Are high-speed cyclists at risk from air pollution?

Behind the Headlines

Monday October 31 2016

Some cyclists use face masks to reduce pollution exposure

Regular cycling brings a range of health benefits

"High-speed cyclists 'breathe in dangerous levels of air pollution'," The Sun reports.

Alexander Bigazzi, a Canadian engineer, put together a mathematical model, and his figures suggest cycling faster than 20km (12.4 miles) an hour increases exposure to pollutants.

He used a series of complex equations to calculate the speed that walkers, joggers or cyclists need to travel at to minimise the level of pollution they potentially breathe in.

Bigazzi applied this to a theoretical population of 10,000 people of different ages, both male and female.

He found that this speed happens to be the level at which most cyclists would normally travel – about 3-8km an hour walking, 8-13km an hour jogging, and 12-20km an hour cycling on flat ground.

Going uphill causes more work, so the speed has to decrease a little, although most people normally slow down when they are cycling uphill anyway.

Going above these speeds may increase the potential harms caused by air pollution.

The harms caused by air pollution are often overlooked. A report we discussed in February estimated air pollution in the UK contributed towards 40,000 deaths a year.

But as sophisticated as this model may be, the study did not look at any real-world outcomes.

And as a related study from earlier this year argued, the health benefits of cycling are likely to outweigh any pollution-related risks, with the exception of cycling in areas of extreme pollution.

Where did the story come from?

The study was carried out by Alexander Bigazzi, a single author from the University of British Columbia. No sources of funding were reported.

The article is currently available in manuscript form and is set to be published in the peer-reviewed International Journal of Sustainable Transportation.

The Sun and the Mail Online both inappropriately stated high-speed cyclists are at risk of lung cancer and stroke, which is certainly not proven by this study, nor the objective of the paper. 

What kind of research was this?

This was a mathematical modelling study where Bigazzi aimed to derive equations to estimate the air pollution inhalation dose pedestrians and cyclists are exposed to when travelling at a particular speed.

Bigazzi explains how there is a balance with high-speed active travel – such as running or cycling – and exposure to pollution because of the trade-off between higher breathing rate (which increases exposure) against the shorter duration time (which decreases exposure).

This study applied exposure levels from the literature to an imaginary population of travellers to calculate individual minimum dose speed (MDS).

MDS is described as the speeds that minimise the air pollution inhalation dose per unit of distance travelled.

But as the study is all model-based, it can't give concrete findings.

What did the research involve?

Bigazzi's methods describe a complex series of mathematical equations that are not possible to explain in-depth here.

In summary, Bigazzi generated a series of equations to determine the steady state pollution dose a person inhales, depending on differing circumstances. He then aimed to determine the speed that would minimise these values.

He made several assumptions for the analysis, including that the pollution concentration cyclists or pedestrians are exposed to is independent of speed, and that breathing rate increases with speed.

Increasing speed was always determined to reduce inhalation dose over a fixed distance. He then calculated breathing rate as a function of speed for cyclists and pedestrians.

He finally calculated MDS for a range of travellers. The author used population distributions from the 2012 US census to imagine a theoretical population of 10,000 people of a range of ages, half male and half female.

For these people he used functions of age, sex and body mass to calculate their resting metabolic rate and oxygen consumption.

He also inputted data on work/power rate during cycling from the American College of Sports Medicine, and additional factors like bicycle mass, road grade and resistance, air density and drag. This was also done for walking and jogging.

What were the basic results?

As the author says, the minimum dose speed (MDS) – the speed that minimises the air pollution inhalation dose – is "within a range of reasonable pedestrian and bicycle speeds".
 
The MDS was calculated to be:

  • 3-8km an hour for walking
  • 8-13km an hour for jogging on flat ground
  • 12-20km hour for cycling on flat ground

Energy expenditure and breathing rate increase with increasing road grade for both cyclists and pedestrians, which decreases the MDS.

However, it has a greater impact on cyclists because of the higher speeds they travel at and the extra weight of the bike.

Each 1% increase in road grade decreases MDS for cyclists by up to 1.6km an hour, depending on the equation model used.

Large deviations from the MDS – for example, more than 10km an hour for cyclists – was calculated to more than double the pollution inhalation dose over a fixed distance.

How did the researchers interpret the results?

The author concluded that, currently, most "pedestrians and bicyclists choose travel speeds that approximately minimise pollution inhalation dose".

But he acknowledged that potential exposure to pollution is unlikely to be the main motivation for the speed cyclists travel at.

Conclusion

This study may be of interest to those in the field of sports medicine. It calculates the speed that walkers, joggers and cyclists may travel at to minimise the pollution they potentially inhale.

It also finds that these values turn out to be those that many pedestrians and cyclists would travel at in any case.

And, somewhat unsurprisingly, this speed would go down as you go uphill because of the increased effort and breathing rate required.

Fitter people who run or cycle fast for sport are, however, naturally likely to exceed this minimum speed requirement, both on the flat and speed gradients.

This could potentially expose them to more pollution, although care should be taken not to speculate too widely on the possible implications of this.

These equations use valid assumptions and previously collected data, but are only estimates. These aren't definite figures or recommendations on the speeds a person should walk or cycle at.

Many things may influence how much pollution a person is exposed to – not least the environment they're travelling in, whether an urban area or countryside.

And pollution exposure doesn't clearly and automatically equate to increased health risks, such as asthma, cancer or stroke.

The benefits of cycling, such as improved fitness and the preventative effect exercise has against a range of chronic diseases, may well outweigh the risks.

Some cyclists now choose to wear a face mask to protect against air pollution. If you decide to buy one, it's recommended you get one that contains sub-micron filters, as this will help protect against the most dangerous types of pollution particles.  

Analysis by Bazian. Edited by NHS Choices. Follow NHS Choices on TwitterJoin the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

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