Duct airtightness classes A to D are defined in European Standard EN 12237 for circular ducts and EN 1507 for rectangular ducts. Class A is the leakiest class. A parallel standard to EN12237, EN 1507 and EN 1751 based on the same leakage classification is EN 15727 which applies to technical ductwork products and specifies the leakage requirements for technical ductwork products. The leakage test method for system commissioning is described in EN 12599. Airtightness classes for air handling units (L1 to L3) are defined in EN 1886. System standards, in particular EN 13779, give further recommendations for airtightness class selection for different purposes. 
G. Guyot, F. R. Carrié and P. Schild, “Project ASIEPI – Stimulation of good building and ductwork airtightness through EPBD,” 2010.