Oxygen might be vital for sustaining life, but scientists from the University of Pennsylvania and University of California, San Francisco (UCSF) have discovered inhaling too much oxygen from the air could in fact be triggering lung cancer.
The researchers analysed the rates of lung cancer occurring in 250 counties across the US and discovered that instances of the disease occurring fell drastically at higher altitudes, where the air is thinner.
For every 1,000 metre (3,281 ft) rise in altitude, the incidence of lung cancer fell by 7.23 cases per 100,000 individuals.
In fact, if the entire population of the US were to move and live in San Juan County in Colorado, which stands at 3,470 metres (11,400 ft), there would be 65,496 fewer cases of lung cancer a year.
Their research, entitled "Lung cancer incidence decreases with elevation: evidence for oxygen as an inhaled carcinogen", is published in the open access journal PeerJ.
Oxygen free radicals
When we breathe, oxygen (O2) is carried by haemoglobin in the red blood cells to the lungs, and from the lungs, arteries deliver the oxygen to the cells, where mitochondria (the power station of the cells) process oxygen and glucose from digested food to provide energy to the cells.
A product of processing oxygen is carbon dioxide (CO2), which would be potentially harmful to the body, but is expelled after the reaction occurs.
However there is another natural by-product – oxygen free radicals, which are highly reactive as the mitochondria reaction leaves the oxygen molecule with one unpaired electron.
Although the body's natural antioxidant defence system can usually handle oxygen free radicals, inhaling a large amount of oxygen could potentially put this system under stress.
If oxygen free radicals are allowed to have their way, they can cause a great deal of damage to cell structures and DNA, which in turn can trigger lung cancer, as well as cardiovascular disease, Alzheimer's and Parkinson's disease.
However, it is important to remember that there are other reasons besides mitochondria why the body produces oxygen free radicals, which include cell oxidation caused by certain drugs.
And not all free radicals are bad. White blood cells for example, produce oxygen free radicals on purpose to act as a defence in killing invading pathogens.
Direct link to elevation
Since other non-respiratory cancers including breast cancer, prostate cancer and bowel cancer showed no link at all to elevation, the scientists believe that the inhalation process is to blame.
They discovered "substantial evidence" that lung cancer triggered by inhalation of oxygen was "tied directly to elevation".
"Viewing our findings through the lens of the literature, atmospheric oxygen emerges as the most probable culprit," the researchers wrote, adding that they took factors like exposure to sunlight and pollution, which are both affected by elevation, into account in their calculations.
"Overall, our findings suggest the presence of an inhaled carcinogen inherently and inversely tied to elevation."
If you don't fancy moving to a higher altitude and are concerned about oxygen free radicals, it's always a good idea to stock up on antioxidants like Vitamin E, which traps peroxy radicals in cellular membranes; or Vitamin C, which reduces all types of free radicals and recycles radicals trapped by Vitamin E.