Citing a heat wave and poor air quality, Harvard University closed its doors on Friday for only the second time in its 352-year history, a spokesman said.

"We've had a few unbroken days of 95-96 degree temperatures," said chief Harvard spokesman Peter Costa. "It's pretty hard to work in that."The university closed all its facilities in Boston and Cambridge, including the medical and business schools on the Boston side of the Charles River, the spokesman said.

Some 6,000 summer school students and about 10,000 of Harvard's 12,000 employees got a three-day weekend, Costa said. Essential personnel and some researchers worked, he added.

The only other time Harvard closed was in February, 1978, when a major northeast blizzard prompted Gov. Michael Dukakis to declare an emergency that effectively shut down eastern Massachusetts for nearly a week.

About 20 percent of Harvard's buildings - some dating back to the 18th century - do not have central air conditioning.

Costa said the university had kept corridor lights off and electricity use down as local utilities continued appeals for energy conservation.