The airman, who was not identified by base officials, was in good condition at Temple University Hospital in Philadelphia, a base spokeswoman said.

He and two other members of an explosive ordnance disposal team were treated after exposure to what Army chemical-weapons experts believe was mustard agent, a dangerous chemical agent used by the United States, its allies and its enemies in World War I.

Analysts from the Army’s Guardian Brigade at Aberdeen Proving Ground, Md., examined material from the unexploded shell and from soil at the Delaware home where the shell was discovered, said Cathy Kropp, a brigade spokeswoman. Those tests led investigators to believe the shell contained mustard agent.

Mustard is a blister agent, causing damage to the skin, eyes and lining of the nose and mouth.

Kropp said the shell was discovered in the driveway of a Delaware home. Though investigators are not certain, she said, it is likely the shell was dredged up from an ocean dumping ground along with clam shells, which Delaware residents often use to pave their driveways.