Characterization of mineral population by index particle: implication for the Stanton hypothesis

Fibers from seven crocidolite samples used by M. F. Stanton et al. (J. Natl. Cancer Inst. 67, 965-975 (1981] were measured from STEM and optical images. Their masses were computed. From these data, the population was normalized to a sample mass of 1 micrograms, and the number of fibers in various size categories of a 1-microgram sample was compared to the tumor probability for each sample. The purposes of this study were (1) to evaluate errors in and the utility of population characterization by index number (log of the number of particles longer than 8 micron with widths equal to or less than 0.25 micron) and (2) to review the Stanton hypothesis in view of this analysis. The study found that in most cases the index number is a reliable population parameter. However, index numbers between 0 and 2.5 cannot be obtained. Therefore, populations of mineral fibers will fall into two groups: those without index particles, which have indeterminant index numbers, and those with index numbers between 2.5 and 6.0. Within Stanton's populations that contain index particles, where y equals logit tumor probability and x equals index number, r equals 0.307 (insignificant). Where y equals tumor probability and x equals index number, r equals 0.503 (significant). The correlation coefficients are low enough to suggest the possibility that factors other than size and shape play a role in mineral fiber carcinogenicity.