A new study suggests that urban cyclists may be exposed to more than twice as much black carbon from vehicle exhausts as pedestrians, Reuters reported.

By examining airway cells in sputum samples from ten people, researchers in London found black carbon levels were 2.3 times higher in the five cyclists compared to the five pedestrians.

The researchers' working theory to explain the soot increase is that cyclists breathe more deeply and rapidly than pedestrians while they are riding in traffic.

Black carbon, a byproduct of gasoline and diesel fuel combustion, is associated with decreased heart rate variability, heart disease and asthma, among other adverse effects.