Tag Archives: Pollution

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.

Exposure to Air Pollution and Cardiovascular Disease

The health effects of air pollution are attracting ever greater international interest. Air pollution is one of the greatest causes of DALY loss according to the Global Burden of Diseases report, accounting for 141.5 million DALYs in 2013, and 5.5 million deaths.[1] However, studies to date have been mostly ecological. That is to say, they compare outcomes for people living in areas with different levels of exposure, not by the individual’s level of exposure.

The US-based Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air) measures exposure to pollution over time. A validated method is used to measure long-term exposure based on sampling in and around individual homes, as well as at the ecological level. Exposure to small (<2.5µm) particles and nitrogen oxide (NO2) was measured over a ten year period. Exposure to each pollutant was related to two markers of atherosclerosis – changes in cardiac artery calcification and in carotid artery intimal-medial thickness.[2] The statistical analysis adjusted for individual factors, such as statin use. Calcification of coronary arteries is a validated surrogate for heart attack, and the prevalence of such calcification was positively correlated with both types of pollution. Cause and effect inference is strengthened by the finding that living near to large road was a risk factor for coronary calcification. No change was noted in the carotid artery, but intimal-medial thickness is a far less valid surrogate for stroke than is carotid calcification for heart attack.

— Richard Lilford, CLAHRC WM Director


  1. Global Burden of Diseases 2013 Risk Factor Collaborators. Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks in 188 countries, 1990–2013: a systematic analysis for the Global Burden of Disease Study 2013. Lancet 2015; 386: 2287-323.
  2. Kaufman JD, Adar SD, Barr RG, et al. Association between air pollution and coronary artery calcification within six metropolitan areas in the USA (the Multi-Ethnic Study of Atherosclerosis and Air Pollution): a longitudinal cohort study. Lancet. 2016; 388: 696-704.

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.