Emissions from the world’s fleet of cars, lorries and buses account for 38,000 premature deaths annually worldwide, claims a study co-authored by University of Colorado Boulder researchers.

Testing inefficiencies, maintenance inadequacies and other factors have led to 4.6 million tons more harmful nitrogen oxides (NOx) than standards allow, according to the study that has been published in Nature.

The findings are said to reveal major inconsistencies between what vehicles emit during testing and what they emit in the real world, a problem more severe that the Volkswagen incident of 2015 when it was found that millions of new diesel cars were fitted with so-called 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.

For the paper, the researchers assessed 30 studies of vehicle emissions under real-world driving conditions in 11 major vehicle markets representing 80 per cent of new diesel vehicle sales in 2015. Those markets include Australia, Brazil, Canada, China, the European Union, India, Japan, Mexico, Russia, South Korea and the United States.

They found that in 2015, diesel vehicles emitted 13.1 million tons of NOx, an umbrella term for gases such as NO, NO₂ and N₂O, which are themselves toxic and are a chemical precursor to other toxic gases such as 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 were the largest contributor worldwide, accounting for 76 per cent of the total excess NOx emissions.

Henze used computer modelling and NASA satellite data to simulate how particulate matter and ozone levels are, and will be, impacted by excess NOx levels in specific locations. The team then computed the impacts on health, crops and climate.

“The consequences of excess diesel NOx emissions for public health are striking,” said Susan Anenberg, co-lead author of the study and co-founder of Environmental Health Analytics LLC.

China is said to suffer the greatest health impact with 31,400 deaths annually attributed to diesel NOx pollution, with 10,700 of those deaths linked to excess NOx emissions beyond certification limits. In Europe 28,500 deaths annually are attributed to diesel NOx pollution, with 11,500 of those deaths linked to excess emissions.

The study projects that by 2040, 183,600 people will die prematurely each year due to diesel vehicle NOx emissions unless governments act.

The authors said emission certification tests, both prior to sale and by vehicle owners, could be more accurate if they were to simulate a broader variety of speeds, driving styles and ambient temperatures. Some European countries now use portable testing devices that track emissions of a car in motion.

Commenting on the study, Dr Paul Young, lecturer and atmospheric Scientist, Lancaster Environment Centre, Lancaster University, said: “This work is a thorough assessment of the size of the gap between real world and lab-estimated emissions, showing that the former can be up to five times higher than the emission standard.

“This means more NOx, more particulate matter and more ground level ozone, all of which have bad effects on health, plants and even our buildings and infrastructure.  One can quibble with the authors’ estimates of the impacts – and they do acknowledge the uncertainty – but they all point in one direction: more deaths and lower crop yields.

“This work points to the need to do two things: (1) get better real world estimates of vehicle emissions, and (2) have regulatory procedures that stipulate meeting real world driving emissions.  This study has made great strides towards number 1), and – as the authors say – regulators are moving towards number (2).”