New international research finds laboratory tests of nitrogen-oxide emissions from diesel vehicles underestimate real-world emissions as much as 50%.

A team of researchers examined 11 major vehicle markets representing more than 80% of new diesel-vehicle sales in 2015.

They found the vehicles emitted 13.2 million tons of nitrogen oxide under real-world driving conditions, which is 4.6 million tons more than the 8.6 million tons expected from vehicles’ performance under official laboratory tests.

The work was led by the International Council on Clean Transportation, working with scientists at the University of York’s Stockholm Environment Institute in the U.K., the University of Colorado and the Austria-based International Institute for Applied Systems Analysis.

The study, published in the journal Nature, estimates excess diesel-vehicle nitrogen oxide in 2015 was linked to about 38,000 premature deaths worldwide – mostly in the European Union, China and India.

At a global level, the study estimates the impact of all real-world diesel nitrogen-oxide emissions will grow to 183,600 early deaths in 2040 unless something is done to reduce it.

Long-term exposure to nitrogen oxide is linked to a range of adverse health outcomes, including disability and reduced life expectancy due to stroke, heart disease, chronic obstructive pulmonary disease and lung cancer.

In some countries, implementing the most stringent standards – already in place elsewhere – could improve the situation substantially, the researchers say.

University of York researcher Chris Malley says the study also shows excess diesel nitrogen-oxide emissions affect crop yields and a variety of human health issues.

“We estimate that implementing Next Generation  (emissions) standards could reduce crop production loss by 1%-2% for Chinese wheat, Chinese maize and Brazilian soy, and result in an additional 4 million tons of crop production globally,” Malley says in a statement.

ICCT researcher Josh Miller says heavy-duty vehicles such as commercial trucks and buses by far were the largest contributor worldwide in the study, accounting for 76% of total excess gas emissions.

“Five of the 11 markets that we looked at, Brazil, China, the European Union, India, and the U.S., produced 90% of that,” Miller says. “For light-duty vehicles, such as passenger cars, trucks, and vans, the EU produced nearly 70% of the excess diesel nitrogen-oxide emissions.”

The study was funded by the Hewlett Foundation, ClimateWorks Foundation, European Climate Foundation, Energy Foundation China, and the NASA Health and Air Quality Applied System Team.