CABIN ATTENDANT RPT; MD80; LGA-DFW; OXYGEN MASKS DEPLOYED AFTER DSCNT TO 10000 FT. CAPT ORDERED CREW AND PAX TO DON MASKS.

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AFTER REACHING 10000 FT; I TURNED TO VIEW THE CABIN AND NOTICED THE OXYGEN MASKS IN THE ENTIRE AIRPLANE HAD DROPPED. SINCE WE WERE AT 10000 FT; I THOUGHT IT ODD THAT THE MASKS WOULD DROP. I ASKED THE #4 FLT ATTENDANT IF HE WOULD OPEN THE COCKPIT DOOR TO INFORM THE CAPT. AS HE DID THIS; I NOTICED BOTH PLTS WERE TAKING OXYGEN. AT THAT TIME; THE ASKED US TO PUT THE PAX ON OXYGEN ALSO. I MADE A PA FOR THE PAX TO DON THEIR MASKS AND ASKED THEM TO STAY ON UNTIL FURTHER ADVISED. AFTER APPROX 5-10 MINS; CAPT MADE AN ANNOUNCEMENT TO ALL THAT IT WAS NO LONGER NECESSARY TO USE OXYGEN. WE THEN PICKED UP THE CABIN AND PREPARED FOR OUR LNDG INTO DFW. MANY PAX WERE CONCERNED ABOUT THEIR OXYGEN MASKS NOT WORKING; SINCE THE BAGS DO NOT INFLATE. OTHER THAN THAT; ALL WAS CALM AND NO ONE WAS INJURED. WE LANDED WITHOUT INCIDENT. AFTER OUR ARR; WE ALL CONTINUED TO OUR NEXT FLT. WE WERE NEVER TOLD WHAT HAPPENED TO THE ACFT THAT WOULD CAUSE THE MASKS TO DROP. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT SHE LATER LEARNED FROM THE FLT TRAINING DEPT THAT THE ACTUAL ALT WAS AT FL230; AND NOT THE STERILE COCKPIT ALT OF 10000 FT THAT SHE THOUGHT IT WAS. THE WHOLE CABIN CREW WAS STANDING BY THE COCKPIT DOOR WHEN IT WAS OPENED; AND THEY ALL SAW THE PLTS WITH THEIR MASKS ON AND WONDERED 'WHY?' SHE OVERHEARD THE CAPT TELL THE MECHS; WHO MET THE FLT; THAT IT WAS PROBABLY AN INADVERTENT OXYGEN MASK DROP. BECAUSE THE CREWS HAD TO RUSH TO OTHER FLTS; THEY HAD NO DEBRIEFING. THE PAX; WHO COMPLAINED ABOUT THEIR MASKS NOT INFLATING; HAD NOT LISTENED TO THE INSTRUCTIONS THAT THE BAG 'DOES NOT INFLATE' IN THEIR PREFLT BRIEFING AND THEY PANICKED. ALL BAGS DID WORK AS DESIGNED.