Taking a taxi may be a relatively luxurious form of transport - but it might also increase your exposure to common pollutants.
Not necessarily the healthy option?
A team at Imperial College London found taxi travel resulted in more exposure to pollutants than travelling by car or bus, riding a bike, or walking.
The study, in the journal Atmospheric Environment, found car travel was the best way to minimise exposure.
The researchers measured exposure to ultrafine pollution particles.
Taxi: over 100,000 pt/cm3
Bus: just under 100,000 pt/cm3
Cycling: 80,000 pt/cm3
Walking: just under 50,000 pt/cm3
Car: 40,000 pt/cm3
Measure is: ultrafine particles counts per cubic centimetre
These particles - less than 100 nanometres in diameter - are particularly dangerous because they are so small that it is possible to inhale them in large quantities, and they can penetrate deep into the respiratory system.
The Imperial team developed a visualisation system which allowed them to play back video images of an individual's activities alongside a read-out of the ultrafine particle concentrations to which they were exposed at any each given moment.
This enabled them to pinpoint which activities were associated with the highest levels of exposure to pollutants.
On average, while travelling in a taxi, passengers were exposed to over 100,000 ultrafine particles counts per cubic centimetre (pt/cm3).
This compared to an average reading of just under 50,000pt/cm3 for walkers.
Lead researcher Dr Surbjit Kaur said: "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.
"Also the fact that taxis are probably on the road for much longer than your average car could cause an accumulation of ultrafine particles."
Dr Richard Russell, of the British Lung Foundation, said his own research had shown higher levels of carbon monoxide and nitric oxide among people who travelled to his clinic by taxi, rather than walking.
"We would encourage people to walk as much as possible, as exercise is good for you," he said.
Richard Massett, of the Licensed Taxi Drivers Association, said: "We have concerns for taxi drivers, who are driving around all day in roads with the highest levels of pollution.
"We would like to see pollution lowered."
The study was carried out as part of the DAPPLE (Dispersion of Air Pollution & Penetration into the Local Environment) project.
This was set up to provide a better understanding of the relationship between air, traffic, people and pollution.