As I walk through our laboratories on campus I am too often finding laboratory personnel improperly using fume hoods. Just as with any device, for it to work properly you must use it properly. You don't expect your house to heat properly if you have boxes sitting on all of your floor registers blocking the airflow, you don't expect your car window to protect you from the wind while driving down the interstate if the window is rolled all the way down, , you don't expect your fireplace to contain the smoke if a lit log is hanging out of the fireplace and you don't expect your heating bill to be cheap or your furnace to operate efficiently if you leave the front door open in the winter due to the fact that you don't want to have to keep opening and closing it. All of the above scenarios are synonymous with the most prevalent mistakes laboratory personnel make when using a fume hood.
Crowded Fume Hood
A fume hood that is filled with materials will not contain vapors, period. Just like materials blocking floor registers if you fill your fume hood full of materials it blocks the air flow which is being pulled through the face of the hood via a fan on the roof pulling the air through the back baffles of the hood. When the hood is full of bottles, equipment, boxes and other materials the air must move around all of these objects and then out the baffles. Once the hood is full or the baffles become blocked then the face velocity drops and the fume hood loses containment. The only materials inside of the hood should be those that are actively in use. Don't use your fume hood as a chemical storage area or fill it full of unused materials as the more items in a fume hood the less efficient it will operate.
Improper Fume Hood Sash Height
All of our fume hoods on campus are certified annually. Once the hood has been certified a certification sticker is placed on the hood. This sticker indicates the average face velocity, the inspection date and the maximum safe operating height for the fume hood sash with a black line as shown to the left.
The fume hood sash should not be raised above this line while actively using the hood because just like the car window in the example the larger the opening the less efficient the fume hood will operate and when the sash is raised above the Maximum Safe Operating Height then the food will not function properly and you can lose containment. The sash not only serves the purpose of containing vapors but is also made of tempered safety glass which can act as a shield to protect you from projectiles and shrapnel should something rupture inside of the hood. The fume hood sash should be raised to a height in which you can comfortably work in the hood but not above the Maximum Safe Operating Height as indicated on the certification sticker.
Failure to Work at least 6 Inches Inside the Face of the Hood
Just like a fireplace will not contain the smoke when a log rolls out a fume hood will not contain vapors when materials are opened at the face of the hood. You must work at least 6 inches inside the face of the hood. Materials located less than 6 inches inside the face of the hood can be exposed to eddy currents which are generated by pulling your hands out of the hood, walking by the hood and other air disturbances in the laboratory. These eddy currents are moving opposite of the air flow into the hood and vapors can escape when trapped in the eddy currents. The image to the right demonstrates the air flow into the hood and the formation of eddy currents within 6 inches of the face of the hood.
Once the material is located 6 inches or more inside the hood then a protective barrier is formed and the exhaust velocity is greater than the eddy current velocity and containment is maintained.
Failure to Keep the Fume Hood Sash Closed when not in Use
Just like leaving your door open to your house, leaving a fume hood sash open when not in use is a tremendous waste of energy. A sash left open can consume the equivalent energy required to heat and cool over 3 average size homes on an annual basis. A closed fume hood sash serves the following purposes:
When the sash is closed it acts as a barrier between you and the materials that may be in the hood. The sash is made of tempered safety glass and can offer added protection from shattered glass, chemical spills, and vapors which will remain contained in the hood if the sash is closed when an "accidental event" occurs.
In case of a power outage or hood ventilation failure, chemical vapors will not back up into the laboratory. If there is an accidental or emergency shutdown of the fume hood ventilation system a closed sash will help contain vapors in the hood. Never open a fume hood if the ventilation system is not operational.
Energy savings. Most fume hoods at IUPUI operated on a "variable air volume" system, which means the volume of air exhausted is reduced as the sash height is lowered.
Save energy and improve the safety of your laboratory by ensuring proper use of your fume hood. It is a Win-Win Solution.