Scientists at Imperial College London found those who walk near the roadside are exposed to 10% more fumes than those who stay away from the edge.

Researchers fitted 11 volunteers with pumps that sucked in air as they took 20-minute strolls along Marylebone Road in central London.

Measurements were taken of the sooty pollution caught in the pumps' filters.

The tiny particles, known as PM10s, become trapped in the lungs and have been linked to respiratory problems and cancer.

Carbon monoxide exposure

Surbjit Kaur, who led the study, told New Scientist magazine: "It is something that susceptible people like asthmatics and those suffering cardiovascular disease should be made aware of."

The study found that walking on one side of the street exposed the volunteers to higher pollution levels than being on the other.

This was possibly due to air sweeping between buildings and concentrating pollution in certain regions.