B737-300 FLT CREW HAS LOSS OF CABIN PRESSURIZATION; DECLARES AN EMER AND DSNDS WHILE ENRTE SEA.

Narrative

AT FL350 THE CABIN ALT WARNING HORN SOUNDED AS THE CABIN WAS CLBING THROUGH 10000 FT. AFTER DONNING OUR OXYGEN MASKS; WE ASKED ATC FOR A DSCNT. ATC STATED WE HAD TFC AT OUR 4 O'CLOCK POS AT FL310. I INSTRUCTED THE FO TO DECLARE AN EMER AS WE BEGAN A DSCNT. ATC GAVE US A TURN OF APPROX 90 DEGS TO THE R OF 030 DEGS. AS WE TURNED; THE FO SAID HE HAD THE TFC IN SIGHT. WE REVIEWED THE QRH ITEMS DURING OUR DSCNT. I SELECTED BOTH PACKS TO HIGH AND PASSING APPROX FL310 THE CABIN PRESSURE RETURNED TO NORMAL AND WE LEVELED OFF AT FL280. ATC HAD GIVEN OUR TFC A L TURN; WHO HAD STATED THEY HAD US IN SIGHT. WE ALSO HAD THE TFC ON TCASII WHICH GAVE US A TA BUT NO RA. AS WE PASSED FL310; TCASII INDICATED OUR HORIZ SEPARATION TO BE APPROX 2 MI. I FEEL THAT WITHOUT DECLARING AN EMER THE CABIN MAY HAVE EXCEEDED 14000 FT HAD WE STAYED AT FL350 FOR MUCH LONGER. AS A RESULT; THE PAX OXYGEN MASKS DID NOT DEPLOY AND THE DSCNT WAS SMOOTH WITH THE AUTOPLT ENGAGED. AFTER BRIEFING THE #1 FLT ATTENDANT; SHE STATED THAT THE CABIN CREW AS WELL AS THE PAX WERE UNAWARE OF ANY PROBS. WITH A STABILIZED CABIN AT FL280 WE CONTINUED TO SEA. ATC REQUESTED THAT I CONTACT THEM UPON OUR ARR IN SEA ON A LANDLINE; WHICH I DID TO CONFIRM THE NATURE OF OUR EMER AS WELL AS OUR TFC SEPARATION ISSUE. IN DEBRIEFING THIS SIT WITH THE FO; HE STATED THAT HE WAS UNSURE WHAT THE PROB WAS INITIALLY WHEN I INSTRUCTED HIM TO DON HIS OXYGEN MASK. I THINK IT WOULD HAVE HELPED TO CLARIFY THE PROB BY STATING 'WE HAVE A LOSS OF CABIN PRESSURE.'