In an open letter published Saturday, NFL Players Association president JC Tretter called for the NFL to take several steps to increase safety measures for players, including the banning of slit film turf.

Tretter wrote that of the three types of turf used at NFL stadiums—slit film, monofilament and dual fiber—slit film is statistically proven to cause more non-contact injuries, lower extremity injuries and foot and ankle injuries, in addition to causing players to miss more time because of injuries.

Tretter added that the NFL and its experts acknowledge that the data is correct, but, "The NFL has not only refused to mandate this change immediately, but they have also refused to commit to mandating a change away from slit film in the future at all."

In discussing the dangers of slit film turf, Tretter noted that NFL player leadership wrote a letter to the NFL this week demanding the immediate removal of that turf from stadiums and practice fields.

Tretter called injuries suffered on slit film "completely avoidable" and said that the NFL has been resistant to making any changes.

There are seven teams that use slit film turf at their stadiums: New York Giants, New York Jets, Detroit Lions, Minnesota Vikings, New Orleans Saints, Indianapolis Colts and Cincinnati Bengals.

In addition to the removal of slit film turf, Tretter called for no games to be played on fields with "clear visible abnormalities," an increase in field standards and safety tests for all field surfaces, and the clearing of excess people and dangerous equipment from the sidelines to make it less likely that players get injured when going out of bounds.

ESPN's Kevin Seifert reported this week on data compiled by IQVIA regarding injuries suffered on natural grass compared to turf:

The study found that the number of non-contact injuries to the lower extremities was virtually the same on natural grass and turf last season, compared to the previous three seasons when more such injuries were clearly suffered on turf.

Tretter seemingly referenced the report, saying the "NFL PR machine" has gone "into overdrive to spin a more favorable narrative to what the union and players know is a problem."

ESPN's Jeremy Fowler reported Saturday that he recently spoke to an NFL executive who said unique challenges come with tailoring turf fields to football players compared to soccer players because of the disparity in weight, as well as the difference in the cleats that are used.

Regardless of the circumstances, Tretter made it clear that he and the players want immediate change, writing: "The NFL has an obligation to provide the safest work environment possible. They are not living up to that standard."

The former Green Bay Packers and Cleveland Browns center also called for the league to "stop with the lip service, stop with the media spin, stop pretending you care."