December 8th, 2017

Suffering from exposure?

5 minute read

by Tanzir Chowdhury

 

If more of us walked and cycled instead of driving short distances, there would be less pollution and we would all be better off. That sort of benefit is well-captured in the types of analyses that have long been used to understand the costs and benefits of transport policies. But what if we want to understand the impact an individuals’ choice of transport method has on their own health?

 

Model questions

There are two well-established ways of modelling the benefits of changes in transport policy that result in more active travel and fewer car journeys:

  • Damage cost models look at the emissions resulting from a journey and place a financial value on them – great for showing the overall societal benefit of emissions reduction.
  • Impact pathway analysis allows for more sophisticated modelling of the impact of pollutants released in a specific location.

 

However, both approaches have significant limitations. Neither the damage costs nor impact pathway analysis approach gives any insight into who in society benefits or loses out from the policy change.

A very different kind of model is needed in order to understand the different impacts of an air quality policy on someone travelling in a car compared with someone cycling or walking; or to understand how much difference there is between cycling on a busy road alongside cars and using a traffic-free cycle path. Let’s call this the ‘personal exposure’ to air pollution of the various types of people affected.

 

Concentration span

Eunomia in partnership with Sustrans has developed a model to evaluate the air pollution impacts of Sustrans’ activities to encourage more active travel, which is explained in a recently published report. The model estimates air pollution benefits for switching from a non-active travel mode to active travel, by calculating the ‘personal exposure’ of a traveller to different air pollutants depending on how they choose to travel. Personal exposure to air pollutants depends mainly on two factors: pollution concentration and inhalation rate. Let’s take a look at what those concepts mean.

‘Pollution concentration’ is the amount of pollution in the micro environment an individual is in (e.g. inside a car, on a bicycle on a busy road, cycling on a traffic-free cycle path). According to research carried out in London, it turns out that cycling along a busy road puts you in a more polluted environment compared to walking adjacent to the road, because of your position relative to the highest concentrations of pollution being generated by the traffic. Once you get away from the main roads, your exposure to pollution becomes two thirds less. However, in all cases, active travellers are in an environment with a lower pollution concentration than those inside a car, bus or taxi, as shown in the table below:

 

Travel Mode Average exposure (µg/m3)
Bus 34.5
Car 38.0
Taxi 41.5
Walk 27.5
Cycle (busy road) 33.5
Urban Background Site (equivalent to cycling away from busy roads) 9.9

 

The lung story

However, that is only half the story. It is also important to understand the volume or air (and consequently the pollutants in the air) a person is breathing in every minute – which of course depends on the activity the person is undertaking. This is the individual’s ‘inhalation rate’. Here research indicates that the active traveller on a bicycle takes in about twice as much air as someone travelling in a car or bus, shown below:

 

Travel Mode Inhalation Rate (litres/min)
Cyclist 23.50
Car Passenger 11.80
Bus Passenger 12.70

 

By combining the pollution concentration with the inhalation rate we can understand the individual’s personal exposure to pollution in the course of their journey. Overall, the personal exposure of someone cycling on a busy main road will be greater than that of someone driving along the same street – mainly because the cyclist will be breathing in a lot more air. However, a person cycling or walking on a traffic-free cycle path will be far better off than the driver.

 

The path of least resistance: active transport away from the main road reduces emissions – and personal exposure to them. Photo: Sam Saunders (CC BY-SA 2.0), via Flickr.

 

Of course, exposure to pollution is not the only relevant factor – the active traveller also experiences health benefits associated with the greater level of exercise involved in their journey. The benefits of physical activity derived from walking and cycling outweigh any exposure to air pollution in the UK. By switching to cycling on a traffic-free cycle path – whether as a change from main road cycling, or from driving – one can both reduce the personal exposure to different air pollutants and receive positive health benefits.

 

The right move

Getting more people on bikes is one of the best ways to reduce road traffic.  This analysis is potentially useful both to individuals deciding how to travel and to policy makers looking at how to maximise the benefits of a switch to more active transport as they seek to include walking and cycling into clean air plans.

Where people have little choice but to cycle or walk along main roads, they will be helping to reduce overall pollution but risk increasing their own personal exposure. Although the impact of that exposure will be more than offset by the health benefits of active travel, it could put some people off, delaying the rate at which the switch to active travel occurs.

If we are to make a major modal shift to active travel, the greatest benefits will be achieved by providing easy access to a network of quieter routes across the UK, in addition to protected cycle routes on roads. By making use of traffic-free paths and back quiet streets where they are available, individuals can minimise their personal exposure to air pollutants and maximise the health benefits.

By integrating the lessons of personal exposure modelling of air quality impacts into local air quality plans, policy makers and urban planners can help organisations like Sustrans put out a clear, simple message: active travel is good for you, and good for those around you.

 

Tanzir Chowdhury

 

 

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