When strolling alongside a busy city street on a smoggy summer day, it may seem as if riding in one of the taxis streaming by might provide a respite from the exhaust-choked air. Instead new research from London reveals that taxi rides take a toll on your lungs as well as your wallet.

In fact, taxi cabins expose drivers and riders to more air pollution than any other form of transportation, according to the results of a survey by Surbjit Kaur and her colleagues at Imperial College London. Armed with particle detectors, volunteers measured their pollution exposure as they took a total of 584 trips either by taxi, car, bus, bicycle or just plain walking on Marylebone Road in central London and in surrounding areas over the course of three weeks in April and May of 2003.

Surprisingly, taxis topped the list of air pollution danger, according to research published in the current issue of Atmospheric Environment. London's Black Cabs exposed passengers to an average of more than 108,000 ultrafine particles--microscopic soot 10,000 times smaller than a centimeter that is particularly dangerous because of its ability to penetrate deep into the lungs--for every cubic centimeter traveled. Public buses came in second with around 95,000 particles per cubic centimeter, followed by cycling at 84,000 particles/cubic centimeter and walking at around 46,000 particles/cubic centimeter.

"It was a surprise the extent to which exposures in a taxi were so high," Kaur says. "I would say that it's got a lot to do with the fact that the taxis are out there everyday. They're stuck in traffic every day with exhaust in front and behind that accumulates to create a higher concentration in the vehicle cabin."

Researchers followed the volunteers with video cameras (including ones mounted in a three-wheeled stroller, prompting passersby to wonder aloud about a lost baby, Kaur recalls) to capture visual evidence of exhaust blasts or lurking smokers that they could then correlate to particle measurements over the course of the trip. A personal car--a 1996 Toyota Starlet--provided the most protection, exposing its passengers to an average of just under 37,000 particles/cubic centimeter.

Of course, that car contributed to the pollution woes of passing pedestrians and prior research from Kaur and her collaborators showed that walking farther from traffic reduced pedestrian exposure to air pollution by up to 10 percent. In fact, when the team compared exposure to all forms of measured air pollution--including larger particles and carbon monoxide--walking proved the best mode of transportation. "Walking is not just good for you from an exercise point of view but also from an exposure point of view," Kaur notes. "Sometimes people have the impression that the vehicle should provide some protection and that's not always the case."