NFL training camps: Guardian Cap players are wearing over helmets, why
Do Padded Helmet Covers Actually Protect Football Players? - The Good Men Project
Guardian Caps: NFL requires use of helmet cover at training camps to reduce head injuries : r/nfl
NFL executive hints that Guardian Caps could eventually be worn in games, along with position-specific helmets
Guardian Caps - Are the soft-shelled football helmet covers effective at limiting head injuries? - ESPN
Guardian Caps: Why some NFL players are wearing them in training camps
NFL CMO Dr. Allen Sills explains importance of mandated guardian caps during training camp
Guardian Cap debuts during minicamps in effort to reduce avoidable head contact
Chicago Bears: Why players are wearing padded helmets