Apoptosis in general is programmed cell death, which sounds bad, but is actually good. It presents cancer from forming (which is basically a collection of cells that won't die and keep proliferating). There are many chemical pathways that induce apoptosis, so it is kind of difficult to answer this question. However, in short, oxygen is actually poisonous to our body. It oxidizes (as the name suggests) many things in our cells which can trigger apoptosis. For instance, DNA can be oxidized which damages it. This triggers the cell to realize there is a problem and it activates transcription factors to start the death process. This is just one simple explanation.