Make sure that a continuous performance monitoring device is present, and check it every time the chemical hood is used. (For further information, see section 9.C.2.8 on testing and verification.)

Box 9.1 provides a list of things to do to maximize chemical hood efficiency.

9.C.2.5 Housekeeping

Keep laboratory chemical hoods and adjacent work areas clean and free of debris at all times. Keep solid objects and materials (such as paper) from entering the exhaust ducts, because they can lodge in the ducts or fans and adversely affect their operation. The chemical hood will have better airflow across its work surface if it contains a minimal number of bottles, beakers, and laboratory apparatus; therefore, prudent practice keeps unnecessary equipment and glassware outside the chemical hood at all times and stores all chemicals in approved storage cans, containers, or cabinets. Furthermore, keep the workspace neat and clean in all laboratory operations, particularly those involving the use of chemical hoods, so that any procedure or experiment can be undertaken without the possibility of disturbing, or even destroying, what is being done.

9.C.2.6 Sash Operation

Except when adjustments to the apparatus are being made, keep the chemical hood closed, with vertical sashes down and horizontal sashes closed, to help prevent the spread of a fire, spill, or other hazard into the laboratory. Horizontal sliding sashes should not be removed. The face opening 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 chemical hoods without face velocity controls (see section 9.C.4.1), 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 laboratory chemical hood testing. Do not raise the sash above the working height for which it has been tested to maintain adequate face velocity. Doing so may allow the release of contaminants from the chemical hood into the laboratory environment.

Chemical hood sashes may move vertically (sash moves up and down), horizontally (sash is divided in panes that move side to side to provide the opening to the hood interior), or a combination of both. Although both types of sash offer protection from the materials within the hood and help control or maintain airflow, consider the following:

BOX 9.1
Quick Guide for Maximizing Efficiency of Laboratory Chemical Hoods

Many factors can compromise the efficiency of chemical hood operation, and most are avoidable. Be aware of all behavior that can, in some way, modify the chemical hood and its capabilities. Always consider the following:

   Keep chemical fume hood exhaust fans on at all times.

   If possible, position the chemical hood sash so that work is performed by extending the arms under or around the sash, placing the head in front of the sash, and keeping the sash between the person and the chemical source. View the procedure through the sash, which acts as a primary barrier if a spill, splash, or explosion should occur.

   Avoid opening and closing the sash rapidly, and avoid swift arm and body movements in front of or inside the chemical hood. These actions may increase turbulence and reduce the containment efficiency.

   Place chemical sources and apparatus at least 6 in. behind the face. Paint a colored stripe or apply tape to the work surface 6 in. back from the face to serve as a reminder Quantitative chemical hood containment tests reveal that the concentration of contaminant in the breathing zone can be 300 times higher from a source located at the front of the face than from a source placed at least 6 in. back. This concentration continues to decline as the source is moved farther toward the back.

   Place equipment as far to the back of the chemical hood as practical without blocking the bottom baffle.

   Separate and elevate each instrument by using blocks or racks; air should flow easily around all apparatus.

   Do not use large pieces of equipment in a chemical hood, because they tend to cause dead spaces in the airflow and reduce the efficiency.

   If a large piece of equipment emits fumes or heat outside a chemical hood, have a special-purpose hood designed and installed to ventilate that particular device. This method of ventilation is much more efficient than placing the equipment in a chemical fume hood, and it will consume much less air.

   Do not modify chemical hoods in any way that adversely affects performance. This includes adding, removing, or changing any of the components, such as baffles, sashes, airfoils, liners, and exhaust connections.

   Make sure all highly toxic or offensive vapors are scrubbed or adsorbed before the exit gases are released into the chemical hood exhaust system (see section 9.C.2.11.1 on chemical hood scrubbers).

   Keep the sash closed whenever the chemical hood is not actively in use or is unattended.

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