For the acid rain, I think that just being under blocks should nullify the debuff (which can be very annoying if you want to fight a boss on the surface) as well. You aren't technically getting hit by rain, so yeah. Also, there's already an accessory that nullifies Irradiated, but I don't know how to get it. (It's a drop or a purchasable item, as you can't craft it)
Useless Fun Fact: It was the main reason I raged against Cryogen because Tremor replaced the Snow biome with the Glacier biome, which did not count as a surface Snow biome. So I went back to my former world, unaware how Acid Rain began. (Should happen to rain post-Moon Lord) My former world already had Moon Lord slain (up to Yharon because this was when Yharon still used the Kyurem theme), which meant that Acid Rain was now possible. I spawned Cryogen and immediately began wondering why I was taking so much damage even though I had Hallowed Armor. Eventually, I raged so hard that I went to my original world and went in the Underground Ice, enabled God Mode, and butchered Cryogen around 60 times.