are they?

I'm pretty sure they aren't.

I've spent a fair amount of time in radioactive places and my dual followers didn't appear to suffer too much. Even though at one point I was using meds AND a rad suit to stay alive and was positively glowing by the time I got out to a safe area.

I'm playing on normal and that may be a factor.

Doubt it. They would have been dead a long time ago considering some of the radiated effected places I've been. I had to stack up for myself.

I wouldn't worry about it.

Their not affected by the enviroment in the same way as the player character nor by wearing faction armor.

They're totally unaffected by any environmental state.. my GF was doing the bit where you have to get to the sunken airplane.. Boone was following her around.. not drowning and [censored].. sometimes even walking on the bottom of the lake like he was superman.. :/

If npcs were actually affected by radiation, a lot of people would die pretty soon after the game starts due to all the irradiated food and water they eat