Data exist that show that chrysotile asbestos does not retain its mineral properties, or biological activity, at temperatures far below the olivine transformation point. Temperatures hundreds of degrees below this point cause the mineral to lose structural water with accompanying crystal structure degradation. The loss of structure is accompanied by modification of its surface and reduction or loss of biological activity. Using heating studies and milling as an approximation of thermal and mechanical shear stress that chrysotile is subjected to on a brake lining, biological blunting is shown to begin much earlier than the olivine transformation process. Minimal degradation of the chrysotile surface structure imparts a disproportionately great effect on its biological activity. Biological and epidemiological data for brake workers exposed to chrysotile asbestos should be viewed in context with the conditions of service to which the product was subjected over a lower range of temperatures than previously considered important.