Exposure is the combined risk of the current environment.
Radiation, Air-pressure (or rather lack there of) ect.
Once you delve far enough down your “Exposure” warning meter will popup and initiate a ticking noise.
Best thing to do is head back up FAST.
To venture into highly “exposed” areas Crafting Armour with a higher exposure rating or sealing the area your in and using an o2 gen are both good ways to proceed.