A truck driver who refused to drive a truck because he smelled fumes informed his co-worker of

Question:

A truck driver who refused to drive a truck because he “smelled fumes” informed his co-worker of this fact. When the employee was disciplined for refusing to take the truck, he alleged that he was engaged in “concerted activity.” What basis does he have for alleging this? [NLRB v. PALCO, 163 F.3d 662 (1st Cir. 1998).]