Human health hazards of exposure to new technology diesel exhaust (NTDE): a literature review

Introduction
Diesel engines have continually improved over the decades and are widely used in commerce due to efficient fuel use. Early diesel engines had low emissions of carbon monoxide and high emissions of nitrogen oxides (NOx) and particulate matter (PM). These characteristics of Traditional Diesel Exhaust (TDE) stimulated research to evaluate health effects of TDE. By the mid-1980s, consensus developed: (a) TDE PM contained numerous complex organic molecules, (b) extracts of TDE PM were mutagenic, (c) lifetime exposures of rats to high concentrations of TDE caused lung cancer and pulmonary disease, and (d) epidemiological studies provided inconclusive evidence for TDE causing cancer. These findings lead to stringent regulations to limit diesel engine emissions which stimulated improvements in engine technology, exhaust after-treatment, fuel, and control systems.

Methods
The most recent published scientific literature concerning the emission characteristics of New Technology Diesel Exhaust (NTDE) was reviewed and compared to that of TDE emissions.

Results
The concentrations of PM and associated chemicals in NTDE are on the order of 1% or less of that found in TDE. Exposure of laboratory animals (up to 3 months) to NTDE did not produce effects at exhaust dilutions similar to those at which TDE produced marked effects. In human exposure studies (approved by an Ethics Committee) one hour TDE exposures has been reported to reduce vasodilatation and increased ex vivo thrombus formation which were not observed after NTDE exposures. Evaluation of chemical and toxicological characterization data indicate that NTDE has a very low human health hazard potential, a determination which has informed policy decisions to accelerate the replacement of old engines with new diesel technology.

Discussion
These new scientific findings indicate that any future evaluation of hazards of exhaust from diesel engines, such as the upcoming evaluation by the International Agency for Research on Cancer, should consider TDE and NTDE separately.