Cheaper cars emit more pollutants, creating inequality in urban air quality

The more expensive the car, the lower the pollution level. That means motorists who own cheaper, higher-emitting cars are contributing disproportionately to the region’s urban air pollution problem, a new study finds.

Research by scientists at the University of Birmingham has highlighted previously overlooked inequalities. In other words, lower-income people are more likely to own cheap, high-emission cars, which contribute disproportionately to air pollution in their local cities.

Analysis by researchers, whose findings are published today in the Journal of Cleaner Production, suggests that spending an extra £10,000 on a diesel car can reduce nitrogen oxide (NOx) emissions by more than 40% per liter of diesel.

Their findings reverse the usual pattern in which wealthy households consume more, thereby causing more greenhouse gas emissions.

Scientists analyzed more than 50,000 vehicles using advanced remote sensing technology that measures emissions from vehicles on the city’s streets in real time.

They combined machine learning-based price estimation with real-world emissions data to link more expensive vehicles to lower pollutant emissions, including NO₂, NOx, carbon monoxide (CO), and particulate matter (PM), among others.

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

Even in the same Euro emissions class, more expensive vehicles emit significantly less pollutants. Average NOx emissions are about 8.8 g per liter of diesel for a 5,000-pound car, compared to 5.6 g/liter for a 15,000-pound car. Diesel cars offer greater emissions reductions for every £1,000 increase in price than petrol cars. Every extra £1,000 you spend on a diesel car reduces nitrogen dioxide (NO₂) emissions by around 0.4g per liter of fuel. Older diesel models (Euro 5) have 1.5 times steeper emissions reductions as a function of price than newer Euro 6 models, suggesting that price is a stronger indicator of emissions for older vehicles.

Professor Francis Pope from the University of Birmingham, co-author of the report, said: “Our research provides the first clear evidence that car prices are a reliable indicator of emissions performance, and provides a powerful demonstration of how citizens’ economic capacity directly influences their environmental impact and a city’s air quality.”

“People in low-income households are more likely to own older, cheaper, higher-emitting vehicles, which can disproportionately contribute to local air pollution.”

The study calls on policymakers to take several steps to reduce emissions while promoting social equity. These include:

A progressive tax structure based on vehicle emissions and price to encourage the adoption of cleaner vehicles. Rebate schemes or waste incentives for low-income households to encourage the transition to cleaner transportation. Enhanced inspection and maintenance programs for older vehicles may provide a cost-effective way to reduce emissions in the short term.

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

The researchers also advocate for further research into how vehicle affordability, emissions and urban planning interact to contribute to urban air pollution. It highlights the importance of integrating socio-economic data into transport and environmental policy frameworks to ensure fair outcomes.

WM-Air has worked with partners to bring greater regional impact from NERC environmental science by connecting research institutions with businesses, policy institutions, and other stakeholders that contribute to region-specific economic development. Previous research from WM-Air has found that wood burning stoves in homes make a significant contribution to PM2.5 emissions and that air pollution in the West Midlands causes up to 2,300 premature deaths each year.