What was found
DURING CLIMB THRU FL210 CABIN ALTITUDE STARTED TO INCREASE RAPIDLY, INTO AMBER RANGE, FOLLOWED EMERGENCY PROCEDURE, DEPLOYED OXYGEN MASKS AND DESCENDED TO 10,000 INITIALLY. THE ACFT RETURNED WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED THE ACFT AND R & R THE NR 1 PACK OUTLET DUCT, EXHAUST GRATE, AND RUBBER SLEEVE. OPS CHECKS WERE PERFORMED WITH NO DEFECTS NOTED AND THE ACFT WAS APPROVED FOR RETURN TO SERVICE.