FLC OF AN LGT MADE AN EMER DSCNT DUE TO LOSS OF CABIN PRESSURIZATION.

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AFTER LEVEL OFF AT FL350 THE SO INDICATED THAT THE CABIN ALT WAS CLBING THROUGH 9000 FT. ALL CREW MEMBERS DONNED OXYGEN MASKS AND THE SO BEGAN THE PROC TO RE-ESTABLISH PRESSURIZATION. AFTER A FEW MOMENTS IT BECAME CLR THAT CABIN PRESSURIZATION WAS UNCTLABLE SO WITH THE FO FLYING I ADVISED ATC OF OUR SIT AND REQUESTED AN IMMEDIATE DSCNT. I BELIEVE HIS REPLY WAS; 'CAN'T LET YOU DSND DUE TO TFC; WILL GET BACK TO YOU IN A COUPLE OF MINS.' I FOUND THAT UNSATISFACTORY. THE CABIN NOW WAS GOING THROUGH 15000 FT AND I ONCE AGAIN TOLD ATC WE NEEDED TO GO DOWN NOW. AFTER ANOTHER REFUSAL I DECLARED AN EMER AND AS SOON AS AN ACFT BELOW US (NOTED ON TCASII) CLRED; WE BEGAN OUR DSCNT. THE CABIN HAD CLBED THROUGH 20000 FT THOUGH THIS WAS NOT AN IMMINENT DANGER SIT. WE DID HAVE ELDERLY AND TODDLERS ON BOARD AND IT WAS MY JUDGEMENT TO GET THE ACFT TO A SAFE ALT AS SOON AS POSSIBLE. AFTER LEVELING WE CHKED THE CONDITION OF THE ACFT AND PAX SEVERAL TIMES AND DECIDED IT BEST TO CONTINUE TO DEST AT THE LOWER ALT SINCE ALL WAS WELL.