In one example, an additive manufacturing system. An unsealed air supply enclosure for clean air is maintained at a first pressure above an ambient air pressure to inhibit unfiltered ambient air from leaking into the enclosure. An unsealed processing chamber is maintained at a second pressure below the ambient air pressure to inhibit processing chamber air from leaking out of the processing chamber. An air pathway is disposed between the air supply enclosure and the processing chamber to provide clean air from the air supply enclosure to the processing chamber.