Compared to the chemical risk assessment process, where hazards are generally identified as a function of an organism's adverse response to a particular mass of a chemical agent (a dose), characterizing the hazards of nanomaterials has been problematic. This is largely due to the many features that may contribute to the toxicity of nanomaterials. For example, an organism's response to a particular nanomaterial may be related to the mass of the administered dose, or it may be related to other factors, including the number of particles, shape, electrical charge, and coating, or a combination of physiochemical characteristics