Diesel driven cars, lorries and buses churn out far more air pollution than standard testing procedures suggest, leading to many thousands of unreported deaths, scientists claim.
The excess emissions of harmful nitrogen oxide (NOx) exhaust gases can be linked to 38,000 premature deaths worldwide, according to the new research.
This is in addition to the World Health Organisation's estimate of 3.7 million deaths caused by outdoor air pollution.
The US scientists argue that there is too little awareness of the impact of "real world" vehicle air pollution.
NOx can damage lung tissue but also reacts with chemicals in the atmosphere to produce ground-level ozone and ultra-fine particles, both of which are harmful.
Ozone irritates the airways and aggravates lung diseases such as asthma and bronchitis, while inhaling fine particles is strongly linked to heart and artery disease.
The study, published in the journal Nature, found that diesel vehicles around the world produced 4.5 million tons more NOx than they should do under international emission standards.
Heavy duty vehicles such as lorries and buses were identified as the major culprits.
Study co-author Dr Susan Anenberg, from the consultancy firm Environmental Health Analytics LLC, said: "The consequences of excess diesel NOx emissions for public health are striking."
The team, which included scientists from the University of Colorado at Boulder (UC Boulder) and the US non-profit organisation the International Council on Clean Transportation, analysed data from 30 studies of vehicle emissions under real-world driving conditions around the world.
In 2015, diesel vehicles generated 13.1 million tons of NOx in the 11 major vehicle markets studied. But had the emissions met the testing standards imposed in those markets, the amount of NOx produced would have been closer to 8.6 million tons, said the scientists.
The impact was strongly felt in Europe, where diesel cars are common, said the authors. Here, 11,500 of the 28,500 deaths each year attributed to diesel NOx pollution were linked to excess emissions.
Dr Daven Henze, from UC Boulder, said the research exposed a much bigger issue than Volkswagen's notorious use of "defeat device" sensors that automatically reduce the pollution emissions of vehicles undergoing tests.
"A lot of attention has been paid to defeat devices, but our work emphasises the existence of a much larger problem," he said. "It shows that in addition to tightening emissions standards, we need to be attaining the standards that already exist in real-world driving conditions."
Dr Henze's team used computer modelling and satellite data to simulate the effect of excess NOx pollution on health, crops and the climate.
The scientists predict that in 23 years time diesel vehicles around the world will be causing 183,600 premature deaths each year unless further action is taken to curb their emissions.
Enforcing more stringent emission limits could prevent 174,000 deaths related to fine particles and ozone in 2040, said the researchers.
Dr Anenberg added: "Tighter vehicle emission standards coupled with measures to improve real-world compliance could prevent hundreds of thousands of early deaths from air pollution-related diseases each year."
British expert Roy Harrison, Professor of Environmental Health at the University of Birmingham, said: "This is a rigorous study which highlights the serious consequences which have resulted directly from the irresponsible actions of the motor manufacturers in producing vehicles which meet regulatory requirements under test conditions, but emit far higher pollutant levels during on-road use.
"The study may well underestimate the full consequences for public health as it quantifies only the effects of particulate matter and ozone formed in the atmosphere as a result of excess nitrogen oxides emissions, but not the direct effects of the oxides of nitrogen themselves."