In a new study co-authored by University of Colorado Boulder researchers, it was found that trucks, cars and buses globally emit 4.6 million tons more harmful nitrogen oxide (NO x ) than standards allow. This happens as a result of of inadequate maintenance, inefficient testing and other factors.

The study was published in Nature and demonstrates that these excess emissions alone lead to 38,000 premature deaths per year globally. This figure includes 1,100 deaths in the United States.

Major inconsistencies were found between what vehicles emit in the real world and what they do during emission testing. The researchers claim that the problem is far more severe than the 2015 incident when federal regulators discovered that Volkswagen had been fitting “defeat devices” to millions of new diesel cars.

These devices detect when a vehicle undergoes testing and then reduces emissions so that these comply with government standards. Studies link about 50 to 100 U.S. deaths annually to excess emissions from defeat devices.

Daven Henze, an associate professor of mechanical engineering at CU Boulder contributed to the study, along with postdoctoral researcher Forrest Lacey. Henze noted that although much attention was given to defeat devices, their work emphasizes the existence of a much bigger problem. It shows that not only should emissions standards be tightened, but the standards that are already in place in real world driving conditions should also be attained.

The researchers assessed 30 studies of vehicle emissions under real world driving conditions for the study. These studies represented 80 percent of new diesel vehicle sales in 2015 in 11 major vehicle markets. Those markets included Brazil, Australia, China, Canada, India, the European Union, Mexico, Japan, South Korea, Russia and the United States.

It was found that 13.1 million tons of NO x was emitted by diesel vehicles in 2015. NO x is a chemical precursor to particulate matter and ozone. Humans exposed to the substance could develop stroke, heart disease, lung cancer and other health problems. Had the emission standards been met, closer to 8.6 million tons of NO x would have been emitted.

Commercial trucks and buses, and other heavy duty vehicles were by far the largest contributor internationally and accounted for 76 percent of the total excess NO x emissions.

Henze and Lacey used NASA satellite data and computer modeling to simulate how particulate matter and ozone levels are influenced by excess NO x levels at specific locations. The team then calculated the expected effects on crops, health and the climate.

Susan Anenberg, co-lead author of the study and co-founder of Environmental Health Analytics LLC, found the consequences of excess diesel NO x emissions for public health alarming.

The greatest health impact is experienced in China where 31,400 deaths are attributed to diesel NO x pollution annually. Of these, 10,700 deaths are linked to NO x emissions beyond certification limits. Diesel passenger cars are common in Europe where 28,500 deaths are attributed to diesel NO x pollution annually, of which 11,500 deaths are linked to excess emissions.

The study predicts that unless governments act, 183,600 people will die prematurely each year by 2040 as a result of diesel vehicle NO x emissions.

The authors of the study feel that emission certification tests, both by vehicle owners and prior to sale, could be more accurate. This could be achieved if the tests were to simulate a broader variety of driving styles, speeds and ambient temperatures. Some European countries currently use portable testing devices that track emissions of a car while it is moving.

Anenberg concluded that measures to improve real world compliance and tighter vehicle emission standards could both prevent hundreds of thousands of early deaths per year from diseases related to air pollution.