Some NFL players are raising concerns about playing football amid the coronavirus pandemic while others are ignoring the advice of medical experts by working out with teammates.

JC Tretter, a center on the Cleveland Browns and president of the NFL Players Association, wrote an open letter to players on Tuesday, saying they have to fight for “necessary COVID-19 protections.”

New Orleans Saints safety Malcolm Jenkins said last week that “football is a nonessential business and so we don’t need to do it.”

Pittsburgh Steelers defensive lineman Cameron Heyward has asthma so he wants to “take every precaution” if he plays.

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“We are not invincible, and as recent reports have shown, we certainly aren’t immune to this virus,” Tretter wrote in his letter. “Underlying conditions like high BMI, asthma, and sleep apnea are all associated with a higher risk of developing severe symptoms and complications when infected with COVID-19. Those conditions are widespread across the league. NFL players are humans — some with immuno-compromised family members or live-in elderly parents. Trust me: we want to play football. But as a union, our most important job is keep our players safe and alive. The NFLPA will fight for our most at-risk players and their families.”

The league informed owners last week that training camp is expected to open as scheduled later this month. Discussions regarding shortening the preseason schedule are ongoing and an announcement is expected soon.

The NFL and the Pro Football Hall of Fame already canceled the opening preseason game between Dallas and Pittsburgh set for Aug. 6. The hall’s induction ceremonies set for Aug. 8, and for mid-September for a special centennial class, were moved to August 2021, when the Cowboys and Steelers will play in the game.

“I want to play football. I think all my peers want to play football. It’s how we make a living,” Jenkins said in a video posted on Twitter. “But there’s so much that we don’t know right now. When we look at what’s happening in the country, cases are going up, projected deaths are going up. ... I know that the bar for the NFL is going to be high when it comes to creating a safe working environment and making it as safe as possible for guys to come back as we keep in mind it’s not just about the athletes and their health and the coaches and staff, but our families, too.”

Dr. Thom Mayer, the union’s medical officer, advised players on June 20 to stop working out together. Still, Tom Brady and Russell Wilson appear to have ignored those warnings, based on social media posts. And a video posted Tuesday showed new Patriots quarterback Cam Newton working out with wide receiver Mohamed Sanu.

Numerous players in the NBA, NHL, and Major League Baseball have tested positive for COVID-19 as those sports have started preparations to return to play. Some players across those leagues have opted out, citing medical concerns.

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“It is the responsibility of the employer to provide a safe work environment,” Tretter told NFL players. “I encourage all workers to hold their employers accountable to high standards. More so than any other sport, the game of football is the perfect storm for virus transmission. There are protections, both short and long term, that must be agreed upon before we can safely return to work. The NFLPA will be diligent as we demand that the NFL provide us the safest workplace possible.”

The NFL and the players’ union haven’t agreed yet on protocols to open training camps. Players are holding a league-wide call on Thursday to discuss the proposals.

Houston Texans wide receiver Randall Cobb wrote on Twitter: “I would advise everyone to get on this call to hear the plans so you’re informed and know what to expect going into camp.”