The Cambridge Fire Department evacuated approximately 150 people from Emerson Hall last night in response to reports of mysterious fumes and abnormal odors in the building.

Fire officials set off the building's alarms at about 7:40 p.m. after the police received complaints of excessive coughing and gagging. Approximately 75 students were taking midterms at the time and were relocated to Lamont Library.

Fire officials closed the building for the night but officials determined more than an hour later that the building would be safe for classes to day.

University engineers and fire officials said they were unable to determine the source or nature of the fumes.

Deputy Fire Chief John J. O'Donoghue said that about 15 fire fighters entered the building to assess the situation and test for the presence of any toxic gasses or combustible fumes.

"We received a report of an irritant odor, andwhen we came, we could sense a little odor but wedid not detect any combustible gasses in thebuilding," O'Donoghue said.

Although they could not identify the odors, thefire fighters secured the heat ventilation and airconditioning shafts and the elevator shaft toprevent any fumes from circulating, officialssaid.

O'Donoghue said the odors definitely abatedwhile the fire companies were inside. "We havedowngraded [the situation] to a nuisance irritanttransient odor," O'Donoghue said.

The alarm interrupted classes inside thebuilding, prompting one professor to lament hisruined lecture. But students left immediatelyafter they were permitted to retrieve theirbelongings from the building in groups of three.

The evacuation could not have come at a worsetime for students in "Legal Reasoning," who werein the middle of their midterm.

"We were about to start a midterm for LegalReasoning and about three minutes after we startedwriting the alarm went off. The professor went outof the classroom to check what was going on and wewere told to evacuate [the building]," saidLorraine Lezama '93.

Neither the fire department nor Harvardengineers would speculate about the source orcomposition of the odor, saying only that it wasan unknown irritant.

Members of the crowd, however, suggested thatit may have been a gas leak.

"It may have been gas because it didn't havemuch of a smell and it made me gag," saidextension school student Joel C. Tillinghast