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Study finds diesel oxidation catalyst eliminates mutagenicity of diesel exhaust in gas phase

Top row: Effects of the DOC on mutagenicity of the condensates in the strain TA98 with and without metabolic activation (+/−S9); n.s.: not significant. Bottom row: Mutagenicity of the particle extracts with and without DOC. Credit: ACS, Westphal et al.Click to enlarge.

A team of researchers in Germany has found that the mutagenicity of diesel engine exhaust (DEE) is eliminated in the gas phase by a Diesel Oxidation Catalyst (DOC), but only slightly reduced in the particle phase. In a study published in the ACS journal Environmental Science & Technology, they investigated the influence of different diesel fuels and the exhaust after-treatment with a DOC on the genotoxicity of DEE using the bacterial reverse mutation assay (Ames test) and a detailed characterization of the emitted particles.

Further benefits of the DOC they found include a reduction of total hydrocarbons of up to 90%; and of carbon monoxide of up to 98%. Total particle mass (TPM) was reduced by 50% with the DOC in common diesel fuel and by 30% in the other fuels.

Diesel engine emissions (DEE) are classified as probably carcinogenic to humans. The classification is based on epidemiological studies of cohorts occupationally exposed to DEE, animal studies and strong mutagenic effects in vitro...Genotoxic and carcinogenic DEE effects were mainly attributed to the particulate matter and the adherent polycyclic aromatic hydrocarbons (PAH). Recently in particular the fine <2.5 μm (PM2.5) and ultrafine (PM0.1) fraction are concerned. The high mutagenic potency of particle extracts from DEE was first described by Huisingh et al. in 1978 and has been confirmed by many other groups. Further investigations showed that condensates of the gas phase are likewise mutagenic.

Epidemiological studies on chronic DEE effects are based on exposures which occurred more than 30 years ago...In order to assess the current situation, methods are needed which yield information in a fairly short time. This can be achieved by direct measurement of the exhaust components. However, during recent years it has occasionally been shown that the available analytical methods do not necessarily show the complete picture of health hazards that are connected to DEE. Data on bacterial mutagenicity of DEE showed that in addition to PAH the directly mutagenic nitrated PAH (nPAH) strongly contribute to DEE toxicity. In face of the rapid technical development it is mandatory to monitor if these alterations can influence the spectrum of toxic emissions.

Therefore, meaningful end points of biological effects reflecting human health hazards due to DEE should be investigated. These parameters should be robust enough to allow comparative investigations even regarding historical data. Concerning these aims the bacterial reverse mutation test has been successfully applied as an assay for suspected carcinogenic effects of DEE by our working group for about 15 years. The test is particularly sensitive for PAH and nPAH. Although the test covers only a part of DEE toxicity, it enables an assessment of the alteration of the mutagenicity of DEE during recent years.

—Westphal et al.

This investigation focused on the effect of a DOC on health hazards from the combustion of four different fuels: rapeseed methyl ester (RME); common diesel fuel (DF); SHELL V-Power Diesel (V-Power); and ARAL Ultimate Diesel containing 5% RME (B5ULT).

The team applied the European Stationary Cycle (ESC) to a 6.4L turbocharged Mercedes-Benz OM 906 LA engine which was equipped with an intercooler, and fulfilling the EURO III emissions standard. The engine was operated with and without DOC. Besides regulated emissions, they measured particle size and number distributions; determined the soluble and solid fractions of the particles and characterized the bacterial mutagenicity in the gas phase and the particles of the exhaust.

They found that the DOC provided benefits for most regulated and non-regulated exhaust constituents and the mutagenicity of the exhaust, including ultrafine particles. In their experiments, the DOC was particularly effective in the elimination of small vaporizable particles, and that the catalytic reduction of mutagenicity was as well particularly effective in the condensates of the gas phase.

NOxemissions were not reduced, and they suggested that the efficiency of the DOC be enhanced by the combination with a DPF to further reduce the particle emissions or a SCR system, which additionally diminish the NOx.


  • Götz A. Westphal, Jürgen Krahl, Axel Munack, Yvonne Ruschel, Olaf Schröder, Ernst Hallier, Thomas Brüning, and Jürgen Bünger (2012) Mutagenicity of Diesel Engine Exhaust Is Eliminated in the Gas Phase by an Oxidation Catalyst but Only Slightly Reduced in the Particle Phase. Environmental Science & Technology doi: 10.1021/es300399e



Diesel is very dangerous, thank God its not popular in the US.


Apparently not true for the new-technology diesel engines.

The ACES Phase 3 study as reported here on Green Car Congress ( did not find any significant health effects in laboratory animals exposed to exhaust from a 2007-compliant diesel engine over several months.

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