All of them being immune is a bit much, what are the chances?
I believe it's simply because they didn't inhale much of the fog, especially the concentrated fog, and even if they were hit, they always wear protection and wash whenever they can.
They are affected, but they mitigate the damage.
The boy transformed so quickly because he inhaled the concentrated fog.
He had no protection; the creature was pressing him to the ground; he couldn't hold his breath and inhaled a lot of concentrated fog.
I think the boy scene was especially to show that.