Question: Why is it that oxygen will one day kill us?

Richard Unwin answered on 12 Nov 2015:

It’s a bit of a leap to say that oxygen will kill us, but it is true that breathing can be a dangerous business!. When we breathe in, we use the oxygen in our cells to chemically convert sugars into energy. A minor side product of this is a form of oxygen called a ‘superoxide radical’. This superoxide is very reactive, and can damage the DNA and machinery in each cell. Cells have a special ‘waste disposal’ system to make these radicals safe, and to repair or remove any damaged DNA or protein. However, bits of damage gets through and contributes to ageing and disease associated with it (this is why radioactivity, which makes more superoxide radicals, is associated with development of some cancers), so I guess in way it’s true that breathing oxygen will one day kill us (although it will take longer than us NOT breathing oxygen!).

The biggest cause of death, therefore, turns out to be life.