A319 FLT CREW DEVIATED FROM THEIR ASSIGNED ALT OF 28000 FT IN RESPONSE TO A TCAS RA.

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FO WAS PF; I WAS CAPT; PLT MONITORING. WE WERE AT FL280 JUST BEGINNING TO REVIEW THE ARR AND APCH FOR IAH WHEN ZHU CALLED TFC TO US AT 2 O'CLOCK POS; 6 MI FROM FL270. WE SEARCHED FOR THE TFC AND I SCANNED MY TCAS DISPLAY. MY TCAS SHOWED TFC IN THAT DIRECTION 700 FT BELOW US. TCAS THEN GAVE US A 'TFC TFC' ALERT AND THE THREAT TARGET TURNED AMBER. A VERY SHORT TIME LATER; WE GOT AN RA TO 'CLB CLB.' FO WAS FLYING AND TURNED OFF THE AUTOPLT AND INITIATED A CLB. WE CLBED TO APPROX FL285 AS TFC FLEW BELOW US. I CALLED CTR AND ADVISED THEM WE HAD A TCAS RA AND WERE CLBING. TFC CLRED BELOW US AND WE DSNDED BACK TO FL280. I THEN NOTICED THAT THE FO'S ALTIMETER WAS SET AT 30.19 INCHES WHICH WAS THE IAH FIELD BAROMETRIC SETTING. I POINTED THAT OUT TO THE FO. HE THEN REALIZED THAT HE HAD BEEN PRESETTING THE IAH FIELD BAROMETRIC SETTING JUST AT THE INSTANT ZHU CALLED THE TFC TO US. THE TFC CALL HAD DISTR HIM AND HE DID NOT RESET THE STANDARD ALTIMETER SETTING (29.92). WE THEN FIGURED THAT HIS MISSET ALTIMETER HAD CAUSED THE TCAS ALT AND FURTHER CAUSED THE TCAS RA. ZHU HAD THE OTHER ACFT VERIFY THEIR ALT. THE ACFT STATED THEY WERE LEVEL AT FL270. CTR ALSO SAID THEIR MODE C READOUT CONCURRED WITH THAT ALT AND WE WERE CLRED TO DSND BACK TO FL280. I DO NOT BELIEVE WE CAUSED ANY TFC CONFLICT WITH ANY OTHER ACFT BY OUR 500 FT CLB. CTR NEVER MENTIONED ANY OTHER TFC TO US. ON THIS TYPE ACFT; YOU CAN PRESET A LCL ALTIMETER SETTING AND THEN PULL THE KNOB TO SET 'STD' OR STANDARD 29.92 INCHES. IN ALL OF MY 15+ YRS OF FLYING AIRBUS ACFT; I HAVE NEVER SEEN AN INCIDENT OF THIS NATURE. BUT I NOW WILL NEVER PRESET THE LCL ALTIMETER SETTING WHEN FLYING AT FL180 OR ABOVE.