Biosafety Cabinet Types

Biosafety Cabinets (BSCs) provide protection from airborne contamination by particulate matter. The most important need for protection is the lab technicians operating the BSC. The cabinets prevent viruses, spores, and bacteria from infecting personnel. Not only does this prevent injury or illness among workers, but it reduces potential liability on the part of the organization that owns or runs the lab.

Cabinets protect the work being done. Without the protection provided by Class II technology, multiple samples, processes, or procedures that occur within a cabinet could potentially undergo cross-contamination, undermining the time and energy already invested and disrupting schedules and completion times.

BSCs also help protect the environment of the lab. Potential contaminants are kept within the cabinet and filtered from discharged air, so they do not pose a risk for personnel or a source of contamination for work located elsewhere in the lab.

To summarize, BSCs are a form of safety control and risk management that have become standard in virtually every lab handling biological materials, whether hospitals and other large healthcare providers, testing facilities, pharmaceutical manufacturers, or other organizations that handle the particulate matter of low-to-moderate-risk.

Class II Biosafety Cabinets

BSCs are ingenious in their design. By using relatively simple mechanisms that rely on the physics of fluid dynamics, these devices maintain barriers to contamination while still allowing the physical access that allows people to do their work.

The Physics behind Biosafety Cabinets

BSCs can seem passive because the most important tool of their effectiveness, air, is invisible. To better understand how Class II containment works take a short review of some basic physics.

Fluids that are not under compression move without gaps through a channel or pipe. The fluid does not bunch up in some places or stretch thinner in others, even when the cross-section of the channel changes. The same volume of fluid must flow by each point. When the channel grows larger in cross-section, the fluid slows, because the volume is directly proportional to both that cross-section and the speed at which fluid moves past the point. Similarly, when the cross-section shrinks, fluid moves more quickly, so the same volume per time flows past. In the 18th century, Swiss mathematician and physicist Daniel Bernoulli applied the conservation of energy to fluid flow that led to the Bernoulli Equation and Bernoulli principle: When a fluid flows through a channel, fluid pressure, fluid speed, and channel cross-section are all directly related. So, when the channel size decreases, the fluid speed increases. Increase the channel size and the fluid speed decreases. Increase the speed and leave the channel size the same and the pressure decreases. The principle is what lets the airfoil of a plane create lift on the wing. It also lets a Class II biosafety cabinet operate and protect personnel, product, and the environment.

How a Class II Biosafety Cabinet Works

A BSC is a box with a blower moving air within. A transparent panel on the front allows personnel to see inside. An opening at the bottom of the panel lets technicians reach in and perform work. This is known as the window sash.

As the blower drives air through the box, the BSC becomes a study of fluid dynamics involving the Bernoulli principle. The moving air column lowers the pressure, creating a negative pressure area about air pressure in the lab.

Even as someone works with hands placed through the bottom of the window sash, air flows from the lab environment into the cabinet through that opening and into an air grill at the front of the cabinet. The air supply creates a moving air curtain in front of the work opening. Because of the negative pressure in the cabinet, air does not move outward through the opening into the lab so long as the person moves hands and arms slowly to avoid disruption of the air curtain.

All or part of the air pulled through the cabinet may be vented out after passing through a HEPA filter. Depending on the BSC type and work being done, the filtered air may move into the lab or can be directed through venting to the building’s exhaust system, eventually to be carried outside. Although they can screen particles from the air, the HEPA filters cannot remove volatile toxic chemical vapors or volatile radionuclides that might be generated.

Any air not discharged is recirculated by the blower that pushes it through the supply HEPA filter and into the workspace. Then the air is again drawn out, continuing the cycle.

Protection and Laminar Flow

Contamination of work within a BSC, while rare, can still occur. The concept of laminar and turbulent flows explains the issue and the solution.

When a fluid, like air, flows past, over, or around an object, it can do so in one of two ways. One is in a turbulent flow, where eddies, whorls, swirls, and other disturbances tend to mix the fluid. If the air movement in the cabinet were a turbulent flow, airborne particulate matter or vapor could move between parts of the work area and contaminate other samples or processes.

The second-way air can move around an object is called laminar flow. A laminar flow reduces to a minimum both turbulence and the potential cross-contamination it can cause as the air moves smoothly past objects in their path.

BSCs are designed to create and maintain laminar flow within the cabinet. The air moves smoothly around objects in the work area, minimizing the turbulence and the chance of cross-contamination. Any airborne contaminants move through the cabinet and are eventually trapped by a filter, whether the air is vented out or recycled to the work area.

To keep laminar flow intact, personnel must move their hands and arms slowly and carefully in the work area. A sudden movement can create turbulence, causing cross-contamination or disruption of the air barrier that prevents contamination of the lab environment.

Class II Biosafety Cabinet Types

There are five types of Class II BSCs: A1, A2, B1, B2, and C1. Here is a brief explanation of each: