Review of animal carcinogenicity studies and comparison of NTDE vs TDE

Exhaust from traditional diesel engines (TDE) contains high concentrations of respirable carbonaceous particles with absorbed organic compounds, some of which are mutagenic, raised concern for the cancer-causing potential of diesel exhaust exposure. Extensive research addressing this issue has been conducted during the last 3 decades. Lifespan animal bioassays demonstrated that chronic inhalation of high concentrations of diesel exhaust caused lung tumors in rats but not in mice or hamsters. Subsequent research has shown that similar exposure to particulate matter generally considered innocuous (such as carbon black and titanium dioxide) also caused lung tumors in rats. Thus, in 2002, the USEPA concluded that the findings in the rats should not be used to characterize the cancer hazard or quantify the cancer risk of diesel exhaust. Increasingly stringent U.S. emissions standards (1988–2010) for particulate matter (PM) and nitrogen oxides (NOx) in diesel exhaust have helped stimulate major technological advances in diesel engine technology and fuel/lubricant composition, resulting in the emergence of what has been termed New Technology Diesel Exhaust, or NTDE. NTDE is defined as DE from post-2006 and older retrofit diesel engines that incorporate a variety of technological advancements, including electronic controls, ultra-low-sulfur diesel fuel, oxidation catalysts, and wall-flow diesel particulate filters. Numerous emissions characterization studies have demonstrated reduction in regulated and unregulated emissions of NTDE compared to TDE. Furthermore, the composition of particles emitted in NTDE is very different from TDE particles and is closer in chemistry to compressed natural gas particles.  Preliminary toxicological data suggest that the changes to the physical and chemical composition of NTDE lead to the complete elimination of the biological responses observed with TDE exposure. There is now sufficient evidence to conclude that health effects studies of pre-2007 DE likely have little relevance in assessing the potential health risks of NTDE exposures.