US comes clean

Diesel becomes a dirty word in the official US environmental lexicon

By Chandrachur Ghose
Published: Monday 30 September 2002

after treating it as a grey area for a decade, the United States Environmental Protection Agency (usepa) has finally declared in black and white that diesel exhaust is a "likely human carcinogen". The usepa officially acknowledged this in the final version of its report, 'Health Assessment Document for Diesel Engine Exhaust', published on September 4, 2002. That it took the agency such a long time to complete the document (five drafts were revised) is a pointer to the scientific rigour which went into its preparation. Ironically, it also reflects in some ways the hectic lobbying resorted to by the diesel industry to obfuscate the issue.

The conclusion comes four years subsequent to diesel particulates being termed "toxic air contaminants" by the California Air Resources Board. Based on an extensive evaluation of loads of evidence generated by painstaking scientific research, the usepa report seeks to identify the most dangerous exposure hazards to public health. It gives a definitive direction to the prolonged debate on health effects of diesel exhaust, which was stoked by the industry, and confirms the findings of numerous other research and regulatory agencies. These primarily deal with establishing the connection between exhaust from diesel vehicles and deadly diseases such as lung cancer (see box: Damning evidence).

The report points out that diesel exhaust comprises a complex mixture of gases and particles. Besides emitting pollutants such as oxides of nitrogen and sulphur dioxide, diesel engines also release aldehydes (including formaldehyde, acetaldehyde and acrolein), benzene, 1,3-butadiene, polycyclic aromatic hydrocarbons (pahs) and nitro-pahs -- all individually known to be toxic.

Diesel particulate matter, consisting of fine particles, has a central core of elemental carbon and adsorbed organic compounds as well as small amounts of sulphate, nitrate, metals and other trace elements. Collectively, these particles have a large surface area which makes them a perfect medium for adsorbing organic compounds. In addition to this, their small size allows them to reach the lower tracts of the lungs. Many of the organic compounds present on the particle and in the gases possess mutagenic and carcinogenic properties.

The most alarming observation, however, is that virtually everyone is at risk from diesel exhaust -- whether exposed for a short period or for a longer duration. On the basis of studies conducted on both humans and animals, the report concludes that even a fleeting exposure to diesel exhaust causes acute irritation in the eyes, throat and lungs. It can lead to neurophysiological symptoms such as light-headedness and nausea, too, and result in respiratory symptoms including cough and phlegm. There is also evidence of exacerbation of allergenic responses to allergens as well as asthma-like symptoms on exposure to diesel exhaust.

It is stated that there is substantial proof to draw a link between diesel exhaust exposure and increased lung cancer risk among workers in varied occupations where diesel engines are used. In fact, the report goes one step further and invalidates the diesel industry's argument that those not occupationally exposed are not vulnerable. It says, "Although the available human evidence shows a lung cancer hazard to be present at occupational exposures that are generally higher than environmental levels, it is reasonable to presume that the hazard extends to environmental exposure levels." There is possibly no safe level of exposure to organic compounds in diesel particulates which can induce mutagenic changes, states the report.

Indeed, the document mentions some ambiguities which need further research. For instance, it is still not clear what level of exposure can induce cancer in humans. Moreover, the impact of improved diesel technology -- introduced to meet stringent norms set by the usepa -- on health risks associated with diesel exhaust needs to be ascertained.

Since this assessment provides an understanding of public health implications of the current fleet of diesel vehicles, it has imparted a sense of urgency to efforts by the usepa and others to formulate tough emission standards for heavy-duty vehicles under the Clean Air Act. The norms, to be enforced from 2007, are expected to reduce emissions by as much as 95 per cent. The agency is also developing a proposal to tackle pollution from diesel-powered non-road vehicles and equipment.

Some environmental groups are, however, sceptical that the George Bush administration may try to water down the effectiveness of the rules laid down by the previous regime. Agency spokesperson Joe Martyak counters activists' charges, pointing out that the usepa is "already sensitive to the importance of the issue, which why we are moving along on the diesel issue with an aggressive schedule".

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