Table 6.

Similarities and differences between RCF and rock wool.

		Comparison
Physical properties	Biopersistence	RCF and rock wool have nearly identical biopersistence as measured by WT1/2 values from short term inhalation exposures with rats.
	Airborne fiber dimensions	Though bulk fiber diameters differ, diameters of airborne fibers from personal monitoring samples are quite similar. Fiber lengths of RCF and rock wool are similar.
	Breakage mechanism	Both RCF and rock wool break transversely rather than longitudinally. Thus, any breakage results in fibers that are more easily removed by macrophages.
Animal studies	IP studies	IP studies of rats show that exposure to both RCF and rock wool result in the development of tumors.
	Inhalation studies	A well-done chronic bioassay of rats exposed (nose-only) to rock wool (MMVF21) resulted in mild fibrosis, but no tumors. A similar study on RCF resulted in both fibrosis and tumors, although the result may have been undermined by overload resulting from exposure to a test article with a non-representative ratio of particles to fibers.
Epidemiological studies	Pleural plaques	Exposure to RCF results in a dose-dependent statistically significant increase in the frequency of pleural plaques. Limited data on rock wool exposure resulted in an increase in the frequency of pleural plaques in exposed workers that was not statistically significant.
	Interstitial Fibrosis	No interstitial fibrosis seen in cohorts exposed to either rock wool or RCF.
	Increased lung cancer or mesothelioma	Mortality studies fail to indicate any increase in either lung cancer or mesothelioma among cohorts exposed to either RCF or mesothelioma. The statistical power of the rock wool studies is much larger, however.