Low emission vehicles, such as the Toyota Prius, promise to dramatically cut levels of noxious fumes on city streets. But, until such vehicles start to dominate the roads, people still have to cope with what spews forth from the tailpipes of their fossil fuel-powered cousins. However, researchers have recently found that pedestrians may be able to reduce the amount of traffic pollution they breathe in simply by crossing the street.

The research, led by Professor of Environmental Modelling Alison Tomlin from the University of Leeds' Faculty of Engineering, has shown that air pollution levels change dramatically within small geographical areas - dependent on wind patterns, the location of traffic queues and the position and shapes of the surrounding buildings.

The findings showed that pollution hotspots tend to accumulate on the leeward side of the street (the sheltered side), in relation to the wind's direction at roof-top level. They also revealed that that carbon monoxide levels were up to four times lower in parallel side streets, compared to the main road.

The team monitored traffic flow and carbon monoxide (CO) levels over an eight week period at one of the busiest junctions in the UK - the intersection between Marylebone Road and Gloucester Place in West London.

"CO levels were highly variable over remarkably short distances," says Professor Tomlin. "As you'd expect, the junction itself showed high levels caused by queuing traffic but, with some wind patterns, these hotspots moved further down the street. However, the leeward side of the street had consistently higher concentrations of carbon monoxide than the windward side. The same trends would be expected for other traffic related pollutants such as ultra-fine particles and nitrogen dioxide."

"Most people would expect pollution levels to be slightly lower away from the main body of traffic, but our figures show a very significant difference," she says.

The research found that pollution could be trapped within the street it is emitted by recirculating winds, but if it escapes above roof-top level it doesn’t tend to be mixed back into neighboring streets very strongly. This suggests that cyclists and pedestrians could massively reduce their pollution exposure by moving just one street away from the main thoroughfares.

The research team says their research has significance for local authorities and other bodies monitoring air quality in urban areas.

"Monitoring stations tend to be sited in what are expected to be pollution hotspots, but our research has shown that hotspots move depending on meteorological conditions, particularly wind direction," says Professor Tomlin. "We need to develop models which take these factors into account, so that the data from monitoring sites can be accurately analyzed to provide a true reflection of air quality across the whole of an urban area."

The research appears in the journal Atmospheric Environment.