Cheaper cars pollute more than expensive cars, leading to emissions inequality

More expensive cars emit lower levels of pollution - meaning that motorists owning cheaper, higher-emitting vehicles contribute disproportionately to their local urban air quality problems, a new study reveals.

Research by University of Birmingham scientists highlights a previously overlooked inequality - that lower-income individuals are more likely to own cheaper, higher-emitting vehicles and, therefore, contribute disproportionately to their local urban air pollution.

Publishing their findings today (14 Nov) in Journal of Cleaner Production, the researchers’ analysis suggests that spending an additional £10,000 on a diesel vehicle is associated with a more than 40% reduction in nitrogen oxide (NOx), emissions per litre of diesel.

Their discovery reverses the usual pattern, where wealthy households typically cause more greenhouse gas emissions through higher consumption.

Scientists analysed over 50,000 vehicles using advanced remote sensing technology measuring in real-time emissions from vehicles on the city’s streets.

They combined machine learning-based price estimation with real-world emission data to connect pricier vehicles and lower pollutant emissions — particularly NO₂, NOx, carbon monoxide (CO), and particulate matter (PM).

The research was supported by the Natural Environment Research Council (NERC) via the WM-Air project. The researchers report that:

• Higher-priced vehicles emit significantly fewer pollutants, even within the same Euro emission class. Average NOx emissions are approximately 8.8 g/litre of diesel for £5,000 cars, compared with 5.6 g/litre for £15,000 cars.
• Diesel vehicles show greater emission reductions per £1,000 increase in price than petrol vehicles - for every additional £1,000 spent on a diesel car, nitrogen dioxide (NO₂) emissions decrease by around 0.4 g/litre of fuel.
• Older diesel models (Euro 5) exhibit emission reductions 1.5 times steeper with price than newer Euro 6 models, suggesting that price is a stronger proxy for emissions in older vehicles.

Report co-author Professor Francis Pope from the University of Birmingham said: "Our study provides the first clear evidence to support vehicle price being a reliable indicator of emission performance - powerfully illustrating how citizens’ economic capacity can directly influence environmental outcomes and urban air quality.

“Individuals from lower-income households may be more likely to own older, cheaper, and higher-emitting vehicles — contributing disproportionately to local air pollution.”

The study calls for policy makers to take several actions to help reduce emissions while promoting social equity. These include:

• Progressive tax structures based on vehicle emissions and price to incentivise cleaner vehicle adoption.
• Rebate schemes or scrappage incentives for lower-income households to accelerate the transition to cleaner transport.
• Enhanced inspection and maintenance programmes for older vehicles may offer a cost-effective way to reduce emissions in the short term.

Report co-author Dr Omid Ghaffarpasand from the University of Birmingham said: “Our findings underscore the need for targeted policy interventions to address environmental injustice. Lower-income communities bear the brunt of local air pollution due to limited access to cleaner vehicles - exacerbating health risks and pollution exposure in disadvantaged urban areas.”

The researchers also advocate for further investigation into how vehicle affordability, emissions, and urban planning interact with each other to contribute to urban air pollution; stressing the importance of integrating socioeconomic data into transport and environmental policy frameworks to ensure equitable outcomes.

WM-Air has been working with partners to bring research organisations together with businesses, policy bodies and other actors contributing to economic development specific to their location, to deliver significant regional impact from NERC environmental science. Previous studies from WM-Air have highlighted major contributions of domestic woodburning to PM_2.5 emissions, and that air pollution in the West Midlands has caused up to 2,300 premature deaths each year.

ENDS

For media enquiries please contact University of Birmingham press office - pressoffice@contacts.bham.ac.uk or +44 (0)121 414 2772.  

Notes to editor:  

• The University of Birmingham is ranked amongst the world’s top 100 institutions. Its work brings people from across the world to Birmingham, including researchers, educators and more than 8,000 international students from over 150 countries.    
• ‘Air Pollution Emissions from Vehicles as a Function of Their Current Real World Market Price’ - Omid Ghaffarpasand and Francis D. Pope is published in Journal of Cleaner Production.