A biosafety cabinet (BSC) is an enclosed, ventilated workspace designed to provide a safe environment for handling materials that may contain biological hazards. Its primary function is to contain infectious microorganisms, preventing them from escaping into the surrounding laboratory and protecting the person using the equipment. The cabinet achieves this containment by utilizing a carefully controlled flow of air and specialized filtration. This ensures that work with bacteria, viruses, and other pathogens can be conducted without risk to the user or the environment. BSCs are indispensable equipment in research, clinical, and public health laboratories globally.
Core Mechanism: Airflow and Filtration
The effectiveness of a biosafety cabinet relies entirely on directional airflow and a high-efficiency filtration system. Air is actively pulled into the cabinet through the front opening, creating an air barrier, or “air curtain,” that captures aerosols and prevents them from moving toward the operator. This inward flow provides essential protection for the personnel working at the cabinet.
Within the cabinet, the air moves downward over the work surface in a smooth, unidirectional stream known as laminar flow. This downward flow of filtered air protects the biological materials being handled from external contaminants, thereby providing product protection. All air that moves through the cabinet, whether it is recirculated within the work area or exhausted out, must pass through High-Efficiency Particulate Air (HEPA) filters.
HEPA filters are the component responsible for environmental protection, capable of trapping at least 99.97% of airborne particles that are 0.3 micrometers in diameter. These filters physically remove biological contaminants like bacteria, viruses, and spores before the air is released back into the lab or outside. It is important to note that while HEPA filters are highly effective against particulates, they do not capture volatile chemicals or gases; these require a different type of filtration or exhaust system.
Classification by Safety Level
Biosafety cabinets are categorized into three main classes—Class I, Class II, and Class III—each offering distinct levels of protection based on its design and airflow pattern. These classifications are established by international guidelines, such as the Biosafety in Microbiological and Biomedical Laboratories (BMBL) manual, to match the cabinet to the risk level of the biological agents being handled.
The Class I cabinet is the simplest design, providing protection for the user and the environment by drawing air inward and filtering the exhaust air through a HEPA filter before it is discharged. This cabinet is often used when product contamination is not a major concern because it does not provide a downward flow of filtered air over the work surface.
The Class II cabinet is the most common type, offering protection for the personnel, the environment, and the product. It achieves this triple protection by using both the inward air barrier and a downward flow of HEPA-filtered air across the work surface. Class II cabinets are further divided into types (A1, A2, B1, B2) based on how air is exhausted and recirculated within the unit.
Class II Types A1 and A2
Type A1 and A2 Class II cabinets recirculate a significant portion of the filtered air back into the laboratory. The A2 type maintains a higher minimum inflow velocity for improved safety. These types are generally not suitable for work involving volatile toxic chemicals, as the recirculated air would carry chemical fumes back over the work area.
Class II Types B1 and B2
Type B1 and B2 Class II cabinets are connected to a building’s exhaust system, venting a higher percentage of air (B1) or 100% of the air (B2) outside. The Type B2 cabinet, known as a total exhaust cabinet, has no internal air recirculation. This makes it suitable for work involving both biological agents and volatile toxic chemicals or radionuclides.
The Class III cabinet represents the maximum level of containment, designed for use with the most hazardous biological agents, such as those requiring Biosafety Level 4 containment. This cabinet is a completely enclosed, gas-tight workspace that operates under negative pressure to ensure no air leaks out. Work is performed using attached, sealed rubber gloves, and both the supply air coming in and the exhaust air going out pass through two HEPA filters or a HEPA filter and an air incinerator.
Difference Between Biosafety Cabinets and Fume Hoods
A common misunderstanding in laboratory settings is confusing a biosafety cabinet with a chemical fume hood, but their functions and mechanisms are fundamentally different. The BSC is designed specifically to contain biological hazards, relying on HEPA filtration to trap harmful particles like microbes. Its purpose is to protect the user, the environment, and the research material itself from biological contamination.
A chemical fume hood, however, is designed primarily to protect the user from hazardous chemical fumes, vapors, and gases. It operates by drawing air from the room and exhausting it directly outside the building, often without any filtration for particulates. Fume hoods do not use HEPA filters and therefore provide no protection against biological aerosols.
Using a fume hood for biological work can put the product and the environment at risk. Conversely, using a standard recirculating BSC for large volumes of volatile chemicals can expose the operator to chemical fumes. The core distinction is the hazard they are engineered to control: a BSC is for biological particulates, while a fume hood is for chemical vapors and gases.