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How to make good fume hoods bad.

by Alexander Koch on August 22, 2017

During our measurement campaigns on-site we – my colleagues and me – very often were asked to help the laboratory people because the smell in the labs is not very nice. Here I collected some examples which may show how one can increase the leakage of a fume hood – where he pays a lot of money to run it safely.

 fume hoods

These examples shall neither be copied nor rebuilt or basics for new designs, although no patents were given for the great ideas. But they may give a hint what is possible to ruin the efforts of safety people and engineers as well as the company who designed the hood when only increasing the airflow through the fume hoods without struggling against the symptoms. 

Example 1: a tested fume cupboard modified from the manufacturer due to the customers wishes 

fume hood side wall

Fume hood with additional sash at the side wall. 

Measurements with tracer gas showed, that even with closed front sash, fume hoods with an additional, open sash in the side wall have a bad containment. Fume hoods with these issues are not any longer defined as fume hood according the standard.

Example 2: a well running fume hood modified by the user

 walk in hood 

walk in hood used as bench type hood with ventilated storage space below. 

This walk in hood was modified by the user by installing a bench top inside. Containment measurements with the ejector above the table showed quite reasonable results. But placing the ejector on the ground – where the open bottles with the used solvents are standing – gave extremely high outbreaks of test gas. Additional byproduct:  the installation of the table top gave the user the chance to prevent definitely the closing of the sash.

Example 3: a tested fume hood used by the most air flow disturbing way the user find 

 fume hood blocking flow

Fume hood with big equipment blocking the flow inside 

In a first view the fume hood below seems to be the worse one (with all the most scientific looking equipment). But containment measurements showed no problems for this hood. But for the hood on the right side: The big “boxes“, which are part of a ozone(!) creating system, and the board above with a small test set up, block the flow inside the hood so that the turbulence pumps the contaminated air into the room. 

fume hood safety

This can be done also in combination with the modification of the fume cupboard by the manufacturer according to the ideas of the user. Here a filled up fume hood with an additional side window to enter the data on the display on the top…

Example 4: Using a fume hood without air 

fume hood storage cabinet

Fume hood as storage cabinet 

This fume hood, standing in the cellar of a university lab, was not only full with old bottles and broken (!) mercury thermometer but also “running“ without exhaust air, as the fume hood was not in use any longer. There is no better way of saving energy…. 

But even running fume hoods can be filled up so that they cannot work:

fume hood wrong gas

Please note: to avoid the wrong gas creeping into the “neighbour hood” it was blocked with porous foam

Example 5: Using a fume hood in a not intended way or “the wrong hood for the correct application”

 fume hood oven

Fume hood as oven

This brand new, standard fume hood is used for the oxidation of probes. With a temperature of the sand bath of min. 350°C. Due to the high convection of the sand bath and the extreme deposit of soot the former white looking fume hood has not only become colored after the use of only four weeks, but also useless, because it cannot contain the convection driven flow inside (Remark: special fume hoods are available which fulfill the requirements for working with high temperatures or for boiling acids; see EN 14175-7) 

Or place an oven in a walk-in hood up: This allows the use as a bench type hood!

 two walk in  hoods

Two walk in hoods – right modified for bench type use. 

Example 6: Use of a hair dryer

 A very common instrument to disturb the intended flow into the fume cupboard is a hair dryer. It already appeared in the official process description for chromatograph papers and works perfectly: the combination of hot air with high speed guarantees best outbreaks of vapors and the used solvent can be spread in the whole lab.

fume hood hair dryer

Fume hood and hair dryer 

But instead of use a mobile unit the placement of blowing equipment can also create disastrous results. The following pictures show very efficient disturbances from outside:

exhaust air fume hood

Exhaust air from a GC (black plate) directly in front of a Fume Hood 

A more hidden possibility is to create a flow under the table. In this case a strong vacuum pump, which has an additional cooling fan, sits under the table just beside the hood. As the air has no chance to escape from the corner it can flow directly beside the opening and take the contaminations out from the hood.

vacuum pump

Vacuum pump blowing cooling air below the table

Example 7: false ceiling air inlet

Another exiting trick to create high draft velocities in front to fume hoods is to place outlets directly in front of fume hood openings. As known from literature even textile low speed air outlets can be “modified” by temperature: with cold air! As cooling air is heavier than the moderated room air it drops down immediately and create a down draft in front of the fume cupboard much higher than the face velocity into the hood. The result: distribution of the pollutants from inside the fume cupboard into the room.

Not so obvious but with a similar result can be openings in the false ceiling to the outside. Because of the under pressure in the room the air from outside comes through the false ceiling to the opening just in front of the sash and can achieve velocities much higher than the entrance velocity of the air in the sash opening.

fume hood false ceiling

Incoming room air from false ceiling opening (red arrow) 

But the easiest way to get a high pollution is to use the fume hood as called: as walk-in hood. It guarantees direct contact with the chemicals and reduces the protection to a minimum

 walk in hood

A real walk-in hood


One of the most important security components inside a laboratory is the fume cupboard. Learn more in our whitepaper "Fume Cupboards: Avoiding Mistakes in Assuming Safety".

Click here and get the whitepaper