Radiation

Pockets of ionizing radiation, or simply radiation for short (areas where the ambient radiation count exceeds 50 mR per hour) can be found all over the Zone. In the outside areas, radiation tends to be dominant in wide, open spaces and on piles of scrap (such as the dirt or scrap piles in Garbage. Also cars, tractors and anything mechanical). Radiation in itself doesn't form artifacts. When you're in an area with radiation, you can see the radiation indication (in your inventory) increase – the more severe radiation, the faster this meter will go up.

When you have accumulated enough radiation, you'll start to lose health; although radiation will decrease by itself (albeit very slowly) when you're outside of a radioactive area, it's often a good idea to use either Vodka, antirads, or a first aid kit to speed up the process. Areas with severe radiation will make your screen go bright/white.

Effects

Shadow of Chernobyl

Radiation can be identified by three different methods in Shadow of Chernobyl;

A bright blue-white hue, similar to second-generation night-vision modules, which becomes closer to a bright white the more copious the radiation.
The Geiger counter in your PDA. The more rapid the clicking, the more radiation.
The radiation bar in your inventory – most radiation sources, without an anti-rad artifact, will fill it up quickly.

Radiation in Shadow of Chernobyl, much like the rare telepathic damage, is extremely devastating – even with 'safe' radiation, around a quarter of the bar, you will likely use several med-kits before it naturally degenerates, and if it climbs to red you can die within ten seconds. Scientific med-kits can be used to neutralize anywhere from 50 to 100% of the radiation in your system, as an alternative to anti-rads.

Infinite Radiation Bug

A common bug in early versions of Shadow of Chernobyl is that moving to another level with radiation in your system caused the game to 'lock' itself, constantly adding a large amount of radiation at a time. Often times the only way to remedy this was to fill the character's belt with anti-rad artifacts, or to simply reload an earlier save and neutralize the radiation poisoning before making the level crossing.

Infinite Radiation Bug Solution:
1. open gamedata/scripts/bind_stalker.script with a text editor
2. locate the line "db.add_actor(self.object)", which should be around line 47
3. add afterwards this block:

-- nv100217 permanent radiation cure
if self.actor_detector.init_time ~= -1 then
self.actor_detector:actor_enter()
end

(source: https://gamefaqs.gamespot.com/boards/540331-stalker-shadow-of-chernobyl/72945676)

Clear Sky

Radiation can only be identified by the PDA's Geiger counter, but the inventory screen is no longer necessary to determine how much radiation poisoning you suffer from. A small scale in the lower-right beside armor condition displays the amount of radiation in the area, and the red bar inside of it shows how much radiation is in your system. Unlike Shadow of Chernobyl, it is not quite as devastating, and vodka is much more efficient in removing it from your system. Scientific med-kits continue to be very effective for countering radiation poisoning.

Unlike Shadow of Chernobyl, most radioactive areas do not come with an obvious 'light' or screen overlay – the Geiger counter is the most obvious clue if you aren't carefully watching the scale.

Call of Pripyat

Radiation is only a mere nuisance in Call of Pripyat – it is nowhere near as dangerous as it was in Shadow of Chernobyl, nor is it as 'stealthy' as it was in Clear Sky. It is more blatantly obvious, as the player's view goes into an amber monochrome, though the night-vision overlays negate this. Radiation poisoning is signified by a glowing green radiation symbol above the health bar, though in certain cases this may turn yellow or red if the poisoning is severe enough. Most radiation sources can be safely countered with a single anti-rad artifact, and anti-rads, vodka, and some other medicinal items can be used to not only flush rad-poisoning out of the system, but will offer brief (up to 1 minute) periods of protection from further radiation.

Trivia

Amazingly, in the real world, nature seems to have adapted to radiation to some degree and has made it a friend rather than enemy. An example of this is the Radiotrophic fungus which was found on CNPP reactor's wall. Read more on Wikipedia.
The radiation sound effect is also heard in the two Metro games: Metro 2033, and Metro: Last Light, also used to signify radiation.