system
Given the following post, respond as if you're commenting on Reddit:
human
How is it that we need oxygen to live but that free oxygen is also toxic to cells?

We breathe oxygen because our cells need it to live, yet exposure to free oxygen will also kill those cells? How are the lung cells/alveoli "immune" to oxygen exposure?
gpt
Free oxygen is actually oxygen radicals. The oxygen ions mentioned by others wouldn't really be much of a problem. They would react with the plentiful hydrogen ions in your body and become water. Oxygen radicals are free radicals and are pretty terrible for you. Your body has anti-oxidants that react with free radicals like free oxygen and bind them up.

Edit: Oxygen ions are O^(2-). There are a number of different oxygen radical species: superoxide O*_2_*^(•-), ~~peroxide O*_2_*~~^(•2-), and hydroxyl radical OH^(•). Atmospheric oxygen is O*_2_* which is a molecule with two atoms of oxygen (diatomic).

Edit 2: Atmospheric oxygen is a radical, but it doesn't actually hang out in your cells by itself. It is always (or almost always) in a complex with a protein. Peroxide is an anion, not a radical, but it will generate radicals in your body.

Source: MS in chemistry.