File picture: Suzanne Plunkett / Reuters.

Diesel vehicles are more than 50 percent dirtier than manufacturers’ say they are – leading to thousands of unreported deaths, a study claims.

The excess nitrogen oxide (NOx) pumped out by car exhausts can be linked to 38 000 premature deaths worldwide, scientists claim.

This is in addition to the 3.7 million deaths caused by air pollution worldwide each year - 40 000 to 50 000 of which occur in the UK.

Researchers found diesel vehicles around the world produced 4.5 million tons more NOx than they should do under international emission standards – and 52 percent more than lab tests suggest.

NOx damages lung tissue when breathed in, but it also reacts with other chemicals in the atmosphere to produce harmful ground-level ozone and ultra-fine particles.

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 by US researchers analysed data from 30 studies of vehicle emissions under real-world driving conditions around the world. It found that in 2015, diesel vehicles generated 13.1 million tons of NOx in the 11 major vehicle markets studied.

But had the emissions met the imposed testing standards, the amount of NOx produced would have been closer to 8.6 million tons, according to the findings published in the journal Nature.

Heavy duty vehicles such as lorries and buses were identified as the main culprits by the team, which included scientists from the University of Colorado at Boulder, the US non-profit organisation the International Council on Clean Transportation (ICCT) and consultancy firm Environmental Health Analytics LLC. The ICCT said the reasons for the gap could range from shoddy maintenance, tampering by owners, deliberate use of ‘defeat devices’ and deficient test procedures,

The impact was strongly felt in Europe, with 11 500 of 28 500 deaths each year attributed to diesel NOx pollution being linked to excess emissions. Dr Daven Henze, from UC Boulder, said the research exposed a bigger issue than Volkswagen’s 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...in addition to tightening emissions standards, we need to be attaining the standards that already exist in real-world driving conditions," he said.

The team also used computer modelling and satellite data to simulate the effect of excess NOx pollution on health, crops and the climate. They 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.

Last night Roy Harrison, a 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 motor manufacturers."

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