The recognition, evaluation and control of dust exposures in occupational environments can be complex. An occupational hygienist must assess the nature of the hazard, the likely target of the dust, and the properties of the dust responsible for the biological response of interest. Dust measurement and dust hazard evaluation is complex because of the need to characterize properties beyond the intensity of exposure, i.e. the dust concentration. It may be necessary to describe the exposure in terms of the particle size distribution, and the often in homogeneous chemical or morphological properties of the dust. For fibrous minerals such as asbestos, particle shape may have profound impact on the toxicity of the material. Several descriptors of the particles may be used to characterize the concentration including: the mass of the particles, the mass of one chemical species in the particles, the active surface area, the number of particles, or the crystalline properties of the particles.