Understanding this, you want to achieve the following then:
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Establishing an area where if a player enters the region that it would trigger an event (Same to be applied when the player leaves the space I would assume)
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A Screen GUI would appear that would give the illusion of the radiation effect while the player is in the region of the radiation
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A radiation sound to play in the area of the radiation zone that wouldn’t be heard if your too far away from the zone
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An damage system that would slightly decrease player’s health away at an interval time given while they are in the region.
Am I getting this logic correct? If so this can kind of establish a plan/step guide towards how you can approach coding it.
For detecting if players are in a specific space/part, there are a number of DevForum topics or YouTube videos that can explain/teach you how to detect if a player is in a certain space (Commonly using Region3). For UIs a probable approach that I can think of at the moment is establishing a UI in StarterGui with the effect and if the player enters and leaves the space, it would enable and disable the effect in PlayerGui (Kinda different from StarterGui but works similarly). For the sound, it’s also a searchable topic to figure out how sound can emulate or play in a certain space only. For the damage script, there are maybe a few that I can find.
I will link to you some places you can check out that may help you with your concern. They may or may not be direct answers but personally feel they may help out:
Sound Regions: Roblox - Play music in different areas - Scripting tutorial (2020 updated version) - YouTube
Constant Player Damage: How to make it so a player constantly takes damage while touching a part - #19 by goldenstein64
UI When Part Touched: How to make a "Gui" appear when you touch a "Part" in Roblox Studio! | Touched Function and other... - YouTube
Detecting if a player enters and leaves an area: How do I accurately check if a player is in a certain area? - #3 by kinkocat
What is a good way to detect if a player has entered/left a Region3?
Detecting Player In a Certain Area Question - #10 by amadeupworld2