What is a biosafety cabinet
A biosafety cabinet (BSC) is a ventilated laboratory workspace that uses high-efficiency particulate air (HEPA) filtration and controlled airflow to protect people, products, and the environment from dangerous microbes. It is essentially an enclosed workspace where air is drawn through filters to trap pathogens before exhaust. All work with infectious materials should take place inside a BSC to prevent the release of contaminants.
In practice, a BSC provides three layers of protection: it shields the laboratory worker, it shields the outside environment, and in most classes, it also shields the samples or products being handled. These cabinets became commercially available in the 1950s and are now standard equipment in microbiology and medical labs.
Biosafety cabinet levels
Biosafety cabinets are categorized into three main classes based on the level of protection they offer. These classes are defined by agencies like the U.S. Centers for Disease Control and Prevention (CDC) and standards organizations. The classes differ in how air flows through the cabinet and which kinds of protection they provide.
- Class I cabinets provide protection for the operator and environment, but not for the product (the samples inside). Air is drawn into the cabinet and filtered before exhaust, protecting the user, but unfiltered room air can enter the workspace from above. Class I cabinets use airflow similar to a fume hood and can be either vented or recirculated to the lab after filtering.
- Class II cabinets are the most common type. They protect the operator, the environment, and also the product. They have an open front with inward air flow to protect the user, and a downward HEPA-filtered laminar flow inside the cabinet to protect the samples. Exhaust air is also HEPA-filtered. Class II cabinets come in several types (A1, A2, B1, B2, C1) that differ in airflow patterns and exhaust methods.
- Class III cabinets are the highest level of containment. These are completely enclosed, air-tight "glove boxes" with sealed gloves attached to the front. All materials enter and leave through special airlocks that can be decontaminated. The cabinet is kept under negative pressure so that air only flows in, and all exhaust is double-filtered or incinerated. Class III cabinets are used for the most dangerous pathogens.
Each class of BSC is safe for use with different biosafety levels (BSL) of agents. Classes I and II are suitable for work with low to moderate risk agents (up to BSL-3), while Class III is designed for the highest level (BSL-4) pathogens.
Class I biosafety cabinet
A Class I biosafety cabinet is the simplest design. It is essentially a ventilated enclosure for operator and environmental protection. Key points about Class I cabinets:
Protection: They protect the lab worker and the environment from aerosols, but they do not protect the product (the samples) from outside contamination. Because room air flows into the cabinet, samples inside can be exposed to contaminants.
Airflow: Air is drawn in through the front opening, passes through the work area, and then goes through a HEPA filter before exiting. This creates a similar airflow pattern to a chemical fume hood but with HEPA filtration on the exhaust. The airflow is not recirculated back into the work area, so all air from the cabinet is filtered and exhausted.
Applications: Class I cabinets are used for low to moderate risk biological agents (Biosafety Level 1–3). They are commonly used to enclose equipment or procedures that generate aerosols, such as centrifuges, but they are not suitable when product sterility is needed. In modern labs, Class I cabinets are considered fairly basic and are less common than Class II.
Class II biosafety cabinet
A Class II biosafety cabinet is designed to protect the user, the samples, and the environment all at once. Class II cabinets have a front opening with inward airflow and a downward laminar flow of filtered air over the work surface. They provide:
- Personnel protection: Inward airflow at the front opening keeps contaminants from escaping the work area.
- Product protection: Downward, HEPA-filtered laminar airflow over the work surface protects samples inside the cabinet from contamination by the user or outside environment.
- Environmental protection: All exhaust air is HEPA-filtered.
Class II cabinets are used for work with pathogens at BSL-1, BSL-2, and BSL-3. There are five types of Class II cabinets (Type A1, A2, B1, B2, and C1). The types differ in airflow patterns, recirculation, and how exhaust is handled.
| Cabinet Type | Airflow | Exhaust | Uses |
|---|---|---|---|
| Class II Type A1 | Inflow from the front at ~75 fpm; downward HEPA-filtered airflow; ~70% recirculated inside | HEPA-filtered air can be vented back into the room or outside via canopy |
For biological agents only; not suitable for volatile chemicals or radionuclides due to air recirculation |
| Class II Type A2 | Similar to A1 but with higher inflow (~100 fpm); ~70% recirculated | Can exhaust back to room or outside through a canopy | Safer than A1; handles small amounts of non-volatile chemicals if vented properly; not for large volumes of toxic gases |
| Class II Type B1 | Inflow ~100 fpm; downflow is mostly recirculated, but rear airflow is exhausted | ~30% of air (rear downflow) is HEPA-filtered and hard-ducted out | Suitable for trace amounts of toxic chemicals or radionuclides in rear work zone; product protection is maintained |
| Class II Type B2 | Inflow ~100 fpm; no recirculation at all; 100% HEPA-filtered downflow | Total exhaust via hard duct; no return to room | Ideal for hazardous chemicals or radionuclides; provides maximum containment and sterility |
| Class II Type C1 | Dual-mode: can operate as recirculating (like A2) or externally exhausting (like B1); marked zones for each | ~60% of airflow exhausted; mode can be changed with duct adjustment and recertification | Flexible use; switchable between A2 mode (general work) and B1 mode (small chemical handling); suitable for versatile lab needs |
Class III biosafety cabinet
A Class III biosafety cabinet is the highest-containment cabinet, often called a "glove box." Key features include:
- Fully sealed enclosure: The cabinet is gas-tight and totally enclosed. Operators reach inside through heavy rubber gloves attached to ports in the front of the cabinet. This makes it impossible for air or microbes to escape into the room.
- Material transfer: There are special pass-through chambers (sometimes using a dunk tank or double-door autoclave) so that materials can be sterilized when entering or leaving the cabinet.
- Airflow: The cabinet is kept under negative pressure. All incoming air goes through HEPA filters, and all exhaust air is double-filtered or incinerated before release. Even equipment placed inside must be specially designed to fit through the sealed environment.
- Uses: Class III cabinets provide maximum protection for the operator, the sample, and the environment. They are used for the most dangerous pathogens (Biosafety Level 4). Any work with highly infectious or lethal agents is done in Class III cabinets.
In summary, Class III BSCs isolate the work inside a sealed box, whereas Class I and II allow an open work area (with filtered airflow). Class III is the only type that completely prevents any direct contact between the user and the material without specialized gloves and transfers.
What are the differences among the three classes
Here are the key differences between Class I, II, and III biosafety cabinets:
- Protection provided: Class I cabinets protect the user and the laboratory environment but do not protect the product. Class II cabinets protect the user, the product (samples), and the environment all at once. Class III cabinets protect everything inside the cabinet because they are fully enclosed.
- Airflow and enclosure: Class I and most Class II models have an open front with inward airflow at the sash. They use HEPA filters on the exhaust. Class III cabinets are sealed with attached gloves, with no open access; they use double HEPA filters and incineration on exhaust.
- Recirculation of air: In Class I there is no recirculation (all air is HEPA-filtered and exhausted). In Class II, some types (A1/A2/B1/C1 in A-mode) recirculate a portion of the air after filtering. Type B2 and Class III do not recirculate any air.
- Chemical compatibility: Class I and standard Class II A1/A2 are not suitable for volatile chemicals, because some or all air is recirculated. Class II B1, B2, and C1 can handle small quantities of chemicals because they exhaust contaminated air outside. Class III can also handle chemicals (in its own sealed system) with appropriate exhaust treatment.
- Biosafety level (BSL) use: Class I and II cabinets are typically used for work requiring BSL-1, BSL-2, or BSL-3 containment. Class III cabinets are required for BSL-4 work with the most dangerous pathogens.
Conclusion
Biosafety cabinets are essential tools in laboratories for protecting people and the environment when handling infectious agents. The three classes of cabinets-Class I, Class II, and Class III-differ in design and in the levels of protection they offer. Class I is the most basic type, providing user and environmental safety but not sterility of the sample. Class II is the most common type, combining protection for the user, the product, and the environment; it comes in multiple types (A1, A2, B1, B2, C1) to suit different applications. Class III offers the highest containment, isolating the work in a sealed, glove-accessed chamber for maximum safety.
When choosing a cabinet, one should consider the risk level of the biological agents and whether hazardous chemicals are involved. For routine microbiology (up to BSL-3), a Class II BSC (often Type A2) is usually appropriate. For work with highly dangerous pathogens or large volumes of toxic chemicals, a Class III cabinet or specialized Class II (B2/C1) may be required. By selecting the proper class of biosafety cabinet and following safety procedures, laboratories can effectively prevent exposure to biohazards and ensure safe research.