Diesel Pollutes More Than Lab Tests Detect

The study, published in Nature, shows these excess emissions alone lead to 38,000 premature deaths annually worldwide, including 1,100 deaths in the US.

The findings reveal major inconsistencies between what vehicles emit during testing and what they emit in the real world --a problem that's far more severe, said the researchers, than the incident in 2015, when federal regulators discovered Volkswagen had been fitting millions of new diesel cars with "defeat devices."

The devices sense when a vehicle is undergoing testing and reduce emissions to comply with government standards. Excess emissions from defeat devices have been linked to about 50 to 100 US deaths per year, studies show.

"A lot of attention has been paid to defeat devices, but our work emphasizes the existence of a much larger problem," said Daven Henze, an associate professor of mechanical engineering at CU Boulder who, along with postdoctoral researcher Forrest Lacey, contributed to the study. "It shows that in addition to tightening emissions standards, we need to be attaining the standards that already exist in real-world driving conditions."

The research was conducted in partnership with the International Council on Clean Transportation, a Washington, D.C.-based nonprofit organization, and Environmental Health Analytics LLC.

Researchers found that in 2015, diesel vehicles emitted 13.1 million tons of NOx, a chemical precursor to particulate matter and ozone. Exposure in humans can lead to heart disease, stroke, lung cancer and other health problems. Had the emissions met standards, the vehicles would have emitted closer to 8.6 million tons of NOx.

Heavy-duty vehicles, such as commercial trucks and buses, were by far the largest contributor worldwide, accounting for 76 percent of the total excess NOx emissions.