system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do biohazard suits protect you from radiation?

I recently went down a virtual rabbit hole on the “Demon Core” incidents in the late 1940’s and it got me thinking about protective equipment in regards to dealing with radiation. How does it work? Are there certain materials or designs that are more effective than others? 

Apologies in advance if this has already been answered.
gpt
They don't, if we're talking about gamma radiation.

An NBC suit would not have helped Slotkin in the slightest.

In short, there are 3 main types of ionizing nuclear radiation. Ionizing means that when it hits something, it has enough power to knock off an electron or break a molecular bond - this is a problem for living things because it tends to damage and destroy cells, particularly by damaging the DNA that gives instructions to the molecular machinery inside.

* Gamma rays are very high energy and pass through matter readily. It is the most dangerous, because it penetrates the body deeply and does a lot of damage, and because the only thing that shields against it is a LOT of mass between you and the emitter.
* Beta radiation can only penetrate a half inch or so at most. Thick clothing or a regular wall will protect against most beta rays.
* Alpha radiation can only penetrate a couple of mm. You can stop alpha radiation with as little as a piece of paper.

So. When Slotkin fumbled the screwdriver and caused a criticality incident with the demon core, everybody in the room was bathed in a high dose of gamma rays. They were dead men walking, even if they'd been wearing full NBC suits. It's a big part of the reason why after this and a couple other incidents, future criticality experiments were conducted remotely - with the scientists behind adequate shielding.

Shielding against gamma radiation (such as you'd use for a fallout shelter) is typically rated in "halving-thicknesses," or the amount of a given material that will deflect or absorb roughly half of incoming gamma rays. Roughly 2.4 in of concrete, 3.6 inches of packed earth, .4 in of lead, etc = 1 halving-thickness. "Adequate" radiation shielding (at least for a fallout shelter) is roughly 10 halving-thicknesses - 24 inches of solid concrete or 3 feet of packed earth overhead. It reduces incoming gamma rays by a factor of about 1000 - generally enough to keep you safe from even something emitting a LOT of gamma radiation.

You can see how wearing 4 inches of lead shielding is... not practical, yes? It's why there IS no wearable "radiation suit" like in video games. ETA: Some kind of futuristic gamma suit would have to have a personal magnetic field strong enough to redirect the radiation away from you or something.

So, why do we even have NBC suits in real life? It's to keep Nuclear-Biological-Chemical substances off our skin.

* In the event of a radiological incident, particles that release gamma rays decay relatively quickly (higher energy release = the "battery" runs down quicker), but alpha and beta emitters last longer.
* Alpha- and beta-emitting particles can't do much more than give you a "nuclear sunburn" *unless* you ingest/inhale them (alpha emitters don't even really do that unless you get them directly on your skin). If they get inside you, they can damage internal organs and increase your risk of cancer. You see where we're going here?
* The suit and mask keeps radioactive dust off your skin and out of your hair, eyes, mouth, and lungs.
* When you leave the hazard area, you remove the suit carefully and go through a decontamination shower (ideally) to make sure you don't bring those particles with you where they can float around and contaminate food and water or be inhaled.

The same principle works for areas contaminated with dangerous pathogens or chemical agents (assuming it's not something that will damage the suit directly). You want to keep them off your skin and out of your lungs until you can decontaminate the outside of the suit and take it off in a safe area.

Bonus: Potassium Iodide (KI) is often touted as a remedy in a radiological disaster, but it **only protects against one specific isotope**. Iodine-131, a radioactive version of Iodine, is produced by nuclear weapons or certain kinds of nuclear disasters. Your thyroid gland likes to store iodine for use in producing certain hormones. It can't tell the difference between regular Iodine and radioactive Iodine in your diet. The reason you take KI is to saturate your thyroid with normal Iodine so it won't absorb any of the radioactive kind that you might ingest in contaminated food and water. If you did get I-131 in your thyroid, it could irradiate and kill your thyroid, damage thyroid function, or just increase your risk of getting thyroid cancer in a couple of decades (depending on the dose you got). KI won't do jack to protect you from gamma rays, or from any other kind of alpha or beta emitter (like Radium, or Cesium-137 or Strontium-90) that you might ingest.