Abstract: Disclosed is an in-line silencer for a clean room breathing apparatus which reduces noise transmitted to a user from an attached air blower. The clean room breathing apparatus includes a headgear assembly worn by the user and an air blower that supplies air to or exhausts air from the headgear assembly through a connecting air hose. The in-line silencer is installed in the air duct between the air blower and the headgear assembly. The silencer has a shell housing with couplings at opposite ends for attaching the silencer in-line with the air blower and the connecting air hose. Within the shell housing of the silencer is a sound-absorbing material that absorbs sound energy from the air flowing through the silencer.