On the , I was struck by the black dust that cakes the tunnels. Much of it must be human skin. What is the annual mass of skin cells shed into the underground system? People make a billion journeys on the tube each year. Are there other factors? And what other material is the dust composed of?
鈥� Two-thirds of airborne dust in the London Underground is iron oxide, mainly produced by abrasion between train wheels and the track. Most of the rest is volatile matter, while quartz accounts for 1 or 2 per cent. Quartz is a component of brake dust, so we can assume that the proportion of quartz would have been higher in the days before electric motors were introduced to decelerate trains. There are also traces of metals such as chromium, manganese and copper.
The amount of airborne dust increases from dawn, reaching a peak at around midday. Then it remains fairly constant, with fine dust carried into the system as it is sucked in by air currents from the surface. It then settles overnight.
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More details can be found in a 2004 paper by Anthony Seaton and colleagues in (). This research was commissioned by London Underground in response to health-scare stories in the press.
It found that underground dust is coarser than that found above ground. The lower proportion of tinier particles is good news for passengers because bigger particles tend not to penetrate as deeply into the lungs and are cleared out more efficiently.
Skin contributes very little to dust in the London Underground. We each shed about 1.5 grams of skin per day. Journeys by 2.93 million people per day take an average of 44 minutes. If passenger density had been at present levels since 1863, when the network opened, there would be about 7500 tonnes of shed skin. However, spread across the 402-kilometre network, this would equate to a depth of only about 5 millimetres, assuming that tunnels have an average width of five metres and that dead skin has the same density as water. Crucially, this also assumes there are no microbes to feed on the detritus.
鈥淪kin contributes very little to dust in the London Underground. We shed about 1.5 grams per day鈥�
Mike Follows, Willenhall, West Midlands, UK
To clarify: 44 minutes is about 0.75 of an hour. 0.75 divided by 24 (hours in the day) and multiplied by 1.5 (grams) equals 0.0469 grams in 44 minutes. Therefore the amount of skin left by 2.93 million people is 137,417 grams. Over 149 years (the length of time the network has been open) this amounts to about 7478 tonnes 鈥� Ed