The effect on the measurement of volumetric breast density of variations in physical and chemical properties of adipose and fibroglandular tissue reported in a number of studies is investigated using the authors' model of mammographic image formation. This model is developed specifically for the measurement of breast density. The effect of varying stromal composition, a popular histopathological explanation of mammographic density, is also discussed. Given the uncertainties in tissue attenuation highlighted by this study, as well as noise, and acquisition model error, the validity of this measurement is discussed, together with alternative measurement scales. Several issues are considered, including the effect of beam quality on normalisation accuracy, and the measurement failure which can occur when clinical data falls outside the limited range defined by 100% adipose to 100% fibroglandular tissue.