In this photograph taken on December 18, 2015, Indian commuters travel on a polluted road near a bus terminus in the Anand Vihar District of New Delhi. (Xinhua/AFP Photo)
WASHINGTON, May 15 (Xinhua) -- Lab-based tests of harmful nitrogen oxide (NOx) emissions from diesel cars, trucks and buses worldwide significantly underestimate the real-world emissions by as much as 50 percent, a new study said Monday.
These excess emissions alone lead to 38,000 premature deaths annually worldwide, according to the study led by the U.S.-based International Council on Clean Transportation (ICCT) and Environmental Health Analytics LLC.
"Since 2015, revelations that Volkswagen and other manufacturers used 'defeat devices' to hide from regulators the fact that their diesel cars were emitting too much NOx helped to heighten public awareness of the problem," the ICCT said in a statement.
"But this is not just a defeat device problem. Both light-duty and heavy-duty diesel vehicles emit more NOx in on-road driving conditions than during laboratory certification testing, for reasons that may range from details of the engine calibration to equipment failure, inadequate maintenance, tampering by vehicle owners, the deliberate use of defeat devices, or simply deficient certification test procedures."
For the new study, researchers assessed 30 studies of vehicle emissions under real-world driving conditions in 11 major vehicle markets representing 80 percent of new diesel vehicle sales in 2015.
They found that in 2015, diesel vehicles emitted 13.1 million tons of NOx, which is 4.6 million tons more than the 8.6 million tons expected from vehicles' performance under official laboratory tests.
Heavy-duty vehicles, such as commercial trucks and buses, were by far the largest contributor worldwide, accounting for 76 percent of the total excess NOx emissions.
NOx is a key contributor to outdoor air pollution in the forms of ground-level ozone and secondary fine particulate matter (PM2.5). Long-term exposure to these pollutants is linked to a range of adverse health outcomes, including disability and years of life lost due to stroke, ischemic heart disease, chronic obstructive pulmonary disease, and lung cancer.
"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 suffers 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, it said.
In Europe, where diesel-passenger cars are common, 28,500 deaths annually are attributed to diesel NOx pollution, with 11,500 of those deaths linked to excess emissions.
The study also projected that by 2040, 183,600 people will die prematurely each year due to diesel vehicle NOx emissions unless governments act.
"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," said Anenberg.
The findings were published in the British journal Nature.