Particulate-matter emissions are one of the major environmental hazards of diesel cars and trucks, but the problem may not be as great as it appears.

A new paper (PDF) from the United Nations Economic Commission for Europe (UNECE) says that vehicular exhaust is only responsible for a small amount of total particulate emissions in the U.S. and Europe.

The paper--titled Diesel Engines Exhaust: Myths and Realities--claims that vehicle-exhaust emissions account for just 3 percent of airborne particulate matter in the U.S. and Canada, and 17 percent in the European Union countries.

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In light of these results, the paper's authors advocate shifting more scrutiny from the heavily-regulated transportation sector to other areas--such as commercial and household sources--that appear to be producing the majority of harmful emissions.

Diesel taxis in London (Image by Flickr user Lars Ploughmann, used under CC license)

However, the paper also called for continued efforts to improve the efficiency of diesel and other road-vehicle powertrains, saying this should be pursued "in an aggressively well-targeted way."

In the U.S., regulators have worked to clean up diesel-exhaust emissions through the implementation of stricter efficiency standards and the introduction of "Ultra-Low-Sulfur Diesel" between 2006 and 2010, which enabled a new generation of more sophisticated exhaust aftertreatement technology to be introduced.

Passenger-car manufacturers have to meet a Corporate Average Fuel Economy (CAFE) standard of 54.5 mpg (the equivalent of 42 mpg on the window sticker) for 2025 models.

But more recently, President Barack Obama has instructed the Environmental Protection Agency and Department of Transportation to draft standards for 2025-model medium- and heavy-duty trucks as well.

In addition, stricter standards for diesel tractors and construction equipment are now taking effect for the 2014 model year.

These "Tier 4" standards call for a 90-percent reduction in particulate matter and nitrogen oxides compared to 1996-model-year vehicles.

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