Letters
Tunnel travelling
EDITOR, - In his interesting editorial on the health risks associated with travelling through a tunnel David W Proops states, "Rapid changes in ambient pressure occur when entering a tunnel at speed, or when two trains pass each other in a tunnel. This increase in pressure deflects the intact tympanic membrane medially."1 The tympanic membrane is actually deflected laterally, not medially.
A moving train (like any other moving object) catches air with it, and a passing train (or the tunnel wall) makes this air move backwards. This air flow causes a negative pressure, which is proportional to the square of the air velocity, according to Bernoulli's principle. As a consequence, air escapes from the cabin and the cabin pressure decreases, making the tympanic membrane move outwards - that is, laterally. The opposite happens when the trains have passed each other. If the passing train or the tunnel is long enough
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- Sound advice for tunnel travellers
- D W Proops
BMJ 1994 309: 426.[Extract] [Full Text]
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