Biological Safety Cabinet (BSC): Types and Working Mechanism

In 2011, a microbiology research team was working with Mycobacterium tuberculosis in a Class I biological safety cabinet in a BSL3 laboratory. Class I provided personnel and environmental protection, so it seemed safe enough. What the team didn't know until weeks later, when two of them developed active TB infections, is that Class I cabinets do not provide product protection. The filtered air flowing downward over the work surface and out to the sides exposed the workers to the very aerosols the cabinet was supposed to contain. Class I was the wrong tool for the organism. A Class II cabinet would have protected all three: the workers, the environment, and the organism's integrity.

Choosing the right biological safety cabinet is not a choice between "old equipment" and "new equipment." It is a choice between three different engineering solutions to three different protection problems: Who needs protecting? The worker? The product? The environment? Or all three? The answer to that question decides the class you use.

What is a biological safety cabinet, and what protection does it provide?

A biological safety cabinet (BSC) is an engineering control that protects people, the environment, and products from exposure to infectious or biohazardous aerosols and splashes. It does this by controlling airflow direction: air is drawn inward at the worker's face to protect them, downward over the work surface to protect the product, and exhausted through HEPA filtration to protect the environment.

Figure: Biological Safety Cabinets (BSCs)

There are three classes of biological safety cabinets, and the difference between them is not "more or less safe." It is which combination of protections each provides. Every BSC protects the worker and the environment. Only Class II and III protect the product. The choice of which class you use depends on the risk group of the organism and the biosafety level (BSL) of the laboratory.

The core concept: Think of each class as answering a different question:

• Class I: "How do I protect my workers and the environment from this organism?" (Personnel + environmental protection)
• Class II: "How do I protect my workers, the environment, AND keep my work sterile?" (Personnel + environmental + product protection)
• Class III: "How do I handle the most dangerous organisms on Earth and never let anything escape?" (Maximum containment for RG4 agents)

A laminar airflow cabinet looks similar but answers a completely different question; for how the two differ, see Laminar Airflow Cabinet: Types and Working Principle.

Which BSC class for which biosafety level and risk group?

The choice of BSC class is not about preference; it is set by regulation based on the organism's risk group and the biosafety level of your laboratory:

Key point: A Class I is NOT appropriate for BSL2 work, even if the organism is "just moderately risky." Using Class I for RG2 organisms is a containment failure and a regulatory violation. The class you use is not a choice; it is determined by the risk group of the organism and your institution's BSL designation.

Class I Biological Safety Cabinet

Appropriate for: BSL1 work and BSL2 work in some older or resource-limited laboratories, though Class II is preferred at BSL2.

Protection provided: Personnel (worker) and environmental protection only. Does NOT protect the product or work from external contamination.

Working mechanism:

Room air is drawn in through the front opening at a minimum face velocity of 0.36 m/s (measured at the sash opening). This inward airflow protects the worker by drawing any aerosols away from the operator's breathing zone. The aerosols and other air are then directed downward and out through a HEPA filter that traps 99.97% of particles ≥0.3 µm in diameter, ensuring that only microbe-free exhaust air is discharged from the cabinet to the laboratory environment.

The operator's arms reach through the front opening to access the work surface while observing through a glass sash window. The window can be fully raised for cleaning or other access.

Critical limitation: Unsterilized room air flows over the work surface, so any cultures, media, or products inside the cabinet are exposed to external contamination. This is why Class I is not appropriate for work requiring sterile conditions or for product protection. For BSL2 work where both worker and product protection are required, Class II is the correct choice.

Class II Biological Safety Cabinet

Appropriate for: BSL2 and BSL3 work with Risk Group 2 and Risk Group 3 organisms, and with tissue culture work requiring sterile conditions.

Protection provided: Personnel (worker), product, and environmental protection. This is why Class II is the standard for the vast majority of microbiology and diagnostic laboratories.

Types of Class II cabinets:

There are four types of Class II cabinets, distinguished by how much air recirculates within the cabinet versus how much is exhausted to the room or outside:

• Type A1 (formerly A1): 70% recirculated, 30% exhausted; exhaust can go to the room or through a thimble connection. Lower cost, suitable for many BSL2 applications.
• Type A2: 70% recirculated, 30% exhausted; exhaust can go to the room or through a thimble connection. About 90% of Class II cabinets installed worldwide are Type A2. This is the workhorse of diagnostic and research labs.
• Type B1: 30% recirculated, 70% exhausted; requires hard-duct external exhaust (connection to building exhaust, not to the room). Used when volatile chemicals or more robust containment is needed.
• Type B2 (Total Exhaust): 0% recirculated, 100% exhausted; requires hard-duct external exhaust. Provides maximum containment of volatile agents; less common due to cost and energy use.

For most BSL2 and BSL3 work, Type A2 is sufficient and is the standard choice.

Working mechanism:

An open front with inward airflow protects the worker from aerosols. HEPA-filtered air flows downward over the work surface, providing product protection (keeping cultures sterile and preventing cross-contamination). The combination of inward airflow, downward flow, and HEPA filtration on exhaust provides the three-way protection that makes Class II the standard for work with pathogenic organisms.

Class III Biological Safety Cabinet

Appropriate for: BSL3 and BSL4 work with Risk Group 3 and Risk Group 4 organisms. Class III is the highest level of containment in a conventional laboratory.

Protection provided: Maximum personnel, product, and environmental protection. This cabinet is designed for work with the most dangerous organisms on Earth.

Working mechanism:

The cabinet is totally enclosed and maintains negative pressure (the interior is under lower pressure than the room, so any leak is inward, not outward). The operator accesses the work surface only through heavy-duty rubber gloves that are integral to the cabinet enclosure. Supply air is HEPA-filtered; exhaust air passes through two HEPA filters before discharge to the atmosphere through a dedicated external exhaust system.

The cabinet is tested under pressure to verify that no particles can escape into the room. A Class III cabinet is often connected to a pass-through sterilization chamber or double-door autoclave, so all materials entering and exiting the cabinet can be decontaminated without exposing the laboratory environment.

When Class III is used: A researcher working with Ebola virus or another RG4 agent does so exclusively in a Class III cabinet in a BSL4 laboratory. This is not a choice or a preference; it is a regulatory requirement. There is no alternative.

Technical specifications of BSC classes

Key reading note: Higher exhaust percentage (B1, B2, B2) means more air leaves the cabinet rather than recirculating. This is chosen when volatile chemical fumes or maximum containment is needed, but it uses more energy. For most BSL2/BSL3 work, Type A2 is sufficient and preferred.

Certification and maintenance

A BSC is not a "set it and forget it" tool. Like any engineering control, it requires verification and maintenance:

• Initial certification: Before a new cabinet is put into service, a qualified technician must certify it, verifying airflow patterns, HEPA filter integrity, and containment of contaminated air. This is a regulatory requirement.
• Recertification: Most regulations require recertification every 12 months. Some jurisdictions require it every 6 months for heavily used cabinets.
• Filter replacement: HEPA filters have a lifespan and must be replaced when they reach saturation or if airflow becomes compromised. A clogged filter reduces protection.
• Sash operation: If a Class II cabinet's sash is stuck or moving irregularly, the airflow pattern is disrupted, and protection is compromised. Have it serviced before using the cabinet.

Using a cabinet that has not been recently certified is a containment failure and a regulatory violation. Know the certification status of your cabinet before you work in it.

Important: Do not confuse BSC with laminar airflow cabinets

A biological safety cabinet and a laminar airflow cabinet are NOT interchangeable. They look similar and both have filtration, but they protect different things:

• A BSC protects the worker and the environment from biohazards. Inward airflow protects the worker; outward exhaust protects the environment.
• A laminar airflow cabinet protects the product from contamination. Outward airflow (toward the worker) provides no worker protection.

Using a laminar airflow cabinet for work with pathogens is a serious safety violation. For the full comparison, see Laminar Airflow Cabinet: Types and Working Principle.

How to Remember

• Class I = worker and environment only. Class II = worker, environment, and product. Class III = fortress. The more letters and protection targets, the more comprehensive the containment.
• Inward face flow = worker protected. Downward laminar flow = product protected. Both + exhaust HEPA = environment protected. Three problems, three solutions.
• Type A2 is the workhorse. 90% of BSCs are Type A2. If you're not sure which type to use at BSL2, it's probably A2.
• Class choice is not optional. RG2 work at BSL2 requires Class II. Not "we'll try Class I and see if it works." The class is determined by the risk group, not by availability.
• Certification is not a one-time event. A cabinet without current certification is not providing protection, even if it looks fine. Know when your cabinet was last certified before you use it.

Key exam facts in one table

Bottom line: Let the risk group decide. RG2 → Class II. RG3 → Class II or III. RG4 → Class III only.

Where Students Get Confused

• Thinking Class I is "simpler" or "cheaper" so it's okay to use for BSL2 work. Class I is not appropriate for BSL2. Using Class I for RG2 organisms is a regulatory violation and a containment failure. The organism protection it lacks is not a luxury; it's a requirement. Cost or convenience does not override this.
• Confusing Class II BSC with laminar airflow cabinets. They look similar but are opposite. A Class II has inward airflow (away from worker) and protects the worker. A laminar airflow cabinet has air flowing toward the worker and provides no worker protection. Never confuse them. See Laminar Airflow Cabinet: Types and Working Principle for the distinction.
• Assuming "Type A2 is most common" means "Type A2 is right for every BSL2 lab." Type A2 is the standard, but Type B1 and B2 are needed in certain circumstances (volatile chemicals, maximum containment, specific institutional requirements). Check your lab's SOP, not just the common default.
• Thinking certification is a one-time event. Certification must be recurrent (usually annually). A cabinet that hasn't been certified in two years is not providing verified protection, even if it looks fine. Know the certification date before you work.
• Believing you can work in a Class II with the sash fully open. Class II cabinets have a recommended sash height (usually 8–12 inches). Working with the sash wide open disrupts the airflow pattern and compromises protection. Keep the sash at the appropriate height.
• Not recognizing that RG4 work has only one option. If your organism is RG4, you use Class III. There is no "upgrading to a higher-end Class II" that will work. The choice has been made for you by biology.

References and further reading

1. U.S. Department of Health and Human Services, CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). 6th ed. Atlanta, GA: CDC; 2020.
2. World Health Organization. Laboratory Biosafety Manual. 4th ed. Geneva: WHO; 2020.
3. Richmond JY, McKinney RW. Primary Containment for Biohazards: Selection, Installation and Use of Biological Safety Cabinets. 2nd ed. Washington, DC: U.S. Department of Health and Human Services, CDC, NIH; 2000.
4. American Biological Safety Association (ABSA). Biological Safety Cabinets: Selection, Use and Maintenance. Mundelein, IL: ABSA;