Chris Borland shocked the sports world when he left the N.F.L. after his rookie season, saying the long-term health risks of playing football was not worth it.
Now, Borland is going further, appearing in a public service announcement that denounces the N.F.L. for hiding the risks of catastrophic brain damage.
In the announcement, Borland discusses his decision to leave the N.F.L., and how the league spent years trying to play down and discredit research on chronic traumatic encephalopathy, a degenerative brain disease known as C.T.E. that is linked to repeated head trauma.
In the one-minute spot, which is now online and will be shown on television , Borland said that the league’s efforts to suppress the risks of the disease “is especially sad when you think about the fact that there are 5-year-old kids out there playing tackle football.