What Is a Class Two Biosafety Cabinet and How Air Is FilteredA Class II biosafety cabinet is a microbiological safety cabinet that protects the operator, the sample and the environment at once, by combining an inward flow of room air at the front with a HEPA-filtered, near-laminar downflow of clean air over the work surface. It is the most common cabinet in clinical, microbiology, virology and cell-culture laboratories because it delivers personnel protection and an aseptic, ISO Class 5 work zone in a single enclosure. A sibling article covers the biosafety cabinet classes in general; this one stays on how a Class II cabinet filters air and how to choose between its types.The engineering rests on splitting and filtering airflow precisely. Understanding that split is what lets a buyer pick the right type, size the exhaust and avoid a cabinet that either fails containment or burns energy needlessly.How the Airflow and Filtration WorkTwo air streams define a Class II cabinet. Room air is pulled in through the front grille at an average inflow velocity typically around 0.5 m/s, forming an air barrier that stops aerosols reaching the operator and immediately capturing anything generated near the opening. Separately, a blower drives air down through a supply HEPA filter as a smooth vertical downflow, usually in the region of 0.25 to 0.5 m/s, bathing the work surface in particle-free air so the sample is protected from contamination.The captured inflow and the used downflow meet at slots in the work surface and are drawn into a plenum. From there the cabinet recirculates a proportion back through the supply HEPA and exhausts the remainder through an exhaust HEPA filter. Both filters are typically H14 grade to BS EN 1822, retaining at least 99.995% of the most penetrating particle size. A critical safety feature in Type A cabinets is that contaminated plenums are held under negative pressure or surrounded by negative-pressure ducts, so any leak draws inward rather than out.The Class II Types and How They DifferTypes differ chiefly in the split between recirculated and exhausted air, the inflow velocity, and whether the cabinet is ducted. This is the selection decision that matters most.Type A1Recirculates roughly 70% and exhausts 30%, with a lower minimum inflow. It may exhaust back into the room and is not suitable for work involving volatile or toxic chemicals.Type A2The most widely specified type: about 70% recirculated, 30% exhausted, with a minimum inflow generally around 0.5 m/s and negative-pressure surrounding all contaminated plenums. It can exhaust to the room or, via a canopy (thimble) connection, to a duct, and tolerates only minute traces of volatile chemicals.Type B1Hard-ducted, exhausting around 70% and recirculating 30%, with higher inflow. The ducted exhaust allows limited work with volatile chemicals and radionuclides, provided they are used at the rear of the work zone.Type B2 (total exhaust)Recirculates nothing: 100% of the air is exhausted through a dedicated duct to atmosphere. This suits volatile toxic chemicals and radionuclides but carries the highest airflow, ducting and energy demands of the group.Choosing the Right TypeThe decision tree is short. If no volatile chemicals are used, a Type A2 (recirculating or thimble-connected) covers most clinical and cell-culture work economically. If small amounts of volatile chemicals are involved, a hard-ducted B1 or thimble-connected A2 may suit. If significant volatile toxic chemicals or radionuclides are handled, a total-exhaust B2 is required. Never assume a canopy connection turns an A2 into a chemical cabinet: it does not increase the volume of volatiles that can be handled safely, only where the exhaust goes.Verifying Inflow and DownflowThe two velocities that define a Class II cabinet are measured directly at commissioning and re-certification. Downflow is checked with an anemometer on a grid across the work surface to confirm both the average and the uniformity, while inflow is derived from the exhaust volume and aperture area or measured at the face. Both must sit within the manufacturer's and standard's tolerances: a downflow that is too strong can splash the work surface, and a too-weak inflow loses operator protection. Airflow smoke-pattern tests then visualise that air actually moves downward and inward as intended, with no dead spots or reverse flow at the sash line.UV Lamps, Decontamination and Filter ChangesMany Class II cabinets include a UV-C lamp for surface decontamination between sessions, though it supplements rather than replaces chemical wipe-down, since UV only reaches exposed surfaces and lamp output falls with age. Before a filter change, relocation or repair, the cabinet must be gas-decontaminated, commonly with vaporised hydrogen peroxide, because the HEPA filters hold viable material. Bag-in/bag-out filter housings allow safe removal of loaded filters for higher-risk agents. Confirm which decontamination method the cabinet supports and that your service provider can perform it, as this affects both safety and downtime.Sash Height and Safe WorkingA Class II cabinet contains at a defined working sash height, usually marked on the front, and protection is only assured at or below that opening. Working with the sash too high raises the aperture, weakens the inflow barrier and can trip the airflow alarm. With a Class II cabinet specifically, keep work at least 100–150 mm behind the front air grille, never rest items on the intake slots, and allow the downflow and inflow to stabilise for a few minutes after switch-on before starting. These behaviours matter as much as the hardware, so staff training should be specified alongside the cabinet.Standards and CertificationIn the UK and Europe, Class II cabinets are performance-tested to BS EN 12469:2000, while many manufacturers also build to the North American NSF/ANSI 49, which formally defines the A1, A2, B1 and B2 types. Cabinets must be validated on installation and re-certified at least annually, testing HEPA integrity by DOP/PAO challenge, inflow and downflow velocities, and airflow smoke patterns. Devices for clinical use must carry appropriate UKCA or CE marking; regulatory status can be confirmed through the MHRA, and siting and safe-use guidance is available from the HSE. Ask for type-test certification against the named standard and a written re-certification plan.Procurement ChecklistWhen specifying a Class II cabinet, confirm the following:Type (A1, A2, B1, B2) chosen on whether, and how much, volatile chemical or radionuclide work is involved.Exhaust arrangement agreed with estates: room recirculation, thimble/canopy, or hard duct, with the required fan interlock.Certified inflow and downflow velocities and evidence of type testing to BS EN 12469 or NSF/ANSI 49.HEPA grade and filter access, with bag-in/bag-out changing where higher-risk agents are handled.Airflow monitoring and alarms for low inflow, low downflow, sash position and filter loading.Working aperture, internal height and services matched to your tasks and pipette or microscope use.Installation validation and annual re-certification included, with calibration traceability.Siting, Energy and Running CostsA Class II cabinet is only as good as its installation. Draughts from doors, corridors and ceiling diffusers disturb the inflow air barrier, so keep the cabinet away from traffic and supply-air outlets. Total-exhaust B2 units consume the most energy because they discharge all conditioned air, so where the work allows, an A2 is markedly cheaper to run. Recurring costs include HEPA replacement driven by loading, UV lamp changes where fitted, and annual certification. Standardising on one type and manufacturer across the laboratory reduces spares and simplifies staff training.ConclusionA Class II cabinet protects operator, sample and environment by pairing an inward air barrier with HEPA-filtered downflow, then splitting the air between recirculation and HEPA-filtered exhaust. Choose the type on your chemical and radionuclide use, confirm certification to BS EN 12469 or NSF/ANSI 49, and plan siting and ducting before you order. To compare Type A2, B1 and B2 cabinets or arrange validation, talk to the MediGear team or register your requirements through our buyer services.DisclaimerThis article is for informational purposes only. It is published by MediGear (medigear.uk) for general information and procurement guidance, and is not clinical, diagnostic, treatment, technical, engineering, legal or regulatory advice, nor a product endorsement, guarantee or substitute for professional assessment. MediGear does not provide medical consultations. Buyers should consult their clinical, biomedical, estates and regulatory contacts, and the manufacturer's documentation, and independently verify all specifications, certifications, compatibility and suitability before purchase. Specifications, certifications and availability are correct at the time of publication and may change without notice. MediGear is a medical-equipment distributor and does not sell medicines or pharmaceutical products.
