Nature of exposure to chrysotile.

Fibres in general, and chrysotile fibres in particular, have properties that set them apart from other airborne particles in terms of their aerodynamic behaviour and the nature of their interactions with cells and fluids in vivo. Their lengths and diameters are primary determinants of their penetration through conductive airways to deposition sites within the lungs, and to their access into translocation pathways; their surface properties and biopersistence affect their retention times and the biological responses that can lead to chronic disease. In the absence of sufficiently detailed knowledge of the specific ranges of lengths, diameters, surface properties and biopersistence of the long, thin fibres most closely related to hazard potential, crude surrogates of relevant exposure have been used. These include: (i) the number concentration of fibres (aspect ratio > 3) longer than 5 microns, as measured by optical microscopes lacking the ability to resolve thin (< 0.25 micron dia.) fibres; or (ii) by electron microscopy of mostly short, thin fibres by scanning a small fraction of the surface of the filter on which they have been collected. The conventional electron microscopy analytical protocols lack the ability to determine the presence of a statistically relevant number of the long fibres having the greatest hazard potential. A strategy for determining the distribution of long fibres of all thicknesses is outlined.