Exposure to traffic fumes can damage human DNA, potentially raising the risk of cancer, research suggests.
There is concern about the health effect of traffic fumes
Scientists at Taiwan's National Defence Medical Center found raised levels of a key chemical, 8-OHdG, in the urine of motorway toll booth operators.
Elevated 8-OHdG levels are a sign of DNA damage caused by particles called oxygen free radicals formed in high numbers by pollution.
Details are published in Occupational and Environmental Medicine.
The research team carried out tests on 47 female motorway toll booth operators working on a particularly busy stretch of road south of Taipei.
The results were compared with tests carried out on female office workers not exposed to traffic fumes in the same way.
The researchers analysed urine samples for levels of 8-OHdG, and a second chemical, 1-OHPG, produced by the breakdown of pollutants found in traffic fumes.
Blood samples were also taken to measure levels of nitric oxide - another sign of possible tissue damage caused by exposure to traffic fumes.
The operators worked in eight-hour shifts for four consecutive days before taking a day off.
During their shifts, they took breaks of between 30 and 45 minutes every couple of hours. They regularly changed lane booths, working a rotation system.
However, they were exposed to high levels of traffic exhausts, up to 20 times the usual exposure in big urban centres, such as London or Birmingham.
Levels of urinary 8-OHdG were on average 90% higher among the non-smoking toll booth operators than they were among the office workers.
Levels of nitric oxide were on average 30% higher.
Levels of 1-OHPG were strongly linked to the levels of 8-OHdG. The higher the 1-OHPG, the higher was the 8-OHdG.
The researchers say their work suggests that traffic fumes boost oxygen free radical activity and therefore cause DNA damage.
Researcher Professor Jouni Jaakkola told the BBC News website: "These findings combined with results from other studies underline the need to control air pollution produced by traffic.
"Although technology has reduced the air pollution load per unit, it has not compensated the increase in the numbers of automobiles."
Traffic fumes are a complex mix of by-products of the combustion process and include hundreds of pollutants.
Research suggest that particulates - tiny airborne particles - are particularly damaging to health, as they can penetrate deep into the tissues of the lung, and even pass directly into the blood.
Dr Jonathan Grigg, from the Leicester University Children's Asthma Centre, has shown that children living near busy roads suffer more coughing attacks.
He told the BBC News website that separate research suggested that pollutants might have a damaging effect on the genetic material of various cells found in the airway.
"It may be that long-term exposure to traffic fumes may be a cause of lung cancer and the like," he said.
"The big question is whether much lower levels of exposure than those experienced by the women in this study have the same effects."
Dr Grigg said that if a genotoxic effect could be confirmed, then it might be that current safe limits on exposure to traffic pollution would have to be revised.
John McGoldrick, of the National Alliance Against Tolls, said: "The effect of traffic fumes on air quality is complex and less than other factors, but if the risk to toll collectors from traffic fumes is significant, then one answer is to get rid of the toll booths."