Air cleaning systems are subjected to major challenges under accident conditions and the potential large source terms require the highest environmental protection from atmospheric release. The objective of this report is to consider the principles involved and to survey existing systems and equipment for the mitigation of the consequences of abnormal plant operations, including severe accidents beyond the design basis. Some experiences under abnormal conditions and international trends in the development of accident air cleaning are also included. Contents: 1. Introduction; 2. Source terms; 3. Design principles; 4. Control of airborne contamination in design basis accidents; 5. Control of airborne contamination in severe accidents; 6. Experience and trends; 7. Conclusions and recommendations; Appendix: Testing of off-gas cleaning equipment for nuclear power plants.