A BE90 AT FL200 DEVELOPED SMOKE IN THE COCKPIT AFTER THE PILOT SELECTED THE VENT BLOWER TO HIGH FROM AUTO. AN EMER WAS DECLARED FOLLOWED BY A LNDG AT A NEARBY ARPT.

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I WAS AT FL200 ENRTE TO ZZZ TO PICK UP PAX AND MOVED THE VENT BLOWER SWITCH FROM AUTO TO HIGH; AS THERE WAS NOT MUCH AIR COMING OUT OF THE OVERHEAD COCKPIT VENTS. DOING THIS; HOWEVER; DID NOT INCREASE AIRFLOW. I STARTED TO NOTICED A SMOKY SMELL; LIKE AN ELECTRICAL FIRE; THEN I STARTED FEELING IT WHEN I COULD BREATHE IN. AT THIS POINT; I INFORMED CTR THAT I WAS EXPERIENCING WHAT I BELIEVED TO BE SMOKE IN THE COCKPIT AND WAS GOING TO TURN BACK TOWARDS THE CITY AND REQUESTED A LOWER ALT. ONCE I MADE THE TURN BACK; I COULD SEE THE SMOKE; SO I TURNED THE VENT BLOWER TO AUTO AND THE ENVIRONMENTAL SWITCH TO OFF. I ALSO PUT ON THE OXYGEN MASK AND SMOKE GOGGLES AT THIS POINT AND COMPLETED BOTH THE ENVIRONMENTAL SMOKE AND ELECTRICAL SMOKE CHKLISTS; WITH THE EXCEPTION OF LEAVING THE AVIONICS ON FOR A LITTLE BIT. AT THIS POINT; I DECLARED AN EMER AND DIVERTED TO ZZZ1. I TURNED THE AVIONICS OFF MOMENTARILY TO SEE IF THEY WERE THE PROB AT THIS POINT. ONCE THEY WERE OFF; THERE WAS NO CHANGE IN THE SMOKE; SO I TURNED THEM BACK ON; BUT ONLY 1 COM; 1 NAV; AND THE XPONDER. I MADE A NORMAL LNDG AT ZZZ1 AND SHUT DOWN WITHOUT FURTHER INCIDENT. HAVING THE SMOKE GOGGLES IN THE BE90; EVEN THOUGH NOT REQUIRED; WAS A TREMENDOUS HELP DURING THIS EVENT.