system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: what is radiation? And how does it kill you?
gpt
Lets say you get 7 Gray of ionizing radiation. That is enough to kill you, on average, even with good and immediate medical treatment.

Radiation is just particles, either massed particles like protons or neutrons, or massless ones like photons, aka light. In the case of photons, what we call radiation (in the bad for you sense) is just photons with way more energy than something like visible light. A visible light photon has an energy of 1.5-3.5 eV; a gamma ray (bad radiation) has an energy of 200,000-3,000,000 eV.

Getting 7 Gray of whole-body radiation means that every kilogram of your body has been exposed to as much as 200,000,000,000,000 of these dangerous photons.

Because the photons have so much energy they are "ionizing radiation". The photon goes shooting though your body, missing a lot of stuff because photons are small and atoms are mostly empty space. But eventually the photon crashes into something, either an electron or an atom's nucleus, and it gets blown out of the atom.

Suddenly the atom is no longer a chilled out atom, making up a molecule. It's now an ion, and it can't be part of the molecule any more because it has a charge. The molecule breaks.

Maybe that molecule was a strand of DNA. Maybe it was a hemoglobin protein in a red blood cell, trying to carry oxygen. Maybe it was a molecule of water, and now there's super reactive hydrogen and hydroxide ions inside a cell, looking for the first thing to react with.

You're exposed to radiation all the time. Your cells have ways of fixing it, including killing the cell because the damage is too great. Not a big deal at low rates, your body makes new cells, everything works out.

But not when each kilogram of your body sees trillions of individual radiation events. Your body only has trillions of cells. Literally every single cell is exposed to hundreds of ionizing particles by itself.

So in the end, too many of your cells are too broken and can't be repaired. You are the sum of your cells, and when enough of them die, so do you.