19 January 2022
In their commentary on the suitability of the use of standardized diesel exhaust particles (DEP) for studying the adverse effects attributed to particulate matter (PM), Block and Kodavanti  refer to our recent publication by Farahani et al.  in which we offered experimental evidence on the dissimilarities in chemical composition of these DEP to ambient PM to which populations are exposed. The specific DEP is diesel soot emitted by a forklift engine (NIST SRM 2975). Parenthetically, we are not the first study to demonstrate this observation. For instance, Braun et al. concluded that “The NEXAFS spectra of the two diesel soot samples appear quite similar, while they differ significantly from the urban PM spectrum, in agreement with X-ray diffraction data published recently” .
Block and Kodavanti stated that “Diesel exhaust is a component of urban air pollution and a prominent source of ultrafine particulate matter”. However, we should note that the specific type of DEP upon aerosolization has a median particle number diameter of about 200 nm and more importantly a mass median diameter of 500 nm, thus they hardly qualify as ultrafine PM. About 50 percent of the mass fraction in DEP is black carbon, which is higher by 5-fold or more than the BC content of PM inside the US freeways that are heavily impacted by heavy-duty diesel truck traffic. As we discussed in our paper, their chemical composition, depleted in several high molecular weight carcinogenic PAHs, does not make them a suitable proxy for traffic emitted PM nor urban PM. The latter include substantial fractions of polar organic compounds, ingredients of secondary organic aerosols (SOA) produced by highly complex photochemical reactions in the atmosphere, the content of which is negligible in the NIST SRM 2975 DEP. Furthermore, the levels of redox-active trace elements and metal species in PM that hold significant importance in health studies (e.g., Ba, Ti, Mn, Fe, Cu, Ni) are almost 20 times greater on Los Angeles roadways than those in the DEP.
We recognize the need for using standardized materials that might facilitate comparability between different studies. But we believe these materials will need to bear stronger resemblance to reality if the outcomes of these studies are to be used for the implementation of regulatory measures to protect the public.
University of Southern California
Los Angeles, California, USA
 Block ML, Kodavanti UP (2021) The use of standardized diesel exhaust particles in Alzheimer’s disease research. J Alzheimers Dis 84, 607-608.
 Farahani VJ, Pirhadi M, Sioutas C (2021) Are standardized diesel exhaust particles (DEP) representative of ambient particles in air pollution toxicological studies? Sci Total Environ 788, 47854.
 Braun A, Mun BS, Huggins FE, Huffman GP (2007) Carbon speciation of diesel exhaust and urban particulate matter NIST standard reference materials with C (1s) NEXAFS spectroscopy. Environ Sci Technol 41, 173-178.