MD88 CREW WAS UNABLE TO PRESSURIZE THE ACFT.

Narrative

AFTER A NORMAL PREFLT; TAXI; DEICE; TKOF AND CLB; THE CABIN ALT ALERT WENT OFF SHORTLY AFTER CLBING THROUGH 10000 FT MSL AFTER DEPARTING CVG TO THE N. I LEVELED THE AIRPLANE AT 12000 FT WITH THE ALT HOLD BUTTON; AND PUT ON MY OXYGEN MASK. THE CAPT SAID THE CABIN ALT SHOWED US UNPRESSURIZED AND DECLARED AN EMER WITH CVG DEP. HE THEN ASKED FOR AND RECEIVED CLRNC TO 10000 FT. I INITIATED DSCNT ON AUTOPLT. THE ACFT FAILED TO CAPTURE AT 10000 FT. I NOTICED THE ALT WINDOW SHOWED 9000 FT. I STOPPED THE DSCNT AT 9800 FT AND ASKED DEP IF WE WERE CLRED TO 10000 FT OR 9000 FT. THEY ASKED WHAT WE WANTED. I RECEIVED CLRNC TO 9000 FT AND CONTINUED THE DSCNT. IN THE MEANTIME; THE CAPT WAS DOING THE 'LOSS OF PRESSURIZATION CHKLIST' AND TALKING TO THE FLT ATTENDANTS IN THE REAR OF ACFT. THEY RPTED A SOUND OF AIR RUSHING FROM A REAR EXIT DOOR (I DON'T KNOW WHICH ONE) WITH A SEAL THAT LOOKED ABNORMAL. THEY COULD SEE DAYLIGHT THROUGH IT. THE CAPT ADVISED THEM TO STAY AWAY FROM THE DOOR. I DON'T KNOW ALL THE DETAILS BECAUSE I WAS BUSY FLYING. AFTER A SMOOTH LNDG; THE HANDLE FOR THAT DOOR WAS DISCOVERED POPPED UP. THERE WERE NO DOOR MESSAGES ON THE OAP OR ANYTHING ELSE ABNORMAL. DSCNT WAS MADE AT 700-800 FPM AND SUBSEQUENT LNDG WAS UNEVENTFUL.