Tag Archives: Bicycles

Small Pollution Particles May Pass Directly into the Brain through the Snout

Yes, they appear to be able to follow the pathway used by smell neurons and thus pass directly from the olfactory membrane into the brain, i.e. not going via the lung and bloodstream. Experiments in rodents using radio-labelled nano-particles show that very small particles really can penetrate directly through the roof of the nose and pass into the brain along olfactory neurons.[1] Here these particles set in motion an inflammatory process, which activates micro-glia (brain type macrophages), which attack neurons and lead to amyloid deposits – the hall mark of dementia. People who are exposed to particles have a high risk of dementia,[2] and animals randomised to be exposed (or not) to pollution particles acquire brain amyloid and manifest cognitive decline. So there you have it – there is growing and quite compelling evidence that pollution particles are bad news for humans and other animals. It is time to act – phase out diesel cars, incentivise car manufacturers to clean up emissions, gradually increase tax on cars/lorries/fuels, incentivise cycling in cities (and make it safer), and build rail lines. But none of this will happen without public support so proselytise and increase susceptibility to the message by increasing science teaching in schools. In the end, lots of things come back to the intellectual sophistication of the average citizen. In the meantime I suspect that an increasing proportion of people will adopt face masks, although I do not know how effective they are in trapping particles.

— Richard Lilford, CLAHRC WM Director


  1. Underwood E. The Polluted Brain. Science. 2017; 355(6323): 342-5.
  2. Chen H, Kwong JC, Copes R, et al. Living near major roads and the incidence of dementia, Parkinson’s disease, and multiple sclerosis: a population-based cohort study. Lancet. 2017; 389(10070): 718-26.

Is it Better to Cycle or Take the Car in Heavily Polluted Cities?

It is often said that it is better to exercise than remain sedentary, even in heavily polluted environments. These statements are based on modelling studies that draw their data from empirical investigations of the dose response curves for increasing activity and increasing pollution (as measured by particulates in the range 5-200 mg/m3):

058 DC - Cycle or Car in Polluted Cities - Fig1

A recent modelling study [1] finds that the balance of harms to benefits does not become adverse except in the cities at the very highest end of the distribution of pollution levels, i.e. levels seldom seen, even in heavily polluted cities. However, these studies do not consider the other risks of certain activities, such as cycling, which, we think, should be included in such models.

— Richard Lilford, CLAHRC WM Director


  1. Tainio M, de Nazelle AJ, Götschi T, et al. Can air pollution negate the health benefits of cycling and walking? Prevent Med. 2016; 87:233-6.

Should Bicyclists Wear Helmets?

Readers of a recent article in the BMJ [1] may have puzzled over the failure of evidence to confirm the intuitive theory that bicycle helmet legislation saves lives. How to explain such a finding when, given a standardised blow to the head, helmets reduce damage in the cranium?[2] One possibility is risk compensation whereby cyclists armed with a helmet are overbold. Another is that drivers are more careful when overtaking a bare-headed cyclist. Support for the latter hypothesis is produced by Walker’s study of 2007, based on telemetric measurements of vehicle distance from bicycles whose riders were, or were not wearing helmets.[3] Beware of theories that are too tidy – subsequent re-analysis of the same data shows that the effect size is tiny after controlling for other variables, and disappears if driver distances are dichotomised into ‘safe’ and ‘unsafe’ at one metre.[4] It is doubtful if the small differences in Walker’s paper could account for the lack of effectiveness observed for bicycle helmets. Should we start again and carry out further empirical work of cyclist-related variables that may affect driver behaviour? Occasionally I see a parent riding with a child to school and I like to think that drivers take extra care in such a situation.

— Richard Lilford, CLAHRC WM Director


  1. Dennis J, Ramsay T, Turgeon AF, Zarychanski R. Helmet legislation and admissions to hospital for cycling related head injuries in Canadian provinces and territories: interrupted time series analysis. BMJ. 2013; 346: f2674.
  2. McIntosh AS, Lai A, Schilter E. Bicycle Helmets: Head Impact Dynamics in Helmeted and Unhelmeted Oblique Impact Tests. Traffic Inj Prev. 2013; 14(5): 501-8.
  3. Walker I. Drivers overtaking bicyclists: objective data on the effects of riding position, helmet use, vehicle type and apparent gender. Accid Anal Prev. 2007; 39(2): 417-25.
  4. Olivier J, Walter SR. Bicycle helmet wearing is not associated with close motor vehicle passing: a re-analysis of Walker, 2007. PLoS One. 2013; 8(9): e75424.