Biosafety cabinets (BSCs) come in three classes. Class I protects the worker and the environment but not the product, Class II protects the worker, the product and the environment, and Class III is a sealed, gas-tight glove box for maximum containment. A laminar flow clean bench is not a biosafety cabinet at all.
Choosing the wrong cabinet, or using the right one badly, is a common audit finding in microbiology labs. Cabinets are expensive, hard to move once ducted, and only as good as the airflow they deliver on the day.
Every BSC relies on directional airflow and HEPA filtration. Air drawn in at the front forms a barrier that keeps aerosols away from the operator, and HEPA filters clean the exhaust. Class II cabinets add HEPA-filtered downflow over the work surface to protect the product.
Class I biosafety cabinets
A Class I cabinet draws unfiltered room air in through the front, across the work surface and out through a HEPA exhaust filter. The operator and environment are protected, but room air passes over the work, so there is no product protection.
Class I cabinets are simple and robust, suiting work where product contamination does not matter, such as enclosing aerosol-generating equipment.
Class II biosafety cabinets
Class II cabinets are the workhorse of clinical, food and pharmaceutical microbiology. Air enters at the front grille, HEPA-filtered air flows down over the work, and the mixed air is partly recirculated through the supply HEPA filter and partly exhausted through the exhaust HEPA filter.
NSF/ANSI 49 divides Class II cabinets into types by inflow velocity, recirculation and exhaust:
- Type A1: mostly recirculated air, a lower minimum inflow, and exhaust to the room or through a canopy connection. Not for volatile toxic chemicals.
- Type A2: the most widely installed type. Higher minimum inflow and contaminated plenums under, or surrounded by, negative pressure. A canopy connection can allow minute quantities of volatile chemicals if the risk assessment supports it.
- Type B1: hard-ducted, with most contaminated downflow air exhausted rather than recirculated.
- Type B2: total exhaust through a hard-ducted building system, with no recirculation. Used for biological work with volatile toxic chemicals or radionuclides, and dependent on reliable building exhaust with alarms.
Newer editions of NSF/ANSI 49 also describe a Type C1. The European standard EN 12469 sets performance criteria for Class I, II and III microbiological safety cabinets without the A and B subtypes, so state in your purchase order which standard the cabinet must meet.
Class III biosafety cabinets
A Class III cabinet is a fully enclosed, gas-tight enclosure. The operator works through attached heavy-duty gloves, the cabinet is kept under negative pressure, supply air is HEPA filtered, and exhaust passes through two HEPA filters in series or a HEPA filter and an incinerator. Materials enter and leave through a pass box, a dunk tank or a double-door autoclave.
Class III cabinets are used for the highest-risk agents, usually in maximum containment facilities.
Biosafety cabinet classes compared
| Class / type | Personnel | Product | Environment | Airflow and exhaust | Typical use |
|---|---|---|---|---|---|
| Class I | Yes | No | Yes | Unfiltered inflow over work, HEPA exhaust | Enclosing aerosol-generating equipment |
| Class II A1 | Yes | Yes | Yes | Mostly recirculated, exhaust to room or canopy | Routine microbiology without volatile chemicals |
| Class II A2 | Yes | Yes | Yes | Higher inflow, mostly recirculated, room or canopy exhaust | Most clinical, food and pharma microbiology |
| Class II B1 | Yes | Yes | Yes | Hard-ducted, most air exhausted | Biological work with small amounts of volatile chemicals |
| Class II B2 | Yes | Yes | Yes | Hard-ducted, total exhaust | Biological work with volatile toxic chemicals or radionuclides |
| Class III | Yes (maximum) | Yes | Yes | Gas-tight, negative pressure, double HEPA exhaust | Highest-risk agents in maximum containment |
| Laminar flow clean bench | No | Yes | No | HEPA air blown towards the operator or down onto the work | Sterile media preparation, non-hazardous work only |
BSC vs laminar flow hood
A horizontal or vertical laminar flow clean bench blows HEPA-filtered air over the work and out towards the operator. It protects the product from contamination but pushes any aerosol straight at the person working. Never use a clean bench for infectious material, clinical specimens or cultures of pathogens. Label clean benches clearly so new staff do not mistake them for cabinets.
Certification and correct use
Field testing
Test the cabinet on installation, after relocation, filter changes or major repairs, and at regular intervals, commonly at least annually or as local requirements and the manufacturer specify. Field tests typically cover inflow and downflow velocity, HEPA filter integrity, smoke patterns and alarms. Label the cabinet with its next due date and stop using it if it fails.
Working safely in the cabinet
- Switch the cabinet on and let it purge for the time stated by the manufacturer before starting.
- Never block the front or rear grilles with paperwork, pipette boxes or arms.
- Work well inside the cabinet, keeping clean items and waste separated across the work surface.
- Move arms slowly and minimise traffic behind you, because rapid movement disturbs the air curtain.
- No open flames. Bunsen burners disrupt airflow and can damage the HEPA filters. Use disposable loops or electric micro-incinerators.
- Disinfect surfaces before and after work with a disinfectant effective against your agents. UV lamps are not a substitute for disinfection.
Placement in the room
Site the cabinet away from doors, walkways and air supply diffusers, since cross-draughts break the inward air barrier, and leave the clearance the manufacturer specifies.
Worked example: choosing a cabinet for a TB lab in Abu Dhabi
A hospital laboratory in Abu Dhabi is expanding its tuberculosis service from smear microscopy and cartridge-based molecular testing to liquid culture and drug susceptibility testing. The biosafety officer runs a risk assessment using the WHO Laboratory Biosafety Manual 4th edition (2020) and WHO TB laboratory biosafety guidance.
Culture work manipulates concentrated Mycobacterium tuberculosis suspensions that can generate infectious aerosols, so all open manipulations must take place in a BSC, in a room with controlled access, directional airflow and respiratory protection.
The team weighs the options:
- Class II A2 with canopy connection: protects staff, cultures and the environment without the tight exhaust balancing of a hard-ducted cabinet. Selected.
- Class II B2: no volatile chemicals are used, so total exhaust adds HVAC complexity and failure risk for no benefit.
- Class I: no product protection, so long incubations risk contamination.
- Class III: not required by the risk assessment.
The cabinet is certified on installation and placed away from the door. Staff are trained and observed before working alone, and the lab confirms Department of Health Abu Dhabi requirements before going live.
Common mistakes
- Using a laminar flow clean bench for cultures or patient samples.
- Assuming a certificate from last year still proves today's performance after the cabinet has been moved.
- Blocking the front grille with forearms or paperwork.
- Relying on UV lights instead of surface disinfection.
- Buying a Class II B2 cabinet without the building exhaust to support it.
- Installing cabinets beside doors or supply vents.
Key takeaway: Choose the cabinet class from a risk assessment of the actual work, not the agent name alone. Class II A2 suits most microbiology, a clean bench is never a substitute, and certification and disciplined technique matter as much as the cabinet itself.
Frequently Asked Questions
What is the difference between Class I, II and III biosafety cabinets?
Class I protects the operator and the environment but not the product. Class II protects the operator, product and environment using inward airflow, HEPA-filtered downflow and HEPA-filtered exhaust. Class III is a gas-tight glove box under negative pressure for maximum containment of the highest-risk agents.
Is a laminar flow hood the same as a biosafety cabinet?
No. A laminar flow clean bench blows HEPA-filtered air over the work and towards the operator, so it protects only the product. It gives no protection to the worker or the environment and must never be used for infectious material, clinical specimens or pathogen cultures.
Which Class II type is most common?
Type A2 is the most widely installed Class II cabinet in clinical, food and pharmaceutical microbiology labs. It protects personnel, product and environment, can exhaust to the room or through a canopy connection, and is simpler to install than hard-ducted Type B cabinets.
How often should a biosafety cabinet be certified?
Certify a cabinet on installation, after it is moved, after filter changes or major repairs, and at regular intervals, commonly at least annually. Follow the manufacturer's instructions and the requirements of your accreditation body and local regulator, which may be stricter. Keep each certificate and test report on file for audits.
Next steps
To build practical skills in risk assessment, containment and safe cabinet use, explore our Laboratory Biosafety and Biosecurity Essentials course.
For related guides, read laboratory biosafety vs biosecurity and biological spill response in microbiology labs. Questions about the right course? Speak to a counsellor.
Looking for structured certification?
Explore Innoryx courses or speak to a counsellor about the right program for you.