Where a car's exhaust pipe is positioned has a direct and significant impact on the amount of toxic air pollution pedestrians breathe, according to groundbreaking new research. A study led by scientists in the UK has found that a simple design change could slash pollution levels at the kerbside.
The Surprising Impact of Exhaust Placement
Researchers from the University of York, led by Professor David Carslaw, made detailed measurements of exhaust emissions from 38,000 vehicles in both Milan and York. The team developed a novel data analysis technique to overcome the major challenge of measuring pollution from individual cars as they pass, where exhaust plumes typically overlap.
The findings were striking. The study revealed that a pedestrian on the pavement experiences roughly 40% more air pollution when a car with an exhaust pipe positioned close to the kerb drives past, compared to a vehicle with its exhaust nearer the centre of the road.
"The main surprise was that the position of the exhaust, on the left or right of a car, has an important and direct impact on air pollutant concentrations close to roads," stated Professor Carslaw. The team calculated that traffic pollution at the edge of UK roads could be reduced by 21% if all diesel cars had their exhaust pipes positioned near the centre of the road.
Diesel Dominance and Manufacturer Disparities
A decade after the diesel emissions scandal, the research confirms diesel vehicles still dominate exhaust pollution. In Milan, diesel was responsible for 81% of nitrogen oxides from local traffic, 61% of black carbon (tiny soot particles), and 55% of small particle counts.
The study also uncovered vast differences between vehicles and manufacturers. The most polluting 5% of cars produced twice as much nitrogen oxides as the median vehicle. For black carbon and small particles, the worst performers emitted more than seven times the median level, hinting at potential issues with some particulate filters.
"One application of our approach is 'market surveillance'; ensuring that air pollutants remain well controlled under real driving conditions and checking that manufacturers are meeting their obligations," explained Carslaw. This comes as several European nations tighten annual vehicle inspections to catch defective filters, putting pressure on the UK to follow suit.
Design Quirks and Unexpected Benefits
Intriguingly, the research highlighted a curious design trend with significant consequences for UK roads, where traffic drives on the left. "We were surprised to find that most diesel cars have their exhausts on the left – closest to the kerb in the UK where there is maximum impact on concentrations – and most petrol and petrol hybrid cars have their exhaust on the right," noted Carslaw.
The study also pointed to an unexpected benefit from electric vehicles. The turbulence created by electric buses and vans was found to help disperse pollution without adding any exhaust emissions of their own, providing a secondary cleansing effect.
While diesel's polluting legacy persists, the impact of stricter legal limits is clear. Newer diesel cars now produce nitrogen oxide levels similar to petrol vehicles and generate far less particle pollution than older models. However, this latest research suggests that alongside cleaning up emissions, where we put the exhaust pipe could be a quick win for public health.
