From Scientific American:
When strolling alongside a busy city street on a smoggy summer day, it may seem as if the multiplicity of 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 the Imperial College London. Armed with particle detectors, volunteers measured their pollution exposure as they took a total of 584 trips by taxi, car, bus, bicycle or just plain walking on Marylebone Road in central London and surrounding areas over the course of three weeks in April and May of 2003.
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 for its ability to penetrate deep into the lungs–for every cubic centimeter traveled. Public buses came second with around 95,000 particles per cm3, followed by cycling at 84,000 particles/cm3 and walking at around 46,000 particles/cm3. “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.”
A personal car–a 1996 Toyota Starlet–provided the most protection, exposing its passengers to an average of just under 37,000 particles/cm3.