I grow old, I grow old, my DNA got rolled.
There is an unregulated toxic gas in our atmosphere, and it is slowly killing us all. Its effects are similar to ionizing radiation, through the same chemical pathways, and it attacks all the key components of our cells - lipid membranes, proteins, and DNA. No feasible methods are known for cleaning our air of it and still providing us with breathable air.
That gas is oxygen, and it constitutes roughly 21% of our atmosphere. The catch, of course, is that we can't live without it and a whole lot of it. Its toxicity was first revealed in its effects on underwater divers breathing pressurized air. Prolonged breathing of concentrated oxygen produces unconsciousness and inflammation and destruction of the lungs.
We do much better with lower concentrations, but there is no safe level. Many believe that it is the principal mechanism of human and cellular aging. How does it do its damage?
Respiration, using oxygen to extract energy from carbon compounds, is how we generate most of the ATP that is the universal currency of cellular energy. A side effect of this process is the generation of OH- radicals, one of the most reactive molecules, which will attack practically anything. Aerobic cells have developed a set of defenses against them, but these are not perfect, and a bunch of them are loosed in each cell every second to attack and destroy whatever they encounter. Ionizing radiation does most of its damage by similar means - the energetic particle or photon splits water, the most abundant molecule in the body, and that produces the destructive OH- radicals.
Interestingly, oxygen produces a lot more such radicals over the course of a year than our typical environmental radiation exposures.