| BSC Class |
Face Velocity |
Airflow Pattern | Applications | |
| Nonvolatile Toxic Chemicals and Radionuclides |
Volatile Toxic Chemicals and Radionuclides |
|||
| I | 75 | In at front; exhausted through HEPA to the outside or into the room through HEPA (see Fig. 2) | YES | YES (1) |
| II, A | 75 | 70% recirculated to the cabinet work area through HEPA; 30% balance can be exhausted through HEPA back into the room or to the outside through a thimble unit | YES | NO |
| II, B1 | 100 | Exhaust cabinet air must pass through a dedicated duct to the outside through a HEPA filter | YES | YES (minute amounts (2)) |
| II, B2 | 100 | No recirculation; total exhaust to the outside through hard-duct and a HEPA filter | YES | YES (small amounts) |
| II, B3 | 100 | Same as II, A, but plenums are under negative pressure to room; exhaust air is thimble-ducted to the outside through a HEPA filter | YES | YES (minute amounts (2)) |
| III | N/A | Supply air inlets and hard-duct exhausted to outside through two HEPA filters in series | YES | YES (small amounts) |
(1) Installation may require a special duct to the outside, an in-line charcoal filter, and a spark proof (explosion proof) motor and other electrical components in the cabinet. Discharge of a Class I cabinet in to a room should not occur if volatile chemicals are used.
(2) In no circumstances should the chemical concentration approach the lower explosion limits of the compound.
