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Diesel engine exhaust (DEE) has been demonstrated to cause several toxic effects in animals and humans. These effects include respiratory, cardiovascular and immune system toxicity. DEE has also been demonstrated to be genotoxic and carcinogenic. IARC and US EPA have described DEE as probably and likely to be carcinogenic to humans, respectively. The California Air Resources Board (CARB) listed particulate matter from diesel-fueled engines as a Toxic Air Contaminant (TAC) in 1998, with a cancer unit risk factor of 3 × 10-4 (µg/m3)-1. Since the diesel exhaust particulate (DEP) TAC listing in 1998, diesel engine manufacturers have developed diesel engines (“new technology engines”, or NTE) which produce substantially lower exhaust levels of DEP and air toxics compared to older engines. Reported reductions in DEP and air toxics vary according to the engine type, fuel, type of emissions controls used, and engine test cycles used to evaluate engine emissions. Experimental data from several NTE engine emissions studies indicate that the reductions of some air toxics such as polycyclic aromatic hydrocarbons, benzene and 1,3- butadiene in NTE exhaust (often 80 – 90%) are not as great as the corresponding reductions in DEP (often 95 – 99%). The resulting air toxics/DEP ratios for NTE exhaust may be greater than or equal to similar ratios found in exhaust from older diesel engines. As an example, an analysis of data from one published review indicated that the average 3-ring PAH, 1,3-butadiene and benzene/DEP ratios increased in NTE exhaust compared to older DEE by 2-, 10- and 4-fold, respectively. These data suggest that while the absolute amount of DEP (and thus estimated cancer risk) and air toxics is much reduced in NTE exhaust, the exhaust composition has not necessarily become less hazardous. Thus, the available data do not indicate that NTE exhaust should be considered to be fundamentally different in kind compared to older DEE for risk assessment purposes and suggests the TAC cancer unit risk value for DEP can continue to be applied to NTE exhaust risk assessments