BioSafety cabinet characteristics are described in "Biosafety in Microbiological and Biomedical laboratories" (BMBL).
BioSafety cabinet class I is rarely being use in Thailand. The weak point is BSC I only protect the operator and Environmental. (by draw the room air horizontally through the opening sash then go through HEPA filter before discharge) . No product protection. Many people confuse BSC I is the same as Clean bench. The answer is NO. Clean bench only protect the product. (no protection for operator and environmental)
The most popular BSC you find in laboratories is BSC II type A2. BSC II offer all round best performance in term of good protection for Product , Operator and Environmental as well as no cross-contamination in the work area inside the cabinet.
The intake air velocity BSC II type A2 is >100 foot/minute (NSF49 standard - US version) or 0.45 meter/second (EN 12469 standard - EU version) We call this portion of air as "in flow". The clean air inside the working area is supplies by main HEPA filter with down flow laminar flow. (for good product protection and prevent cross-contamination) We call this portion of air as "Down flow".
There are at least two sets of HEPA filters installed in the BSC II. Main HEPA filter is the bigger piece of HEPA filter supply the downflow clean air to work surface. The smaller piece of HEPA filter is installed on top of the safety cabinet. It filter particle in the exhaust air before release to the environment. (Environmental protection). The modern BSC II now use DC motor/blower because it consume much less energy than the classic AC motor. The good controller shall have "active" air flow control. In case the HEPA absorb lots of dirts. The blower can increase the speed to deliver same air velocity of both in flow and down flow. The video below show you how the BSC II adjust the air flow automatically.