Four firemen and six Fairfax County public works employes were injured yesterday when a pipe blew off a chlorine gas tank at a sewage plant and released poisonous gas into the air, according to a county spokesman.

Edmund Castillo, the county spokesman, said the 10 men were injured around 3 p.m. at the Westgate sewage treatment plant at 6801 Fort Hunt Rd., in the Penn Daw section of the county.

Castillo said the gas leak occurred when the plant workers were going through a routine procedure of hooking up a new one-ton tank of pressurized liquid chlorine. One worker began opening the valve to tap the new cyclinder when the pipe running to the cyclinder blew off, Castillo said.

He said it took 30 minutes to stop the leak. School officials at nearby Belle View Elementary School were notified at the gas leak and the school was evacuated, he said.

All of the injured were taken to Mount Vernon Hospital, where they were treated for chlorine inhalation. Five of the victims were admitted to the hospital for precautionary reason, according to hospital spokeswoman Mary Howland.

Admitted to the hospital were four firemen - Capt. James Adams, 38; Lt. Andrew Gent, 38; firefighters Floyd Ellmo, 27, and Charles F. Phillips, 27, - and one public works employe, Timothy Gentry.

Treated for smoke inhalation at the hospital and released were James Stempel, Harold Wright, Henry Dunigan, Harold Waterholper and Leo Reynolds, all public works employes.