A woman exercising on the fourth floor of an Upper East Side fitness center was nearly killed when a flying jackhammer chisel crashed through a double pane window yesterday morning around 8:45 a.m. Robin Cocking, 59, was doing pelvic thrusts on the floor when the 10-pound, two-foot-long piece of steel smashed through the window right by her head. "I saw this thing come through the glass," Cocking tells the Post. "I thought: if it’s breaking the glass it has to be heavy metal, and sharp."

Cocking sustained cuts to her right hand, right foot, and left arm, and was treated at Lenox Hill Hospital and released.

The chisel flew off a jackhammer being used in construction of the controversial Upper East Side waste transfer station on East 91st Street, and the incident has emboldened critics of the project who say its location—next to a recreation area where children play—is ill-conceived. The Asphalt Green fitness center, where Cocking also works, overlooks the construction site.

"Of course this happened," former Asphalt Green director Carol Tweedy told the Daily News. "We have been saying for years, this is not the right site for an industrial facility. There are 34,000 children who use Asphalt Green every year."

"Thank God she rolled left, because the jackhammer chisel landed right where she was and would have impaled her if she had not moved,” Kelly-Mimmo-Guengher of the Pledge 2 Protect community group told the Post. "Had it just gone the other direction, it could have killed a child."

All work has stopped at the site pending an investigation. Rep. Carolyn Maloney, who represents the Upper East Side, released a statement on the incident:

"I am horrified to learn that a drill bit crashed through a fourth floor window at Asphalt Green showering at least one person with glass. I fully support the decision by the city to stop construction on the MTS project following today’s incident and it needs to stay shut down pending a full investigation and until safety at the work site can be examined and improved. The drill bit could just as easily have caused greater damage within the facility or flown onto an Asphalt Green field filled with children at play. This ill-conceived project should not be taking chances with our safety."

The Post has video of the moment of impact.