1-nitropyrene.

Carcinogenicity of diesel fumes – how will it affect the industry?

Published:  09 October, 2012

How bad are engine exhaust fumes for your health? Dr. Roger Klein explains the consequences for the industry, based on the recent report published by the WHO.

Press Release 213# from the World Health Organization (WHO) dated 12 June 2012 highlights the findings of a recent meeting of a Working Group of experts held in Lyon, France, organised by the WHO International Agency for Research on Cancer (IARC)[1]. The Working Group formally classified diesel engine exhaust as carcinogenic to humans (Group 1) based on sufficient evidence that exposure is associated with an increased risk for lung cancer, as well as classifying gasoline engine exhaust as possibly carcinogenic to humans (Group 2B)[2,3]#.

Dr. Christoph Portier, Chairman of the IARC Working Group, stated that “….The scientific evidence was compelling and the Working Group’s conclusion was unanimous: diesel engine exhaust causes lung cancer in humans….” Dr. Christopher Wild, Director of the IARC, commented that “….today’s conclusion sends a strong signal that public health action is warranted….”

How serious a problem is this for the heavy plant and transport vehicle industry? Potentially very serious as evidenced by attempts to prevent publication. As reported by Bryant Furlow in the journal Lancet Oncology, “…Several scientific journals, including The Lancet titles, have received letters from the industry-funded Mining Awareness Resource Group (MARG)[4]#, warning against “publication or distribution” of papers from a US government study of diesel exhaust and lung cancer…”[5].#

Diesel produced by refining petroleum contains approximately 75% saturated hydrocarbons, consisting of straight-chain n-alkanes, branched-chain iso-alkanes and cyclic cycloalkanes, together with 25% aromatic hydrocarbons including benzene, toluene and xylene (BTX fraction) as well as polycyclic aromatic hydrocarbons (PAHs) such as naphthalene and its higher homologues. The average chemical composition for petroleum-derived diesel is C12H23, with range of about C10H20 to C15H28, indicating that it is a high boiling point fraction with a considerable degree of unsaturation due to the aromatic content by comparison with motor gasoline.

The high sulphur levels found in diesel represent a potentially serious problem for the environment because neither can catalytic particulate filters – the sulphur will poison the catalyst – nor can absorption technology for the nitrogen oxides (NOx) produced be used. Additionally oxidation of the sulphur content during combustion gives rise to sulphur dioxide and sulphur trioxide, which then form sulphuric acid in the presence of water vapour contributing to the production of “acid rain’’ Attempts to reduce sulphur levels - regulations in many countries such as the United States and the European Union mandate that sulphur content be greatly reduced or completely eliminated – require the inclusion of additives in order to maintain fuel lubricity and prevent excessive engine wear.

Alternative sources of diesel fuels aimed at avoiding the problems of high sulphur and aromatic hydrocarbon content, especially the PAHs, include synthetic diesel manufactured by the Fischer-Tropsch process from almost any carbonaceous material and containing mainly paraffins with zero sulphur content and extremely low in aromatic hydrocarbons, as well as biodiesel or FAME.

Biodiesel or FAME (fatty acid methyl ester) which is manufactured from vegetable oil or animal fats, suffers from a lower energy yield than diesel due to its oxygen content. It also produces high levels of NOx, may contain residual methanol, glycerol and glycerides, and is suspected of giving rise to polymerisation and corrosion problems. FAME is mostly used mixed together with petroleum-based diesel, e.g., as a product Bxx where xx represents the percentage of biodiesel. On the other hand biodiesel exhaust gases do have significant advantages in that they are low in sulphur dioxide and trioxide, polycyclic aromatic hydrocarbons (PAHs) and nitrated PAHs.

Overall ten nitro-polycyclic aromatic hydrocarbons have been identified in diesel-engine exhaust in the Benbrahim-Talla et al. study3. 1-Nitropyrene (1NP) is the predominant nitrated polycyclic aromatic hydrocarbon (PAH) present in the exhaust gases produced by combustion in diesel engines. It is categorized as an IARC Group 2A carcinogen indicating potential carcinogenicity in humans, together with 6-nitrochrysene (6NC). Nitropyrene is produced, together with other nitrated PAHs all of which are toxic, through oxidation with oxygen O2 in the presence of atmospheric nitrogen N2 at the high temperatures and pressures found in the engine cylinder. Other Group 2B nPAHs identified as showing strong mechanistic evidence for genotoxicity include 3-nitrobenzanthrone (3NBzA). Nitrated PAHs are all potentially carcinogenic to humans, for example, including those in tobacco smoke, because they are able to initiate free-radical damage to the DNA of cells that are exposed to them, e.g., in the lung. 2-Nitrofluorene (2NF, another IARC Group 2B carcinogen) and 1-nitropyrene have been specifically targeted as nitrated PAHs in exhaust emissions and dramatic reductions of 90% have been achieved with biodiesel fuels.

How does typical gasoline compare with diesel? Gasoline is more volatile than diesel and contains typically hydrocarbons with C4-C12 carbon atoms (typical boiling point range 35-200°C compared to 160-370°C for diesel). Straight-run gasoline distilled from crude oil is low in aromatics and unsaturated hydrocarbons (olefins), consisting primarily of alkanes and cycloalkanes with some olefins. Its low octane rating (RON) means that additives are required; traditionally this used to be lead tetraethyl but this has now been replaced by less toxic oxygenates such as ethanol, MTBE or ETBE. The oxygenates reduce the amount of carbon monoxide and unburnt fuel in the exhaust gases. Treatment of straight-run gasoline to increase the iso-alkane content and thus the octane rating are carried out as part of refinery streams such as alkylate or isomerate. Benzene content, itself a carcinogen affecting the bone marrow, is strictly regulated in many countries. Many gasolines are now blended with 5-10% or more ethanol, for example, E10. Gasoline comes in two grades, winter and summer gasoline. Winter gasoline has a higher butane content than summer gasoline to increase volatility at low temperatures.

Because gasoline is much lower in aromatic hydrocarbons than diesel, especially benzene, as a result of regulatory limits in many countries together with an increasing preference for high octane blends containing predominantly branched-chain alkanes, there is much less likelihood of highly toxic nitrated aromatic hydrocarbons (nPAHs) being produced as seen with diesel. Gasolines are generally much lower in sulphur than diesel resulting in reduced emission of sulphur dioxide and trioxide in the exhaust gases, contributors to the production of acid rain.

The risk of exposure to high concentrations of dangerous combustion products will be greatest for those occupations involving either continuous driving in heavy traffic where re-breathing of contaminated air in the cab occurs, or in situations where heavy diesel plant is used in poorly ventilated conditions such as underground in the mining industry or in other enclosed, poorly ventilated spaces. Whether the diesel used is low in sulphur as used for road vehicles or high in sulphur as used for fixed plant, also has a profound effect on the toxicity of the exhaust emissions produced.  What can be done about protecting against occupational exposure to diesel and, to a lesser extent, gasoline exhaust fumes?

Clearly the best solution, which is also environmentally advantageous, is to reduce aromatic hydrocarbon and sulphur content as far as possible. This may not be commercially viable, however, especially if high sulphur feedstocks such as Venezuelan Crude are being used.

2-nitrofluorene.

Alternatively exhaust gases may be removed using a ventilation system. This is only practical for fixed installations such as large compressors or pumping units or in vehicle appliance bays at engine start-up, a solution long used by fire departments and other vehicle fleet managements worldwide.

Air filtration using HEPA filters and activated charcoal may be effective within vehicles, i.e., in the driving cab, so long as filters are properly maintained and replaced regularly. Air conditioning systems can be switched to re-circulation mode to prevent drawing in contaminated air from the outside especially in heavy traffic or in summer where smog conditions may occur with high ambient temperatures and UV levels.

With the classification of diesel and, to a lesser extent, gasoline exhaust fumes as carcinogenic by the WHO IARC, where does this leave the industry? Certainly in need of developing measures to control the exposure of the population at risk, with all the cost implications that this involves. Additionally employers will be at risk of prosecution by national health and safety regulatory authorities as well as being exposed to potentially expensive legal or industrial action taken by the unions. The time is certainly ripe for the introduction of standard risk management measures to reduce, control or eliminate the risk posed?

6-nitro-chrysene.

References

[1] World Health Organization (WHO) International Agency for Research on Cancer. Press Release             213, 12th June 2012, ”, Lyon, France, “IARC: Diesel Engine Exhaust Carcinogenic”  to appear in IARC Monograph 105.  http://terrance.who.int/mediacentre/audio/press_briefings/>

[2} Silverman, D.T., et al. 2012; http://jnci.oxfordjournals.org/content/early/2012/03/05/jnci.djs034.abstract

http://jnci.oxfordjournals.org/content/early/2012/03/05/jnci.djs035.abstract.

[3] Benbrahim-Tallaa, L., et al. (2012) “Carcinogenicity of diesel-engine and gasoline-engine exhausts and some nitroarenes” The Lancet Oncology 13(7) 663-664.

[4] See the court papers of the US District Court for the Western District of Louisiana Lafayette-Opelousas Division for correct name.

[5] Furlow, B. (2012) “Industry group “threatens“  journals to delay publication“ The Lancet Oncology 13(4) 337.

3-nitro-benzanthrone.

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