A B737-300 CREW; IN CRUISE AT FL350; EXPERIENCED A CABIN PRESSURE LOSS WHEN ONE OF THE 2 AIR CONDITIONING PACKS WAS INADVERTENTLY TURNED OFF.

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CRUISING FROM BHM TO MSY AT FL350. THE FLT ATTENDANTS CALLED AND ASKED US TO WARM UP THE CABIN. THE FO WARMED UP THE CABIN AND TURNED OFF THE GASPER FAN. BEGAN A DSCNT TO FL240. AFTER A SHORT TIME; CABIN ALT WARNING HORN BEGAN TO SOUND. HORN WAS SILENCED; CABIN ALT WAS AT 10000 FT; AND DIFFERENTIAL PRESSURE WAS INDICATED. CABIN WAS IN A SLIGHT CLB. BEGAN EMER PROC FOR RAPID DEPRESSURIZATION. AS THE FO WAS GOING FOR THE QRH; 1 NOTICED THAT THE R-HAND AIR CONDITIONING PACK SWITCH WAS IN THE 'OFF' POS. IT WAS PLACED 'ON' AND THE CABIN IMMEDIATELY BEGAN TO DSND. DURING THIS TIME WE HAD NOTIFIED ATC THAT WE WERE REQUESTING A LOWER ALT AS WE WERE EXPERIENCING A PRESSURIZATION PROB. THEY CLRED US FOR FURTHER DSCNT AND ASKED IF WE WERE DECLARING AN EMER. OUR INITIAL THOUGHT WAS TO GET THE AIRPLANE BELOW 14000 FT TO KEEP THE OXYGEN MASKS FROM DEPLOYING. ONCE THE CABIN WAS UNDER CTL THE DSCNT WAS SLOWED AND WE REPLIED THAT WE WERE NOT DECLARING AN EMER AND OUR SIT WAS UNDER CTL. AT NO TIME DID WE DSND BELOW A CLRED ALT OR DEV FROM OUR ASSIGNED RTE. THE SEQUENCE OF EVENTS THAT LED TO THIS BEGAN WHEN THE FO TURNED OFF THE R-HAND AIR CONDITIONING PACK SWITCH INSTEAD OF THE GASPER FAN DURING CRUISE BY MISTAKE. THE CABIN SLOWLY CLBED DURING CRUISE AND THEN SLIGHTLY MORE ONCE THE PWR WAS REDUCED TO BEGIN THE DSCNT ALLOWING IT TO FINALLY REACH 10000 FT SETTING OFF THE WARNING HORN. SUPPLEMENTAL INFO FROM ACN 556501: IN TRYING TO REGULATE THE CABIN; I TURNED OFF THE GASPER AND RECIRCULATION FAN SWITCHES AND EVIDENTLY BUMPED OR INADVERTENTLY PLACED THE R PACK SWITCH TO 'OFF.'