University Safety and Assurances

Fume Hoods: Procedures and Practices

 

Fumehood

General

Laboratory fume hoods serve to control exposure to toxic, offensive or flammable vapors, gases and aerosols. Fume hoods are the primary method of exposure control in the laboratory.

Use the correct hood for the job:

  • General Purpose Hoods:
    • Standard Fume Hood
    • Bypass Hood, or Constant Volume Hood
    • Variable Air Volume (VAV) Hood
    • Auxiliary Air Supplied Hood (Note: At UWM, found only in the Chemistry Building)
  • Radioisotope Hood--These hoods have been authorized by Radiation Safety for use with volatile radioactive materials.
  • Biosafety Cabinet--Specialized hoods to prevent or minimize the exposure of humans or the environment to biohazardous agents or materials.
  • Perchloric Acid Hoods must be used when working with PCA (e.g., acid digestion procedures). These hoods prevent the formation of perchlorates which could lead to explosions. They are constructed with special materials and have water-wash capability.
  • Hoods are labeled for special use when practical.

Equipment Use

  • Place apparatus and equipment as far back as possible in hood for safety and optimal performance. Equipment should be placed a minimum of 8 inches inside the hood. Keep electrical connections outside of hood.
  • Ensure that equipment or materials do not block the baffle vents in the back of the hood.
  • When using a large apparatus inside the hood, place the equipment on blocks, when safe and practical, to allow air flow beneath it.
  • Do not place electrical apparatus or other ignition sources inside the hood when flammable liquids or gases are present. Keep in mind that liquids with low flash points may ignite if they are near heat sources such as hot plates or steam lines.

Good Work Practices

  • When using the fume hood, keep your face outside the plane of the hood sash and remain alert to changes in air flow.
  • Work at least 6 inches back from the face of the hood. A stripe on the bench surface is a good reminder.
  • Always use splash goggles and wear a full faceshield if there is possibility of an explosion or eruption.
  • Do not make quick motions into or out of the hood, use fans, or walk quickly by the hood opening. All of these will cause airflow disturbances which reduce the effectiveness of the hood. 
    • A demonstration of quick motion out of the hood is featured in this video clip. (courtesy of University of Nevada EHS Department)
    • A demonstration of the effects of walking past the fume hood is demonstrated in this video clip. (courtesy of University of Nevada EHS Department)
  • Substitute less hazardous or less volatile chemicals where possible.
  • Look for process changes that improve safety and reduce losses to the environment (e.g. more accurate chemical delivery systems vs. pouring volatile chemicals from bottles).
  • Develop a process to evaluate research proposals ahead of time for potential emissions and look for opportunities to reduce them.

Waste Disposal 

Do not use the hood as a waste disposal mechanism. Apparatus used in a hood should be fitted with condensers, traps, or scrubbers to contain and collect waste solvents, toxic vapors or dust. Please contact the UWM Environmental Protections Office for additional information on waste disposal or refer to the Waste Disposal Guide.

Good Housekeeping Practices

  • Limit chemical storage in fume hoods. Keep the smallest amount of chemicals in the hood needed to conduct the procedure at hand.
  • Store hazardous chemicals such as flammable liquids in an approved safety cabinet.
  • Keep caps on chemical reagent bottles tight and check fitting on laboratory glassware to minimize vapor loss.

Always use good housekeeping techniques to maintain the hood at optimal performance levels. Excessive storage of materials or equipment can cause eddy currents or reverse flow resulting in contaminants escaping from the hood.

Proper Sash Use

  • The sash should be kept closed, except when working within the hood is necessary, to contain and protect from chemical vapors, splashes, or explosions.
  • Use horizontal sliding sash for partial protection during hazardous work.
  • Do not remove the sash or panels except when necessary for apparatus set-up. Replace sash or panels before operating.
  • Keep the slots of the hood baffles free of obstruction by apparatus or containers. 
  • Keep the hood sash closed as much as possible to maximize the hood's performance. Keep the sash closed when not in use to maximize energy conservation. This video clip (courtesy of the University of Nevada) demonstrates the effects of raising and lowering the sash height.
Sash in Full-Open Position

Demonstration of fume hood capture efficiency with the sash in the full-open position. Notice escape of the visualization smoke from the bottom of the hood.

Sash Partially Closed

Notice the improved capture efficiency with the sash in the partially-closed position.

Fume Hood Testing and Maintenance

Hoods should be evaluated by the user before each use to ensure adequate face velocities and the absence of excessive turbulence.

In case of exhaust system failure while using a hood, shut off all services and accessories and lower the sash completely. Leave the area immediately.

The required face velocity is 100 feet per minute (0.5 m/sec). This velocity is capable of controlling most low-velocity cross drafts and turbulence created by normal work practices at the face of the hood. All hoods should have a sticker designating this maximum safe sash height. Keep the sash at the identified appropriate level to ensure optimal face velocity. When working with open chemicals, reduce the sash as much as possible to maximize hood performance. Low flow hoods offer the same protection, but operate at a lower face velocity (50-60 fpm) to conserve energy.

Hoods are evaluated at installation using the ASHRAE 110 Method of Testing Performance of Laboratory Fume Hoods. In this sophisticated test, a tracer gas (sulfur hexaflouride) is released at a known rate inside the hood. Samples are taken in the breathing zone of a mannequin standing at different positions in front of the hood. The DILHR mandated pass criteria is 100 parts per billion (ppb) of tracer gas at the breathing zone of the mannequin. As with conventional style hoods, keep the sash at the appropriate level as indicated by the test label to ensure optimal hood performance. The objective is to minimize the sash opening to achieve proper containment.

Regular testing of the fume hood should be done by Physical Plant Services or University Safety and Assurances staff to ensure that it is operating properly. Hoods are labeled to indicate the last inspection date. If your hood has not been tested within the past year, please contact Zack Steuerwald, x5808.

Here is the latest DCOMM proposal:

32.24

(6) VENTILATION FOR LABORATORY FUME HOODS.

  1. General.
    1. Except as provided in par. (b), laboratory fume hoods during use shall be operated with a minimum average 100 feet per minute face velocity at full open sash or sash stop position. When determining the minimum flow rate through the fume hood, the sash stop position may not be lower than 18 inches above the work surface.

      Note: When operating the fume hood, the sash should be positioned to maximize the protection to the user.

    2. Vertical sash fume hoods operated at sash stop positions shall have an alarm that gives a warning when the sash is raised above the sash stop position. Combination vertical/horizontal sash fume hoods shall have an alarm that gives a warning when the sash is vertically raised from the fully lowered position.
  2. Alternate operation.
    1. Fume hoods operating at minimum average face velocities less than 100 feet per minute shall achieve a spillage rate less than 0.1 ppm at 4.0 liters per minute gas release for an "as used" condition in accordance with the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) standard 110 - Method of Testing Laboratory Fume Hoods. The minimum allowable average face velocity for fume hoods achieving the ASHRAE 110 containment criteria shall be 40 feet per minute at full open sash.

    2. Fume hoods operating at minimum average face velocities less than 100 feet per minute shall have a continuous flow meter with an alarm.

      Note: For further information regarding fume hood operation, see ANSI/AIHA standard Z9.5 - Laboratory Ventilation.

  3. Testing.

    Operable fume hoods shall be tested annually for minimum average face velocity.

    History: Cr. Register, February, 1999, No. 518, eff. 3-1-99; am. (2) (a), (4) (c), (5) (a) and (c) 2., r. (2) (b) and Figure 32.24, renum. (2) (c) and (d) to be (2) (b) and (c) and am. (c), cr. (5) (d) and (6), Register, June, 2000, No. 534, eff. 7-1-00; CR 01-139: am. (5) (c) 1.
    Register June 2002 No. 558, eff. 7-1-02.

The OSHA Laboratory Standard (29 CFR 1910.1450) does not specify safe hood operation, flows or face velocities. However, it does mandate a chemical hygiene plan and lists requirements for the plan, including "a requirement that fume hoods and other protective equipment are functioning properly and specific measures that shall be taken to ensure proper and adequate performance of such equipment." The non-mandatory Appendix A states: "General air flow should not be turbulent and should be relatively uniform throughout the laboratory, with no high velocity or static areas; airflow into and within the hood should not be excessively turbulent (200); hood face velocity should be adequate (typically 60-100 lfm)"

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