Scientists have warned motorcyclists and people travelling in less air-tight vehicles about the dangers of toxic pollutants in tunnels.
Australian researchers measured the levels of ultra-fine particles along a four-kilometre road tunnel in Sydney and found their concentrations were 1,000 times higher than in other urban locations.
Sooty particulate matter entering the lungs has been associated with respiratory and cardiac problems.
The team made nearly 300 trips along the M5 East Tunnel with journeys lasting up to 26 minutes.
The study, reported in the journal Atmospheric Environment, said motorists in older vehicles that are not sufficiently air tight and bikers are particularly at risk.
Study leader Professor Lidia Morawska from the Queensland University of Technology, said: "People who are driving older vehicles which are inferior in terms of tightness and also those riding motorcycles or driving convertibles, these people are exposed to incredibly high concentrations."
She said urban designs should consider the impact on people's health and the environment before building road tunnels.
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