Researchers combined the most recent vehicle emissions, air pollution, and epidemiological models to find that vehicle tailpipe emissions were responsible for 385,000 premature deaths worldwide in 2015, up from 361,000 in 2010. Exhaust from on-road diesel vehicles was responsible for nearly half of these premature deaths globally, and fully two-thirds in India, France, Germany, and Italy.

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For their study, researchers at the International Council on Clean Transportation (ICCT), University of Colorado Boulder, and George Washington University assessed the health impacts of emissions from four major transportation sectors: on-road diesel vehicles, other on-road vehicles, shipping, and non-road engines (tractors used in agriculture, generators, construction equipment etc.).

The health impacts of transportation emissions such as PM2.5 — atmospheric particulate matter (PM) that have a diameter of less than 2.5 micrometers — and ozone were immense but unevenly distributed geographically. According to the authors, the global cost of transportation-attributable health impacts was approximately $1 trillion. Two-thirds of the impact was felt in the four largest vehicle markets in the world: China, India, the European Union, and the United States.

The global health burden of on-road diesel vehicles is especially large because of their higher levels of particulates — microscopic bits of soot left over from the combustion process, which can penetrate deep into the lungs, causing irritation and potentially triggering asthma attacks. Compared to previous research, the new study attributed a 68% higher health burden to diesel vehicles because it includes the effects of tailpipe PM2.5. A 2017 study published in Nature found diesel cars emitted 50% more nitrogen oxides (NOx) under “real-world” driving conditions than expected.

While diesel vehicles have become more efficient and their sales have fallen in developed countries such as the US, Japan, and those in the EU, such reductions have been offset by growing impacts in China, India, and other parts of the world.

“Unless the pace of transportation emission reductions is accelerated, these health impacts are likely to increase in the future as the population grows, ages, and becomes more urbanized,” said Susan Anenberg, lead author of the study and an associate professor at the George Washington University Milken Institute School of Public Health.

In order to reduce the health burden of diesel emissions, the authors have proposed “soot-free” transportation guidelines. These involve using engines equivalent to Euro 6/VI or US 2010, coupled with fuels containing no more than 10 parts per million sulfur, and particulate filters that effectively eliminate fine particle and black carbon emissions.

“The high public health burden of diesel vehicles in Europe underscores the need for world-class emissions standards to be accompanied by robust compliance and enforcement,” says Joshua Miller, co-author of the study and a senior researcher at the ICCT. “The long lifetime of vehicles and equipment and the increasing health burden in regions without adequate protections stress the urgency to introduce world-class standards, develop compliance programs, and adopt in-use measures that accelerate the replacement of high-emitting vehicles.”

Overall, ambient air pollution is responsible for 3.4 million premature deaths annually worldwide, primarily due to heart disease, lung disease, and diabetes. Almost 95% of the world’s population is breathing air that’s deemed unhealthy by the World Health Organization (WHO).