NFLPA president JC Tretter is calling on NFL franchises to change all of their field surfaces to natural grass.

In an open letter shared Wednesday on the NFLPA's website, Tretter, who is also the starting center on the Cleveland Browns, wrote that "The unforgiving nature of artificial turf compounds the grind on the body we already bear from playing a contact sport.

"NFL clubs should proactively change all field surfaces to natural grass."

Tretter cited injury data collected from 2012 to 2018 that found that players have a 28% higher rate of non-contact lower extremity injuries when playing on artificial turf compared with grass. He found that of those non-contact injuries, players have a 32% higher rate of non-contact knee injuries on turf and a 69% higher rate of non-contact foot and ankle injuries on turf compared with grass.