Why Work Inside a Box With the Front Open? Air Moving the Right Way

The enclosure is not sealed and does not need to be, because the protection comes from moving air rather than from a barrier. A fan draws air out of the top of the cabinet and exhausts it above the building, which means air is continuously drawn inwards across the open front at a controlled speed, typically around half a metre per second. Anything released inside is carried away from the worker and up the duct rather than escaping into the room. The sash at the front can be raised and lowered, and lowering it both reduces the opening, which raises the inward speed for the same fan, and puts a physical screen between the worker and anything that might splash or explode.

Almost every failure is a matter of use rather than equipment:

• •Raising the sash too high, which drops the inward speed below what is needed
• •Putting large objects inside, which disturb the flow and create eddies
• •Working too close to the opening rather than well inside
• •Rapid movements by the worker, which push air outwards
• •Draughts from doors, windows, walkways and other ventilation
• •Blocking the slots at the back, which are what draw air through evenly

Air is invisible and the flow inside a cabinet is not intuitive, so testing uses visible tracers and the results are frequently surprising. Smoke released at the opening should be drawn straight in, and testing commonly shows it rolling outwards at the sides or over the top of a large object placed inside. A standing worker creates a wake in front of them in which air moves back towards them, which is why the position of the hands and the body matters. Formal testing measures the face velocity at many points across the opening and releases a tracer gas while a mannequin stands at the front with a detector, which quantifies how much escapes under realistic conditions.

Several enclosures that look similar work in the opposite direction and confusing them is dangerous. A laminar flow cabinet blows filtered air outwards over the work, which protects the material from contamination and protects the worker from nothing at all, and it is used for sterile preparation and cell culture. A biological safety cabinet of the commonest class draws air inwards for worker protection and passes it through a filter before recirculating, and filters the exhaust as well, which protects the worker, the material and the environment together. A glove box seals the interior entirely and is entered through sealed gloves, which suits work requiring an atmosphere without oxygen or moisture. Choosing the wrong one for a task fails in exactly the direction that matters.

The equipment protects the worker and does several things it is frequently assumed to do and does not. It does not protect the material inside from the worker, since air flows inwards carrying dust and skin, so sterile work requires the opposite arrangement in which filtered air flows outwards. It does not contain an explosion, though a lowered sash limits the consequences. It does not clean the exhaust in most installations, which discharges to the outside air, so what is released still reaches the environment unless a treatment system is fitted. And it does not substitute for handling less material, since the safest response to a hazardous procedure is usually to do it at a smaller scale.

Protection comes from air drawn inwards across the open front at a controlled speed and exhausted above the building, not from any barrier. Raising the sash, placing large objects inside, working near the opening and moving quickly all defeat it, and smoke testing routinely shows air rolling outwards where nobody expects. It protects the worker rather than the material, and most installations discharge untreated to the outside air.