A B747-400 HAD A CONDITIONED AIR DUCT LEAK THAT WAS VENTING WARM AIR INTO THE AFT CABIN. LEAK WAS DEFERRED. CABIN ATTENDANTS RPT ILLNESS IN CABIN ATTENDANTS AND PAX. CABIN ATTENDANTS WERE UNAWARE OF DEFERRED ITEM.

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15 MINS AFTER TKOF I WAS NOTIFIED BY A FLT ATTENDANT ABOUT UNUSUAL TEMP AROUND ROW 23-35; R SIDE OF THE AIRPLANE. A VERY HIGH FLOW OF HOT AIR WITH TEMP OF 95 DEGS F WAS BLOWING OUT FROM THE AIR INLET R LOWER SIDE ROW 23 TO 35. EVEN THE OXYGEN BOTTLES AND DOOR 3R WERE HOT. WE FELT THIS WAS AN UNSAFE CONDITION; THEREFORE; ALL PAX AND ANY FLAMMABLE ITEMS WERE MOVED FROM THIS AREA. THE COCKPIT AND PURSER WERE NOTIFIED. AFTER 45 MINS; I STARTED TO FEEL NAUSEOUS; INCLUDING ALL 4 FLT ATTENDANTS WORKING IN THE SAME CABIN. JUST ABOUT 50 MINS BEFORE LNDG; WE WERE ALL FEELING FAINT AND AS A RESULT 2 BOTTLES OF OXYGEN WERE USED 15-20 MINS EACH AMONG THE 4 OF US WORKING IN COACH. IT IS MY UNDERSTANDING THAT THIS AIRPLANE WAS WRITTEN UP 2 TIMES ALREADY; AND FINALLY IT WAS DEFERRED. THIS WAS CLRLY A CABIN ITEM ISSUE -- WHY WAS THIS NOT LISTED ON THE BOARDING INFO SHEET? CAN SOMEONE IN ZZZZ ADVISE WHY AND HOW COME WE WERE NOT NOTIFIED ABOUT THIS ON THE BOARDING INFO SHEET? THIS AIRPLANE WAS FINALLY GNDED IN ZZZZ ON THE NEXT DAY. WHY ARE WE FLYING AN UNSAFE AIRPLANE WHEN WE KNOW THIS CAN CAUSE HEALTH AND SAFETY ISSUES FOR PAX AND CREW. THIS TYPE OF ACTION NEEDS TO BE STOPPED. SUPPLEMENTAL INFO FROM ACN 673722: PAX SITTING AT ROW 35 CAME INTO THE GALLEY AND QUESTIONED WHERE AND WHAT THE HEAT IS ABOUT AND WHETHER IT WAS SAFE. TWO FLT ATTENDANTS FROM ECONOMY WENT UP TO ROW 32 TO CHECK WHILE THE OTHER FLT ATTENDANTS AND I STAYED AT THE AFT GALLEY TO MONITOR OTHER PAX. FIVE MINS AFTER THE HEAT OCCURS; SOME KIND OF BAD SMELL CAME UP AS WELL FROM THE SAME AREA. IT SMELLS LIKE SOMETHING BURNING AND COCKPIT WAS INFORMED BUT NOBODY FROM THE COCKPIT CAME DOWN AND CHECKED FOR THEY KNEW THE PROB ALREADY. ON OUR WAY BACK; WE WERE TOLD THAT THE AIRPLANE WE FLEW IN WAS GROUNDED THE NEXT DAY AND FOUND OUT THE PROB WAS 'LEAK OF AIR DUCT.'