George Johnson in The New York Times:
Epidemiologists have long been puzzled by a strange pattern in their data: People living at higher altitudes appear less likely to get lung cancer. Associations like these can be notoriously misleading. Slice and dice the profusion of data, and there is no end to the coincidences that can arise. There is, for instance, a strong correlation between per-capita cheese consumption and the number of people strangled accidentally by their bedsheets. Year by year, the number of letters making up the winning word for the Scripps National Spelling Bee closely tracks the number of people killed by venomous spiders. These are probably not important clues about the nature of reality. But the evidence for an inverse relationship between lung cancer and elevation has been much harder to dismiss.
A paper published last year in the journal PeerJ plumbed the question to new depths and arrived at an intriguing explanation. The higher you live, the thinner the air, so maybe oxygen is a cause of lung cancer. Oxygen cannot compete with cigarettes, of course, but the study suggests that if everyone in the United States moved to the alpine heights of San Juan County, Colo. (population: 700), there would be 65,496 fewer cases of lung cancer each year. This idea didn’t appear out of the blue. A connection between lung cancer and altitude was proposed as early as 1982. Five years later, other researchers suggested that oxygen might be the reason.