Fume Hood Use
Chemical exposure may be controlled by dilution ventilation or by capture of chemical vapors and particles by local exhaust ventilation. The primary means of control should be by local exhaust ventilation with dilution ventilation as a back up to remove contaminants that escape local exhaust fume hoods.
When working with perchloric acid, specialized ventilation may be required. When working with cold perchloric acid (less than five percent), a specialized independent fume hood is not required; however, the work must be performed in a properly functioning fume hood. When working with heated or concentrated perchloric acid, a specialized independent hood (perchloric acid hood) with a built in wash down system must be used.
When perchloric acid is heated above ambient temperature, it will vaporize and condense on hood, duct, and fan components. In addition to being highly corrosive, condensed vapors can react with organic materials such as gaskets, greases, and chemical residues to form explosive perchlorate salts and esters. By washing down the hood following each use, materials deposited in the perchloric acid hood are removed, preventing the buildup of shock, heat, and friction sensitive perchlorates.
The hood should be dedicated to perchloric acid use only. Organic materials and those incompatible with perchloric acid should not be used in the perchloric acid hood. The user is responsible for ensuring that perchloric acid hoods are washed down after each use. The hoods are certified on an annual basis.
- The effectiveness with which contaminants are captured from the air by a fume hood is seriously reduced by cross drafts and eddy currents. Cross-drafts occur when people walk in front of a fume hood, when nearby windows or doors are open, or when directional air supply diffusers are placed too close to the fume hood. Eddy currents also occur around the person using the fume hood and around objects inside it. To limit these effects, fume hoods should not contain unnecessary objects and equipment should be placed as far to the back of the fume hood as practical. Work should be performed at least six inches inside the opening. Hoods should be operated with the front sash drawn down as far as practical, so the face and upper body of the hood user are protected from splash, explosion, or fume inhalation.
- In some laboratories, fume hoods are used not only as local exhaust facilities, but also serve as the sole air exhaust for the laboratory. It is necessary for laboratories dependent on fume hoods as the sole air exhaust to assure these hoods are continuously operating (on a 24-hour basis) as long as hazardous chemicals are present in the laboratory. The need for operation of the hood during periods when workers are not present in the laboratory is for the removal of fumes from an accidental chemical spill or release which, if unventilated, may impact other rooms and personnel in the building.
- Operations such as ongoing reactions, heating or evaporating solvents, and transfer of chemicals from one container to another should normally be performed in a hood.
- If especially hazardous or corrosive vapors will be evolved, these exit gases should be passed through scrubbers or absorption trains.
- Control the rate and velocity of the released vapors and particles from chemical reactions in order to minimize risk of exposure.
- Sliding sashes should be kept closed to improve overall performance of the hood. Note that reacting chemicals placed in a fume hood with the sash closed places a physical barrier between workers and chemical reactions as well. Use a separate safety shield when appropriate.
- The fume hood also acts as a containment device for accidental spills of chemicals.
- There should be an adequate number of fume hoods to accommodate research needs. Recommended guidelines are: One fume hood for each two workers, and the fume hoods should be large enough to provide each worker with at least 2.5 linear feet of working space at the face.
- Fume hoods should not be regarded as the means for disposing of chemicals. Apparatus used in hoods should be fitted with condensers, traps, or scrubbers to contain and collect waste solvents or hazardous vapors or dusts. Highly hazardous or offensive vapors should always be scrubbed or absorbed before the exit gases are released into the fume hood exhaust.
- Fume hoods are tested annually by Facilities Operations. Labels are placed on the side airfoil on the front of each hood indicating the measured velocity at specific sash heights. To ensure that adequate personal protection is provided, make sure the fume hood velocity, as posted on the label, is adequate for the processes being carried out in the hood. It may be necessary to use a closed system such as a glove box or bag for highly hazardous chemicals/materials. Call EH&S for assistance at 335-3041. If the annual test results do not meet specification, Facilities Operations will affix a sign to the fume hood warning laboratory personnel not to use the hood until it has been repaired.
- Whenever practical, chemicals or apparatus should be moved from fume hoods to vented cabinets for storage until needed.
- An emergency plan should be prepared for the event of ventilation failure or other unexpected occurrences that could disrupt fume hood function. The minimum contents of such a plan should include:
- Procedure for warning laboratory users that ventilation protective devices are non-operational.
- Instructions to laboratory users as to the risk of their continued presence in the laboratory.
- Instructions to laboratory users about which processes and/or activities are to be curtailed or eliminated during the period of time ventilation systems are disrupted.
- Instructions for use of alternative laboratories or laboratory facilities, including safety orientation toward the use of these alternate facilities.
- This emergency plan/information should be included in your Laboratory Safety Manual / Chemical Hygiene Plan. The site specific information may be kept as an integral part of your Chemical Hygiene Plan or filed under Section VI. Laboratory Specific Information Provided, in your laboratory manual.
- For Power Failure in Laboratories, see EH&S Fact Sheet – “Power Failure: Laboratory Procedures.”
- Use only explosion proof electrical equipment in fume hoods where flammable liquids/vapors are present.