This Hazard Assessment document has been published by HSE. It is aimed at a technical audience and reports on the scientific information which underpins the hazard assessment of a specific substance. This document addresses two issues (i) whether or not the fibrogenic potency (i.e. ability to cause silicosis) of crystalline silica is variable, and if so, what are the factors that influence its variability (ii) in the light of currently available information, what is the most reliable view that can be formed of the exposure-response relationship(s) for the development of silicosis. Contents: Front summary; Background; Section 1: Studies conducted in vitro; Section 2: Studies conducted in vivo; Section 3: Epidemiological evidence.