FLC OF EMB120RT NOTICES PRESSURE CHANGE WHEN CLBING THROUGH FL190. EMER DSCNT EXECUTED AND FLT CONTINUES AT HABITABLE ALT.

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IFR FLT FROM DFW TO MLU. WHILE LEVELING FROM CLBOUT AT FL190 (PNF) WITH ZFW; I NOTICED A SHARP PRESSURE CHANGE IN MY EARS. GLANCING DOWN AT PRESSURE CTLR; I NOTICED CABIN ALT RISING IN EXCESS OF 2000 FPM. CAPT NOTICED SIT AND ATTEMPTED TO RESET PRESSURE MANUALLY. FLT ATTENDANT WAS IN PROCESS OF OPENING COCKPIT DOOR WHEN CAPT CALLED FOR DONNING OF OXYGEN MASKS. I DONNED MY MASK AND ESTABLISHED COMS WITH THE CAPT. CABIN ALT CLBED THROUGH 10000 FT. CAPT INITIATED EMER DSCNT; CALLING FOR GEAR AND BRINGING PWR TO IDLE. HAVING BEGAN TO DSND; I CALLED CTR AND ASKED FOR LOWER ALT BECAUSE OF A LOSS OF PRESSURIZATION. CTR CLRED US TO DSND TO 10000 FT. AS I SET THE ALT IN THE ALERTER; THE CABIN ALT WARNING LIGHT ILLUMINATED TELLING US THE CABIN ALT NOW EXCEEDED 14000 FT AND THE OXYGEN MASKS IN THE CABIN DROPPED; AND OXYGEN WAS FLOWING. WE QUICKLY REACHED 10000 FT; STABILIZED WE RECONFIGURED FOR CRUISE; AND THE CAPT CALLED FOR THE REMOVAL OF OUR MASKS. (CABIN ALT WAS NOW BELOW 10000 FT.) WE PULLED AND VERIFIED WITH OUR CHKLISTS THAT WE HAD COMPLETED THE DSCNT AND THEN ADVISED CTR OF OUR CONDITION. AFTER DISCUSSING OUR SIT WITH CTR; WE BRIEFED THE PAX OF OUR SIT. WE CONTINUED ON COURSE TOWARDS MLU WHILE WE CONTACTED COMPANY FLT CTL AND MAINT. WE DECIDED TO CONTINUE ON TO MLU UNPRESSURIZED. THE FLT ATTENDANT CAME FORWARD (THE DOOR TO THE COCKPIT HAD BEEN LEFT OPEN) AND TOLD US THAT A PAX IN ROW XX HEARD A POP AND RUSHING AIR BEFORE THE LOSS OF PRESSURE. THE REMAINDER OF THE FLT WAS UNEVENTFUL. UPON POSTFLT INSPECTION; THE CAPT AND I NOTICED THE AFT BAGGAGE DOOR SEAL WAS LOOSE AND HANGING FREE. THIS WAS MY FIRST TRIP WITH THIS VERY SENIOR CAPT. I AM A JUNIOR FO AND THE FLT ATTENDANT WAS ALSO VERY NEW. THIS WAS ALSO LATE NIGHT WITH A FULL LOAD OF PAX. DESPITE OUR LACK OF FAMILIARITY WITH EACH OTHER; AT ALL TIMES COM WAS SUPERB. THE CAPT'S SKILLS IN LEADING CRM KEPT MYSELF AND THE FLT ATTENDANT IN CONSTANT; CLR; AND CONCISE REACTIONS. ALTHOUGH EMERS ARE NEVER OPPORTUNE; THIS TOOK US BY COMPLETE SURPRISE. THIS WAS THE CAPT'S FIRST EMER IN 12 YRS OF LINE OPS. THE SIT WAS DEALT WITH QUICKLY AND PROMPTLY AND MOST PAX SEEMED CALMED BY HOW QUICKLY THE FLT WAS RETURNED TO 'NORMAL.' DESPITE BRIEFINGS AND PLACARDS; THE FLT ATTENDANT SAID THAT WHEN THE OXYGEN MASKS DROPPED; AMAZINGLY MOST PAX WERE UNAWARE OF WHAT TO DO AND NEEDED ASSISTANCE!