In a recent video by MrMattyPlays on Youtube, he pointed out a screen of the player HUD where the RAD meter was red, engulfing the health meter, and was confused as to how it works.

Here's my theory. In Fallout Shelter, a vault dweller who is irradiated also has a red bar engulfing their HP. This means that the further the red bar goes into the HP, the more irradiated the person is, and the more their maximum health decreases (It cannot regenerate past the red threshold)

This could mean that, this is how it works in Fallout 4 or, further, its a feature of Fallout 4 that accompanies the regular stat debuffs.

What do you guys think?