Narrative:

We were in cruise at FL240; about 25 minutes out of ZZZ and about 100 NM from landing when the 'lavatory smoke' message appeared on the EICAS. We immediately ran the 'lavatory smoke' QRH checklist and called the flight attendant. She was already in the lavatory checking it because her smoke bell had also rung. She reported seeing or smelling nothing in the lavatory. The captain told her to lock the lavatory; make an announcement for the passengers not to use it; and to keep us advised if she noticed anything else. She called back moments later to tell us that she was seeing some haze in the cabin but was not sure if it was severe enough to declare an emergency. The captain asked her to come into the cockpit and switch seats so he could inspect the cabin; as he could not assess anything definite through the peephole. I was already wearing my oxygen mask as a precaution (we were already descending to 10;000 ft). As soon as the captain stepped out of the cockpit; he said he clearly saw heavy haze hovering near the top of the cabin; mid aircraft; and smelled an electrical fire. He immediately told the flight attendant to get back and let him back into the cockpit and told me we had smoke in the cabin. He declared an emergency with ATC and asked to land at the nearest suitable airport which was ZZZ2 (long runway with emergency equipment). I performed an emergency descent while the captain ran the 'smoke in cabin' and 'smoke evacuation' checklists. We landed without incident at 35;000 pounds in ZZZ2 and taxied. The captain decided to go with a precautionary landing and advised the flight attendant the passengers were not braced on landing. The trucks were waiting for us as we exited the runway. The fire department did not report seeing any smoke coming outside the aircraft and the flight attendant reported most of the haze had cleared. The captain decided the risk of injury that an emergency evacuation would produce without airstairs against the slim chance of an actual cabin fire; he decided; in conjunction with the fire department to proceed with a normal deplaning of the aircraft. Airstairs were brought promptly after we parked and secured the airplane; and the passengers calmly exited the airplane with their belongings. Nobody appeared to be or reported any discomfort or injury. The fire department decided; without consultation; to inspect the aircraft and unplug the batteries. We left the aircraft unpowered and secured with the airport police. Callback conversation with reporter revealed the following information: the reporter states that his aircraft type was an EMB145 and that the source of the smoke was not determined.

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Original NASA ASRS Text

Title: EMB145 First Officer reports LAV SMOKE EICAS at FL240 and diversion to nearest suitable airport.

Narrative: We were in cruise at FL240; about 25 minutes out of ZZZ and about 100 NM from landing when the 'Lavatory Smoke' message appeared on the EICAS. We immediately ran the 'Lavatory Smoke' QRH checklist and called the Flight Attendant. She was already in the lavatory checking it because her smoke bell had also rung. She reported seeing or smelling nothing in the lavatory. The Captain told her to lock the lavatory; make an announcement for the passengers not to use it; and to keep us advised if she noticed anything else. She called back moments later to tell us that she was seeing some haze in the cabin but was not sure if it was severe enough to declare an emergency. The Captain asked her to come into the cockpit and switch seats so he could inspect the cabin; as he could not assess anything definite through the peephole. I was already wearing my oxygen mask as a precaution (we were already descending to 10;000 FT). As soon as the Captain stepped out of the cockpit; he said he clearly saw heavy haze hovering near the top of the cabin; mid aircraft; and smelled an electrical fire. He immediately told the Flight Attendant to get back and let him back into the cockpit and told me we had smoke in the cabin. He declared an emergency with ATC and asked to land at the nearest suitable airport which was ZZZ2 (long runway with emergency equipment). I performed an emergency descent while the Captain ran the 'Smoke in Cabin' and 'Smoke Evacuation' checklists. We landed without incident at 35;000 LBS in ZZZ2 and taxied. The Captain decided to go with a precautionary landing and advised the Flight Attendant the passengers were not braced on landing. The trucks were waiting for us as we exited the runway. The Fire Department did not report seeing any smoke coming outside the aircraft and the Flight Attendant reported most of the haze had cleared. The Captain decided the risk of injury that an emergency evacuation would produce without airstairs against the slim chance of an actual cabin fire; he decided; in conjunction with the Fire Department to proceed with a normal deplaning of the aircraft. Airstairs were brought promptly after we parked and secured the airplane; and the passengers calmly exited the airplane with their belongings. Nobody appeared to be or reported any discomfort or injury. The Fire Department decided; without consultation; to inspect the aircraft and unplug the batteries. We left the aircraft unpowered and secured with the airport police. Callback conversation with Reporter revealed the following information: The Reporter states that his aircraft type was an EMB145 and that the source of the smoke was not determined.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.