from the impact zone of the South Tower (floors 77 to 85) after it was struck by United Airlines Flight 175 at 9:03 am, while a further four escaped from the floors above it. Individuals escaped from the South Tower impact zone using stairwell A in the northwest corner, the only stairwell left intact after the impact. Investigators believe that stairwell A remained passable until the South Tower collapsed at 9:59 am. Because of communication difficulties between 911 operators and FDNY and NYPD responders, most of them were unaware that stairwell A was passable and instructed survivors above the impact zone to wait for assistance by rescue personnel. Despite the relatively few survivors from the impact zone and above, the 9/11 Commission did bring up the possibility of others who may have descended from the point of impact but were unable to make it all the way down before the tower collapsed and killed everybody still within. After collapses After the towers collapsed, only 20 individuals in or below the towers escaped from the debris, including 12 firefighters and three Port Authority police officers. Only 16 individuals who were inside the collapsing North Tower survived and were rescued, and they were all trying to evacuate via stairwell B, located in the center of the building. Four people who were in the concourse area between the Twin Towers survived and either saved themselves or were rescued. Nobody who was in the South Tower at the time of its collapse survived. The last survivor removed from the WTC collapse debris was found in the ruins of the North Tower 27 hours after its collapse. An unknown number of other people survived the initial collapse, but were buried in air pockets deep beneath the rubble and could not be rescued in time. Some were able to rescue themselves and others from the rubble by climbing through the rubble or digging and listening for sounds of life in order to safely remove the victims from the rubble. Survivor advocacy As of September 28, 2008, a total of over 33,000 police officers, firefighters, responders, and community members have been treated for injuries and sickness related to the 9/11 attacks in New York City, including respiratory conditions, mental health problems like PTSD and depression, gastrointestinal conditions, and at least 4,166 cases of cancer; according to one advocacy group "more cops have died of illness linked to