How does radiation work in Metro Exodus?
Being in a radioactive hot-zone and then leaving it alive does not have any long-term effect on the gameplay, just short term damage to your health.
Wearing a gas mask is not related to the radiation hot-zones, it used to prevent inhaling radioactive dust (thus preventing Artyom from suffocating). In the hot-zones the direct radiation is so high it will kill you regardless of you wearing a mask or not.