Sixteen children and four adults were overcome by a cloud of chlorine gas yesterday and taken to hospitals after pool chemicals were inadvertently mixed in a pool shed at the Glenelg Country School in central Howard County, fire and rescue services spokesmen said.

The victims, mostly 7- to 9-year-old youths at the school's summer camp, were taken by helicopter and ambulances to hospitals in Baltimore and in Howard and Montgomery counties after the 10:45 a.m. accident. All but a few were later released, rescue officials said.

Most seriously injured was the pool manager, Brian Vanesco, who apparently mixed an acid cleaning material with chlorine. The gas cloud exploded in his face, rescue commander Donald Howell said. The cloud billowed about 50 feet into the pool area and then started to disperse, he said.

Vanesco, a college student in his early twenties who has managed the pool for two summers, was in fair but stable condition at the Maryland Shock Trauma Center in Baltimore after being treated for respiratory irritation and other problems. Investigators had not been able to interview him by late yesterday afternoon.

Four children who were airlifted to Shock Trauma or Johns Hopkins University Hospital also were being treated for exposure to the gas and were recovering quickly, officials said.

Several were expected to be hospitalized overnight, Howell said. Rescue workers from Howard, Montgomery and Carroll counties responded to the scene, he said.

About 200 other children who were in buildings far from the pool area, including students at Glenelg's summer school session and other campers, were unharmed, said school spokeswoman Nancy M. Zimmerly. School officials called the parents of the students and campers immediately after the mishap, she said.

One mother who was carrying her daughter away from the school said the child had been frightened.

Zimmerly said that letters were sent home with the children to assure parents that "everything was in control . . . and there was no further danger."

Camp and classes will open as usual this morning on the Folly Quarter Road campus, she said.