Info 101: Basic things I need to know about Fume Hoods

Earlier today, a colleague who is new to our sales team approached me and coyly asked ‘What a Fume Hood is for’. Quite honestly, the question came like a surprise. I was expecting her to know since she is a chemical engineering graduate and she have used this equipment with her class experiments before.

I don’t abhor people from asking questions. In fact, I appreciate those who has the courage to ask. Through her, I realized two important things: first, never to assume, not even to those whom you’ve expected to know and secondly, we (as safety advocates) should be doubling our efforts toward educating lab people since misconceptions like this has a huge toll on the lives of the people who are directly exposed to toxic chemicals.

So I will make a little talk about the basic things you need to know about this box-liked fancy equipment called FUME HOOD.

Dr. Einesco working in front of a fume hood. Courtesy of Esco.

Fume Hood Defined

Laboratory fume hoods are a type of ventilation system with the primary function to exhaust chemical fumes, vapors, gases, dust, mist and aerosol generated during analysis. Fume hoods also serve as physical barriers between reactions and the laboratory, offering a measure of protection against inhalation exposure, chemical spills, run-away reactions and fires.

A typical fume hood has a box-like structure with a moveable sash window. Experimental procedures are performed within the hood which is consistently and safely ventilated, usually by means of an extract blower and ductwork. Chemical fumes are exhausted and diluted many times over in the atmosphere and have a negligible effect on human health. When environment concerns are of importance, an extract treatment system, often referred to as a scrubber is installed to remove most of the vapors from the exhaust air stream.

How does it Work?

The hood functions by maintaining a relatively negative pressure in the interior of the hood to prevent any contaminant from escaping while drawing air in through the hood opening at a consistent rate. Air is drawn into the hood under and through the opened sash and is exhausted through openings in the rear and top of the cabinet to a remote point such as an exhaust stack on the roof of the building.

Airflow illustrated

A suitable hood face velocity (the speed at which air is drawn into the hood) is of importance to the safe and effective operation of a fume hood. While excessive face velocities can often result in turbulence and reduce containment, insufficient velocities can also compromise hood performance.

In general, a hood’s face velocity is recommended to be between 0.3 m/s (60 fpm) and 0.5 m/s (100 fpm), however it is important to check with local safety regulations on the face velocity recommendation before using the fume hoods. Most hoods are commonly sized for a minimum face velocity at full sash opening; but as means to conserve energy some hoods size the minimum face velocity of the hood at half-sash opening creating new low flow fume hoods, which are now present in the market.