Police and firefighters who risked their lives at the World Trade Center site will attend private screenings of Oliver Stone's new Sept. 11 movie, but one police union is warning the experience could be too traumatic for the rescuers.

Stone's movie, called "World Trade Center," stars Nicolas Cage as one of two police officers who survived the towers' collapse and were rescued from the trade center's rubble after 22 hours. It opens in the U.S. on August 9.

Police and firefighters have been invited to free, private screenings at area multiplexes this week, the New York Post reported Sunday.

Said Michael Shamberg, the film's co-producer; "Emotionally, it's important that these men and women have the opportunity to see the film first, though only if they feel comfortable."

But the union representing Port Authority of New York and New Jersey police officers wants ground zero rescue workers to be aware that the movie could cause post-traumatic stress disorder. Symptoms of the condition include depression, mood swings and panic attacks.