For isometric particles the mass is and has been the most convenient descriptor. Biological effects have been related to the mass dose, and concentration standards are given in terms of suspended mass per volume of air. Mass can be easily determined by gravimetric and/or chemical analysis. Moreover the measurement techniques themselves are less dependent on a priori assumptions related to factors such as mono- or poly-dispersibility, the particle shape and brief fluctuations of the concentration. An empirical mass-based lung burden of 1-2 mg dust per gram lung was identified early on as a threshold above which relatively benign dusts showed significant prolongation (> doubling) of the pulmonary retention half-time in Fischer F344 rats (Morrow, 1987). More recently, Pauluhn proposed the use of the mass-based pulmonary burden rather than actual mass concentration or surface area concentration as a critical denominator of dose and dose-related pulmonary toxicity (Pauluhn, 2009a).