[Taf]

Evidence has accumulated implicating that atmospheric oxygen can be considered as an important environmental mutagen, whose action can be potentiated by a variety of environmental factors. Under normal conditions oxygen genotoxicity is largely prevented by a cellular antioxygenic defense system, which must be extremely efficient, but might not be entirely safe. A better understanding of this defense system may help to suggest how the incidence of carcinogenesis can be minimized.

http://www.ncbi.nlm.nih.gov/pubmed/6646012

For every 1000 meter rise in elevation, lung cancer incidence decreased by 7.23 [99% CI: 5.18–9.29] cases per 100,000 individuals, equivalent to 12.7% of the mean incidence, 56.8. As a predictor of lung cancer incidence, elevation was second only to smoking prevalence in terms of significance and effect size.

https://peerj.com/preprints/587/

Hold your breath or live up a mountain?