and S. Cornelius (vol. 88, no. 11–12, pages 1970–1978, 2003).
On the first page of this paper, toward the end of the second paragraph, this paper says that W.R. Grace provided an estimate. The statement should read:
Recently, reports in the media estimated that this product [Zonolite] may be in as many as 15 million homes in the United states.