As with any experiment, choosing the right personal protective equipment is essential. In this series we’ll take a look at what different types of hoods add to your arsenal of PPE, what they do and how you can benefit by using them.

First, why do I even need a hood?

It’s common to run a variety of assays at an open bench. But, when you begin to work with microorganisms, vectors, shuttles for RNAi, or others that are classified with health hazards similar in scope to an assortment of everyday chemical reagents, (not to mention organisms that are infectious enough to spread from person to person), you need the added protection that only a specific type of hood can give!

Simply put, a laboratory hood helps protect you from being exposed to bad bugs by containing them behind a forcefield of air – and it helps keep those bad bugs, and other harmful reagents, from cross contaminating and from cross-reacting with anything else.

The Conventional Fume Hood

One of the the most common hoods you’ll utilise is the Constant Air Volume (CAV), ducted hood, which vents unhealthy air out of the lab through a chimney.

There are quite a few variations on this theme, so here are some of the more popular types you should know about:

Non-bypass Hood

For these hoods the rule of thumb is the lower the sash, the more protection you gain. But, the smaller the opening, the faster the air is pulled into the hood to keep the same volume flowing. If you raise the sash too high the hood isn’t effective; close it entirely and the air flow is so forceful that it can actually cause bad air to spill back out towards you!

Bypass Hood

Like a non-bypass CAV hood but with an additional opening built above the sash. As the sash opening decreases, the bypass opening increases to allow air to also be drawn through the top of the hood to while keeping flow rate constant.

Low Flow/High Performance Bypass Hood

This is a newer style of hood that adds a few modifications like sash stops, horizontal sashes, electronics to adjust the baffles and fans to help wash the air where you breathe.

Reduced Air Volume Hood

This hood utilises a smaller blower, sash stops and bypass blocks to reduce air volume into the hood. It’s an energy saver for the lab, but it also means there’s a smaller range of experimentation you can use it for, compared to conventional fume hoods.

Canopy Fume Hood

The laboratory equivalent of your home’s over-the-stove vent fan: for use only with non-toxic, non-hazardous items that evolve inert vapors, steam or smoke.

Specialized hoods

For more specific applications you may see:

  • Acid digestion hoods that are built with polypropylene and polycarbonate instead of only epoxy and metal, for greater acid resistance
  • Downflow hoods that suck air through the workbench, saving you from heavier-than-air gases.
  • Radioisotope hoods lined with stainless steel and built strong enough for your lead shields.
  • Waterwash hoods that help keep you safe by spraying away any leftover residues on the walls and bench.

It’s surprising how many different fume hoods exist, so you should take care you use the right hood for your needs. Pay special note to any signage on the face of your hood; whether you’re looking at a conventional hood – or another kind – it will help you determine what containment is available and what items can be used inside of it.

Hoods for Bugs & Other Biological Material

Laminar Flow/Clean Bench

If you care about clean air over your sample, but not “clean” air to you, the clean bench is an appropriate workspace. Rebecca mentioned how this works in an earlier article on ‘Biological Safety Cabinets and Culture Hoods: Know The Difference‘: lab air is pulled in laminar flow (all in one direction) through a HEPA filter, across your samples and vented out towards you. Unfortunately this offers you zero protection from cells, tissues and bad bugs so we will need to use a biological safety cabinet instead if you need protection yourself.

Biological Safety Cabinets

Biological Safety cabinets are now categorized based on what protection they afford you, your samples and the environment.

Class 1 BSC is the opposite of a clean bench: lab air instead flows into the workspace, over your specimen, up through a HEPA filter and is then recycled back to the room. You’re likely to find these in use with centrifuges, homogenizers and where aerosols are an issue, but not where you’ll work hands-on with live cells. Since it vents back into the lab, avoid using hazardous reagents here!

Class 2, A1 BSC  is your familiar tissue or cell culture hood. Lab air is drawn in through the front grille, then directed up the back of hood before passing through a HEPA filter, where some clean air vents out to the laboratory and the rest flows back down to – and around – the work space, repeating the cycle again.

Class 2, A2 BSC. Much like the Class II A1 except you are able to work with extremely small volumes of volatile chemicals near the back wall of the hood. Exhaust air is sent out through a duct, rather than recycled into the lab.

Class 2, B1 BSC. Unlike the A-type hoods, B-type hoods exhaust more air than they put back through the cabinet. Here, about 30% is recycled to the cabinet while 70% is filtered and vented out a duct. You can work with small amounts of volatile chemicals in this hood too, but take care to use those that won’t eat away at the mechanics.

Class 2, B2 BSC. Similar to the B1 hood, but it exhausts 100% of air that flows through it.

Class 3 – For everything biosafety level-4 (which is beyond most academic research settings). Air is filtered at least once through a HEPA filter as it goes into a sealed glove box and at least twice as it goes out, and even in some cases through an air incinerator.

Tissue culture rooms

To work safely with particularly dangerous things like HIV, TB and other contagions, the workroom itself should be built as a gigantic hood. Enter negative air pressure rooms, where air flows inwards through a small ½ inch opening underneath the door and out through a high-flow exhaust system, eliminating the opportunity for these bad bugs to escape out into the wild.

Hopefully this has help you understand the different hoods available, and highlighted the need to check that the hood you are using is suitable for your needs.

In my next article we will discuss how to work effectively in your conventional fume hood.


Further reading:

Wikipedia: Fume Hood, Biosafety Cabinet Appendix A – Primary Containment for Biohazards: Selection, Installation and Use of Biological Safety Cabinets

University of Nebraska, Lincoln – SOPs for Biosafety Cabinets