Hailing a taxi can increase a travellers exposure to pollution, that’s according to researchers at Imperial College London.

Out of five modes of transport—walking, cycling, cars, taxis and buses—sitting in a cab was found to be the worst, according to a report in the scientific journal Atmospheric Environment.

Passengers in taxis were exposed to twenty times more pollution than those who walked—100,000 ultrafine particle counts per cubic centimetre—and double the levels inhaled by automobile drivers and passengers.

“It was a real surprise to find the extent to which walking resulted in the lowest exposure,” Surbjit Kaur from Imperial College, London told the Reuters news agency.

In a bus and on the road air circulates more frequently than in a taxi.

“The higher exposure from travelling in taxis may come from actually sitting in the vehicle while being stuck in traffic where you are directly in the path of the pollutant source,” Kaur was reported as saying in the scientific journal—Atmospheric Environment.

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