Taxi ranks highest in pollution exposure

Researchers from Imperial College have published their findings in science journal Atmospheric Environment and show that exposure to ultrafine particles when travelling can vary dramatically depending on what method of transport is used.

Ultrafine particles – those less than 100 nanometres in diameter – are particularly hazardous, as due to their tiny size they can be inhaled deep into the lungs

According to the article, taking a taxi is worst for the lungs closely followed by a bus ride.

In a further body-blow for public transport, as well as efforts to reduce traffic and curb pollution, it the data suggests that sitting in your own car is among the safest ways to travel when it comes to dodging ultrafine particles.

The team from Imperial College used standard equipment to measure the levels of ultrafine particles but linked the data with images recorded on video cameras making it possible to track the impact of different events on the journey, such as being trapped on a traffic island, stuck in traffic or waiting at lights.

The scientists looked at five ways of getting about – walking, cycling, by car, by taxi and by bus – and found significant variations.

According to a statement issued by the college passengers travelling by taxi were, on average, exposed to over 100,000 ultrafine particles counts per cubic centimetre (pt/cm3), travelling in a bus resulted in exposure to just under 100,000 pt/cm3, travelling in car caused exposure to 40,000 pt/cm3, cycling was around 80,000 pt/cm3, and walking was just under 50,000 pt/cm3.

The high reading for cycling would seem to corroborate the findings of research commissioned by the British Heart Foundation last year (see related story) which suggested the health benefits of city cycling were being undermined by the cocktail of chemicals and smog riders were exposed to.

According to lead author of the study, Surbjit Kaur, walking generally resulted in the lowest exposure when the results were taken in context.

There were, however, many factors that impacted on the results for pedestrians from walking behind a smoker to passing skips full of construction waste could lead to peak readings.

As shown by previous research at the college the position on the pavement was also a key factor (see related story) with those hugging the buildings and steering clear of the kerbside recording much lower readings.

“It was a real surprise to find the extent to which walking resulted in the lowest exposure,” said Kaur.

“The taxi readings are probably high due to the taxi constantly being in use and in the road – the direct pathway of emissions,” she told edie.

“Also the fact that taxis are probably on the road for much longer than your average car could cause an accumulation of ultrafine particles.”

While the results of the study provide a useful snapshot there is still plenty of room for further investigation, said Kaur.

“At this stage it is very difficult to identify causes of high and low exposure experienced – it is definitely an area requiring further study,” she said.

“Most studies have examined different modes of transport generally, but we need further research that focus on individual modes in detail.”

Dr Mark Nieuwenhuijsen , from Imperial College London, added: “The particular strength of the system is the visual aspect. The new monitoring and visualisation system is an effective environmental risk communication tool that can be used to identify, visualise and avoid hotspots of pollution.”

The study was carried out as part of the DAPPLE (Dispersion of Air Pollution & Penetration into the Local Environment) project, which looks to provide a better understanding of the physical processes affecting street and neighbourhood scale flows of air, traffic and people, and their corresponding interactions with the dispersion of pollutants.

The project consortium includes the University of Bristol, the University of Cambridge, Imperial College London, University of Leeds, University of Reading and the University of Surrey.

By Sam Bond