Part 2 – Biosafety cabinets and HEPA filters

Now, let’s turn our attention to the biological safety cabinets. We will spend the rest of the module just looking at those. They are considered primary containment because they contain biological hazards. They provide protection in a variety of different ways using directional air flow that protects the personnel because they draw air away from you. They provided environmental protection because they filter the air before it’s exhausted. They provide product protection because they filter the air that’s pushed back down to the work surfaces. A biosafety cabinet can therefore provide personnel, environmental and product protection, depending on the class of biosafety cabinet you’re working with.  

Biosafety cabinets are designed to protect us from infectious aerosols in the laboratory. Aerosols are a suspension of solid or liquid particles in a gas. Good laboratory practices should be used to try and prevent their creation. However, this cannot always be avoided and therefore we need to use engineering controls, like the biosafety cabinet to protect ourselves.

There are three classes of biosafety cabinets – class I, class II and class III. Class I only protects the worker and the environment. Class II cabinets are the most popular and widely used. They protect the worker, the product and the environment. Class III, are totally enclosed cabinets that have gloves built into the front for manipulation of the product inside.

As you know, from previous modules, many of the normal activities that we do in the laboratory generate aerosols. Things like, sonication, blending, mixing or vortexing all create aerosols. Almost any kind of liquid manipulation, like centrifuging, pouring plates, pipetting, loading syringes, giving injections or even opening containers can create micro splashes and small aerosols that are released into the air that we breathe. If you do these activities inside the biosafety cabinet while it’s running, you will be protected from the aerosols and may have been created.

To learn more, watch this short movie about aerosols in the laboratory.

Video

The reason BSC can protect you, the product and the environment from infectious aerosols, is because they move the air through high efficiency particulate air (HEPA) filter. They are made of pleated borosilica glass fibers held together with aluminum separators. They typically provide 99.97% filtration of airborne particles at 0.3 microns.  In the middle picture, you can see a filter that’s probably about four to five inches thick with an aluminum frame around it. On the left-hand side, you can see a biosafety cabinet that’s been opened up. In that picture, you can see the blower motor in the top right-hand side and the two (supply and exhaust) wood-framed HEPA filters. On the right-hand side you see a closeup of the borosilica pleated fibers of the filter. It’s important to recognize that the filter is fairly thick which creates a large surface area through which the air has to flow.

In addition to HEPA filters, some companies are now offering ultra-low protection and super ultra-low protection filters. The ultralow protection air (ULPA) filters remove 99.99% of particles at 12 microns. In addition, there are super ultra-low protection air (SULPA) filters which offer filtration at 99.9999% efficiency. Personally, I don’t think you need these in the microbiology laboratory. 99.97% filtration efficacy is just fine for the work that we’re doing in most microbiology laboratories. So, I would not recommend you spend the extra money for the ultra-low or super ultra-low protection filters.

Here you see some particle size ranges to give you some idea of where 0.3 microns is, as shown in red in the scale on the bottom. So, 0.3 microns is somewhere between a typical virus and bacteria in size. Many viruses are smaller than 0.3 microns and most bacteria are typically larger. You can also see that fungi, droplet nuclei and aerosolize bacteria are also large than 0.3 microns.

So how do HEPA filters operate? Basically, they have three different methodologies – impaction, interception and diffusion. These methods provide filtration of 99.97efficiency at 0.3 microns, but particles that are larger or smaller are filtered even more effectively.

Let’s look at each filtration method separately. First, inertial impaction. Here you see the particles flying in from the left-hand side and as they pass through the filter, they hit the fibers that are within these borosilicate pleats and they’re getting trapped.

Next is interception, where the particles are flying in and then by electrostatic attractions are attracted to the pleats of borosilicate and trapped.

Finally diffusion. Some particles are very, very small and are moving in Brownian or random fashion through the air. You can see them drifting around here. Some of them get trapped by inertial impaction, some of them get trapped by interception and some of them escape. That’s the small percentage (0.03%) that get released and back into the air.

In this graph, you see the overall efficiency of a HEPA filter, as shown by the red line. This comes from a combination of the three methods of filtration – the inertial impaction shown in yellow, the interception shown in green and the diffusion in blue. As you can see, the overall efficiency is lowest at 0.3 microns giving a 99.97% efficiency. When the particle sizes are smaller, like we see with most viruses, we get even greater efficiency. Also, larger particles, like anthrax spores, are also filtered out with greater efficiency than 99.97%.

Now I want you to watch this video, that’s going to show you these same filtration principles. In this video they’re filtering out smoke particles, but the it’s the same for infectious particles. So please watch this video.

Video

I want to remind you again that HEPA filters will not filter out gases and vapors. They’re meant to filter out particular materials and infectious aerosols. If you have volatile organic chemicals in the air, smells, or other gases, those will go straight through the HEPA filters and potentially back into your breathing zone. If you have these volatile organic chemicals and you need to work with them, you should be working with those in the fume cabinet or Class II B or C biosafety cabinet.