X-ray characteristics of body tissues are of crucial importance for developing and optimizing x-ray imaging techniques, in particular for dosimetry and spectral imaging applications. For breast imaging, the most important tissues are fibro-glandular, adipose and skin tissue. Some work has and is being done to better characterize these tissue types, in particular fibro-glandular and adipose tissue. In the case of breast skin, several recent studies have been published on the average skin thickness, but with regards to x-ray attenuation, the only published data, to the knowledge of the authors, is the elemental composition analysis of Hammerstein et al (1979 Radiology 130 485-91). This work presents an overview of breast skin thickness studies and a measurement of the effective atomic number ([Formula: see text]) of breast skin using spectral mammography. [Formula: see text], which together with the density forms the attenuation, is used to validate the work by Hammerstein et al, and the dependence of clinical parameters on [Formula: see text] is explored. Measurements were conducted on the skin edge of spectral mammograms using clinical data from a screening population (n  =  709). The weighted average of breast skin thickness reported in studies between 1997 and 2013 was found to be [Formula: see text] mm. Mean [Formula: see text] was found to be 7.365 (95% CI: 7.364,7.366) for normal breast skin and 7.441 (95% CI: 7.440,7.442) for the nipple and areola. [Formula: see text] of normal breast skin is in agreement with Hammerstein et al, despite the different methods and larger sample size used. A small but significant increase in [Formula: see text] was found with age, but the increase is too small to be relevant for most applications. We conclude that normal breast skin is well described by a 1.56 mm skin layer and the elemental composition presented by Hammerstein et al (1979 Radiology 130 485-91) and recommend using these characteristics when modelling breast skin.