OVERWATER B777-200 DEVELOPED SMOKE AND FUMES IN THE CABIN. THE SOURCE COULD NOT BE IDENTIFIED; BUT THE INTENSITY DID NOT INCREASE. THE FLT CREW DECLARED AN EMER AND DIVERTED.

Narrative

DURING CRUISE OVER WATER; WE STARTED TO SMELL AN ODOR THAT COULDN'T BE IDENTED BUT HAD AN ACRID SMELL TO IT. SHORTLY THEREAFTER; THE FO CREW REST SMOKE ALARM WENT OFF WITH THE ASSOCIATED EICAS. THE RELIEF PLT AND I WERE IN THE COCKPIT AT THE TIME AND DONNED OUR MASKS; ESTABLISHED COMS AND CALLED THE CAPT; WHO WAS IN THE CREW REST FACILITY. THE CAPT DID A CABIN WALKAROUND BEFORE COMING UP TO THE COCKPIT AND NOTICED A SLIGHT HAZE. THE CAPT CAME INTO THE COCKPIT AND TOOK COMMAND OF THE ACFT AND I RAN THE SMOKE/FUMES AIR CONDITIONING CHKLIST. THE ODOR REMAINED BUT LESSENED WHEN WE ISOLATED THE R SIDE. I THEN RAN THE SMOKE/FUMES/ODOR REMOVAL CHKLIST AND WHEN I SELECTED THE EQUIP COOLING OVERRIDE SWITCH TO OVERRIDE THE ODOR SEEMED TO LESSEN. WHILE I WAS RUNNING THE CHKLIST THE CAPT WAS COMMUNICATING WITH THE COMPANY AND AFTER I FINISHED WITH THE SMOKE REMOVAL CHKLIST WE DECIDED TO DIVERT TO ZZZ. THE ODOR NEVER COMPLETELY LEFT THE CABIN OR COCKPIT. WHEN WE LANDED AND TOOK THE MASKS OFF; I NOTICED I HAD A SLIGHT COUGH FROM IRRITATION. IT SEEMED TO IMPROVE AS I BREATHED CLEAN AIR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT THE SOURCE COULD NOT BE IDENTIFIED; AND THE FUMES WERE PRESENT FOR THE REMAINDER OF THE FLT. OXYGEN MASKS WERE DONNED AND WORN UNTIL TOUCHDOWN. AN EMER WAS DECLARED.