People who live at higher altitudes are less likely to suffer from lung cancer than those who live in the plains, a new US study has claimed.
Researchers found lower rates of lung cancer at higher elevations, a trend that did not extend to non-respiratory cancers, suggesting that carcinogen exposure occurs via inhalation.
Oxygen is highly reactive and even when it is carefully and quickly consumed by our cells, it results in reactive oxygen species (ROS) which can lead to cellular damage and mutation.
“While essential to human life, aspects of oxygen metabolism may promote cancer,” researchers noted in the journal PeerJ.
While oxygen composes 21 per cent of the overall atmosphere, lower pressure at higher elevations results in less inhaled oxygen – an effect which notoriously frustrates athletes at high altitudes.
To investigate whether inhaled oxygen could be a human carcinogen, researchers compared cancer incidence rates across counties of the elevation-varying Western US.
They found that as county elevation increased, lung cancer incidence decreased.
The effect was dramatic with incidence decreasing by 7.23 cases per 100,000 individuals for every 1,000 metre rise in elevation, equating to approximately 13 per cent of the mean lung cancer incidence of 56.8 cases per 100,000 individuals.
A variety of statistical techniques attested that the association was not due to chance.
The observed association does not prove that oxygen causes lung cancer. The study looked at groups of people rather than individuals and many variables in addition to oxygen levels are correlated with elevation.
Accordingly the researchers performed a thorough analysis to investigate confounding potentials. Their model accounted for important risk and demographic variables, such as smoking prevalence and education.
The association was consistent across population subgroups, states, and models that included a range of additional factors.
The researchers also evaluated breast, colorectal, and prostate cancer.
Elevation’s association with these non-respiratory cancers was either weak or absent, supporting the hypothesis of an inhaled risk factor.
Furthermore, environmental correlates of elevation, such as sun exposure and fine particulate matter (a measure of pollution), produced vastly inferior predictions of lung cancer incidence compared to elevation itself.