A cloud of irritating gas that leaked from a tank at a Baltimore factory last night drifted into southeastern Pennyslvania before dissipating.

About 100 Baltimoreans sought treatment at hospitals for eye irritations and breathing problems. Pennyslvania authorities advised senior citizens and persons with respiratory problems to stay in their homes as the cloud of sulfur trioxide gas drifted past. No serious injuries were reported.

Civil defense officials tracked the bluish-white cloud through York County, but lost sight of it about midnight when it reached the Susquehanna River community of Pequea in Lancaster County.

It had "a strong odor of sulfur - rotten eggs," said a spokeswoman for the York County Emergency Center. "People who have called said it is stinging their eyes. Other people who have seen it say it is like a fog cloud."

A Maryland official said today that the source of the leak at the Procter & Gamble plant in the Locust Point area of Baltimore might have been a hole "the size of a pinhed."

Dr. O'Neill Banks of the Maryland Division of Labor and Industry said the leak appeared to have come from the tank's discharge pipe. He said the gas forms sulfuric acid when mixed with water, and the acid could have corroded the discharge pipe. He stressed that his findings were preliminary and an investigation is continuing.

Procter & Gamble sent metallugists, electricians and technicians to investigate the tank today. Routine work at the plant resumed without incident, and city and state health officials said the air was back to normal.