Secondary biological safety cabinets are currently the most widely used
Biological safety cabinets can be divided into three categories: first-level, second-level and third-level to meet different biological research and epidemic prevention requirements. The first-level biological safety cabinet can protect the staff and the environment without protecting the samples. The airflow principle is the same as the laboratory fume hood, except that the exhaust port is equipped with a HEPA filter. All types of biosafety cabinets use HEPA filters in the exhaust and air inlets. The first-level biological safety cabinet itself does not have a fan, and relies on the fan in the external ventilation pipe to drive the airflow. Because it cannot protect the products in the cabinet, it has been rarely used at present. The secondary biological safety cabinet is currently the most widely used cabinet type. According to regulations, secondary biological safety cabinets can be divided into four levels according to the inlet air velocity, exhaust mode and circulation mode: A1 type, A2 type (formerly B3 type), B1 type and B2 type. All secondary biological safety cabinets can provide protection for staff, environment and products. The minimum amount or average value of the airflow velocity of the front window of A1 type safety cabinet shall be at least 0.38m / s. 70% of the gas is recirculated to the work area through the HEPA filter, and 30% of the gas is filtered out through the exhaust port. The minimum amount or average value of the airflow velocity of the front window of A2 type safety cabinet shall be at least 0.5m / s. 70% of the gas is recirculated to the work area through the HEPA filter, and 30% of the gas is filtered out through the exhaust port. Type B biological safety cabinets are all safety cabinets connected to the exhaust system. The fan connected to the exhaust duct of the safety cabinet is connected to the emergency power supply, and the purpose is to maintain the negative pressure of the safety cabinet even when the power is off, so as to avoid the leakage of dangerous gases such as a laboratory. The minimum amount or average value of the airflow velocity of the front window should be at least 0.5m / s (100fpm). Type B1 70% of the gas is discharged through the exhaust port HEPA filter, and 30% of the gas is recirculated to the work area through the air supply port HEPA filter. Type B2 is a 100% full-row safety cabinet with no internal circulating airflow, which can provide biological and chemical safety control at the same time. The three-level biological safety cabinet is designed for the level 4 laboratory biological safety level. The cabinet is completely airtight. The staff operates through the gloves connected to the cabinet, commonly known as the glove box, and the test article enters and exits through the double-door transfer box. Cabinet to ensure no pollution, suitable for high-risk biological tests. 
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