The Size Distribution of Airborne Bolivian Crocidolite Fibers

Introduction

Current Environmental Protection Agency (EPA) risk models and asbestos fiber counting techniques for monitoring airborne exposures treat all fibers as being equivalent and ignore the effect of dimension on toxicity. While this may have been appropriate when modern analytical methods were not available, the measurement world is now fully capable of making the appropriate distinctions. The importance of width as a determinant of carcinogenicity has been recognized for nearly 30 years within the published literature. Stanton,¹ in a landmark study of a suite of fibrous materials, found that durable mineral fibers with widths less than 0.25 µm and lengths longer than 8 µm were causally associated with the induction of tumors. Pott,² in a later study, postulated a fiber toxicity model that agreed reasonably well with the Stanton findings, but extended the concept to include varying toxicity as a function of length and width. Wylie et al³ reanalyzed samples used by Davis et al⁴ and found that tumor generation was strongly and inversely correlated with fiber width. Crump⁵ testified increasing fiber length was correlated with disease occurrence. Lippman⁶ reviewed a number of studies and suggested that mesothelioma was closely associated with long, thin fibers. Berman et al⁷ published a risk model, which found that disease was correlated almost exclusively with fibers longer than 40 µm and width less than 0.25 µm. Recently, Loomis et al⁸ reanalyzed textile mill samples and found that aspect ratios greater than 40:1 were highly correlated with disease.