Primary Particle Properties Joel Cohen, Harvard School of Public Health 22 January 2015 The indicative primary particle diameter of TiO2 nanoparticles listed in Table 1 is 24 nm as detailed in Xia et al. 2013. The Fe3O4 nanoparticles were prepared by chemical co-precipitation, and have an indicative primary particle diameter of 12.8 nm as reported in Teeguarden et al. 2014. References: Xia, T.; Hamilton, R. F.; Bonner, J. C.; Crandall, E. D.; Elder, A.; Fazlollahi, F.; Girtsman, T. A.; Kim, K.; Mitra, S.; Ntim, S. A.; Orr, G.; Tagmount, M.; Taylor, A. J.; Telesca, D.; Tolic, A.; Vulpe, C. D.; Walker, A. J.; Wang, X.; Witzmann, F. A.; Wu, N.; Xie, Y.; Zink, J. I.; Nel, A.; Holian, A., Interlaboratory evaluation of in vitro cytotoxicity and inflammatory responses to engineered nanomaterials: the NIEHS Nano GO Consortium. Environ Health Perspect 2013, 121 (6), 683-90. Teeguarden, J. G.; Mikheev, V. B.; Minard, K. R.; Forsythe, W. C.; Wang, W.; Sharma, G.; Karin, N.; Tilton, S. C.; Waters, K. M.; Asgharian, B.; Price, O. R.; Pounds, J. G.; Thrall, B. D., Comparative iron oxide nanoparticle cellular dosimetry and response in mice by the inhalation and liquid cell culture exposure routes. Part Fibre Toxicol 2014, 11 (1), 46. Competing interests None of the authors have any competing financial interests in the work described in this comment.