• Make sure that all highly toxic or offensive vapors are scrubbed or adsorbed before the exit gases are released into the hood exhaust system (see section 8.C.8.1 on fume hood scrubbers).

8.C.4.2 Fume Hood Performance Checks

It is necessary to check if the hoods are performing properly. Observe the following guidelines:

  • Evaluate each hood before use and on a regular basis (preferably once a year) to verify that the face velocity meets the criteria specified for it in the laboratory's Chemical Hygiene Plan.

  • Also verify the absence of excessive turbulence (see section 8.C.4.4 below).

  • Make sure that a continuous monitoring device for adequate hood performance is present, and check it every time the hood is used.

(For further information, see section 8.C.5 on testing and verification.)

8.C.4.3 Housekeeping

Laboratory fume hoods and adjacent work areas should be kept clean and free of debris at all times. Solid objects and materials (such as paper) should be kept from entering the exhaust ducts of hoods, because they can lodge in the ducts or fans and adversely affect their operation. Also, the hood will have better airflow across its work surface if there are minimal numbers of bottles, beakers, and laboratory apparatus inside the hood; therefore, it is prudent to keep unnecessary equipment and glassware outside of the hood at all times and store all chemicals in approved storage cans, containers, or cabinets (not in the fume hood). Furthermore, it is best to keep the work space neat and clean in all laboratory operations, particularly those involving the use of hoods, so that any procedure or experiment can be undertaken without the possibility of disturbing, or even destroying, what is being done.

8.C.4.4 Sash Operation

Except when adjustments to the apparatus are being made, the hood should be kept closed, with vertical sashes down and horizontal sashes closed, to help prevent the spread of a fire, spill, or other hazard into the laboratory. Sliding sashes should not be removed from horizontal sliding sash hoods. The face opening of the hood should be kept small to improve the overall performance of the hood. If the face velocity becomes excessive, the facility engineers should make adjustments or corrections.

For hoods without face velocity controls (see section 8.C.6.3.2), the sash should be positioned to produce the recommended face velocity, which often occurs only over a limited range of sash positions. This range should be determined and marked during fume hood testing. For hoods with face velocity controls, it is imperative to keep the sash closed when the hood is not in use.

8.C.4.5 Constant Operation of Fume Hoods

Although turning fume hoods off when not in use saves energy, keeping them on at all times is safer, especially for fume hoods connected directly to a single fan. Because most laboratory facilities are under negative pressure, air may be drawn backward through the nonoperating fan, down the duct, and into the laboratory unless an ultralow-leakage backdraft damper is used in the duct. If the air is cold, it may freeze liquids in the hood. Fume hood ducts are rarely insulated; therefore, condensation and ice may form in cold weather. When the fume hood is turned on again and the duct temperature rises, the ice will melt, and water will run down the ductwork, drip into the hood, and possibly react with chemicals in the hood.

Fume hoods connected to a common exhaust manifold offer an advantage. The main exhaust system will rarely be shut down; hence, positive ventilation is available to each hood on the system at all times. In a constant air volume (CAV) system (see section 8.C.6.3.1), "shutoff" dampers to each hood can be installed, allowing passage of enough air to prevent fumes from leaking out of the fume hoods and into the laboratory when the sash is closed. It is prudent to allow 10 to 20% of the full volume of the hood flow to be drawn through the hood in the off position to prevent excessive corrosion.

8.C.5 Testing and Verification

All fume hoods should be tested, before they leave the manufacturer, by using ASHRAE/ANSI standard 110, Methods of Testing Performance of Laboratory Fume Hoods. The hood should pass the low- and high-volume smoke challenges with no leakage or flow reversals and have a control level of 0.05 parts per million (ppm) or less on the tracer gas test. ASHRAE/ ANSI 110 testing of fume hoods after installation in their final location by trained personnel is highly recommended. The control level of tracer gas for an "as installed" or "as used" test via the ASHRAE/ANSI 110 method should not exceed 0.1 ppm. Periodic per-

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