A RETURN LAND WITH AN OVERWT LNDG MANDATED FOR AN MD80 FLT CREW WHEN LOW FLOW LIGHT INDICATES A CABIN PRESSURIZATION AND COOLING PROB FOR SICK PAX AFTER DEP FROM DFW.

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ACFT DEPARTED WITH THE L AIR CONDITIONING PACK DEFERRED. THE R AIR CONDITIONING PACK WAS TURNED ON AT 400 FT AGL. THE OUTFLOW VALVE IMMEDIATELY CLOSED AND THE FLOW LIGHT CAME ON AND STAYED ON THROUGHOUT THE CLB AND CRUISE. AT CRUISE OF FL240; THE FLOW LIGHT WAS ON AND THE CABIN WAS CLBING AT 500 FPM. THE FLOW LIGHT ON INFLT PROC WAS ACCOMPLISHED DURING THE CLBOUT WITH NO IMPROVEMENT IN MAINTAINING PRESSURIZATION. WE RETURNED TOWARD DFW; DSNDED TO 9000 FT MSL; AND PLANNED TO HOLD UNTIL THE NORMAL LNDG WT OF 130000 LBS. AFTER ENTERING HOLDING; THE CAPT AND THE FLT ATTENDANTS DISCUSSED HOW HOT IT WAS IN THE CABIN AND THAT AT LEAST 1 PAX FELT ILL. THE CAPT; WITH MY AGREEMENT; DECIDED WE SHOULD DECLARE AN EMER AND MAKE AN OVERWT LNDG. THE FLOW LIGHT DURING DSCNT AND APCH WAS MOSTLY CUT; BUT THE OUTFLOW VALVE WAS STILL ABNORMALLY ALMOST CLOSED. THE ACFT REMAINED PRESSURIZED UNTIL LNDG. REGIONAL APCH SAID TO EXPECT RWY 17R; BUT THE CAPT AND I REQUESTED AN ILS TO RWY 17C BECAUSE THE GS WAS OTS ON RWY 17R. WE BOTH WANTED A STABILIZED APCH. WE TOUCHED DOWN ON RWY 17C AT 137500 LBS AT 600 FPM. PARAMEDICS WERE REQUESTED TO MEET US AT THE GATE. SUPPLEMENTAL INFO FROM ACN 625026: WE WERE AT 9000 FT AND HOLDING AT UKN; WHEN FLT ATTENDANTS CALLED TO SAY THAT IT WAS VERY HOT IN THE BACK. PAX WERE STARTING TO FEEL FAINT. WE WERE UNABLE TO COOL THE CABIN ANY MORE. PARAMEDICS WERE CALLED TO MEET THE FLT DUE TO DETERIORATING CONDITION OF 1 PAX. SHE MANAGED TO GET OFF THE FLT; BEFORE THE PARAMEDICS ARRIVED; ON HER OWN PWR. SHE FELT BETTER ONCE ON THE GND.