London Underground air pollution

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UCL '20 minutes' myth

One London Underground-related statistic that turns up occasionally is that 20 minutes on the Tube equals one cigarette. In all but the narrowest of interpretations, this comparison is false.

The original calculation is by Dr Ben Croxford, of the Faculty of Built Environment at University College London. The calculation was carried out at the request of a journalist in 2001 and is given on the part of his UCL website that relates to the Underground.

Dr Croxford estimated the weight of particulate matter that is breathed in when a person smokes a cigarette, then estimated the time a person would have to spend in the most polluted place in London Underground to breathe in the same weight of matter.

The calculation explicitly states that the health effects of breathing cigarette smoke and the health effects of breathing dust on the underground are not considered. The only comparison is of the weight of matter inhaled.

Cigarette smoke consists of products of combustion containing oxides of carbon, nitrogen and sulphur; various alkaloids, aromatic hydrocarbons and tar. Dust in the Underground tunnels is mainly iron (from the wheel–rail interface), skin cells, hair cells and clothing fibres (from passengers), and quartz silica (from brakes). Weight for weight, tunnel dust has far less impact on human health than cigarette smoke.

Despite being a simple comparison of weight inhaled, after it was published, the fact was reproduced as if it were something much more alarming. It was quoted twice by the BBC and at least three times by the London Evening Standard, in the following general form-

  • Scientists from University College London said a 20-minute journey on the Northern line through Central London had the same effect as smoking a cigarette.

This statement is true: breathing the air in the most polluted part of the Underground for 20 minutes draws the same mass of particulate matter past your lips as smoking a cigarette. The implication that the cigarette and the journey have the same impact on your health is untrue.

As such, the claim can be considered a classic example of how to 'lie' with statistics. It's true, there's no doubt about that. But it is true in a limited sense, and it leads the casual reader to an apparently obvious and entirely incorrect conclusion.

It has become an established urban myth used to pique the interest of those recently arrived in London, and has been repeated on web sites such as Tube Facts, Ananova and—ironically—, a website devoted to debunking urban myths. It was even quoted by the British Lung Foundation in a 2003 report.


This is not to say that the air in London Underground is good for you. Three or four studies have been carried out on the pollutants found in modern metros and London's oldest 'deep' tube lines (Piccadilly, Northern, Bakerloo & Central) have the highest concentrations of particulates. Transport for London and their predecessors examined the health impacts extensively, and TfL host two dust reports on their website, as well as transcripts of public discussions of the effects.

In more recent years, the health effects of high dust levels have been examined by detailed analyses of the composition of dust in the air of the Underground. In each report, the authors have failed to come to a final verdict and it's fair to say that although the jury consists of well-qualified health professionals, it still can't decide.

On the one hand, the concentrations of particulate dust in the Underground are higher than one would like. On the other, the particulates are mostly formed from things that have little effect on human health (iron and skin), especially when compared to the effects of traffic exhaust or cigarettes.

Anyone concerned about the effect of breathing the air on London Underground trains is advised to read the 2005 paper by Seaton et al as it may provide the most balanced examination of the subject. A more in-depth discussion is given in the 2003 report by the Institute of Occupational Medicine which covers the same ground in greater detail.