How to Choose Chemistry Fume Hood?

1.  General: Factors to consider when selecting a fume hood:

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• Room size (length x width x height)
• Number of room air changes
• Lab heat load
• Types of materials used
• Linear feet of hood needed based on

1. number of users/hood
2. frequency of use
3. % of time working at hood
4. size of apparatus to be used in hood, etc.

A facility designed for intensive chemical use should have at least 2.5 linear feet of hood space per student.

Good Practice per Stanford EH&S

Evaluating the operational and research needs of the users will ensure that the appropriate type and number of hoods are integrated into the laboratory.

2.  Constant Volume Hoods

These hoods permit a stable air balance between the ventilation systems and exhaust by incorporating a bypass feature. If bypass is 100% this allows a constant volume of air to be exhausted through the hood regardless of sash position.

3.  Variable Air Volume (VAV) fume hoods

These hoods maintain constant face velocities by varying exhaust volumes in response to changes in sash position. Because only the amount of air needed to maintain the specified face velocity is pulled from the room, significant energy savings are possible when the sash is closed. However, since these hoods cost more than up front and more maintenance, effective sash management (e.g., pull sash closed when not using hood) is necessary.

4.  Supply or auxiliary air hoods

These hoods are not permitted, unless an exception is granted by EH&S.

Good Practice per Stanford University EH&S

It is very difficult to keep the air supply and exhaust of supply hoods properly balanced. In addition, the supply air is intemperate, causing discomfort for those working in the hot or cold air stream. As a result, the supply vent is often either shut or blocked off, which eliminates any potential benefit of this type of hood. Finally, the presence and movement of the user’s body in the stream of supply air creates turbulence that degrades the performance of the hood.

5.  Ductless Fume Hoods: Portable, non-ducted fume hoods are generally not permitted; however, a portable hood may be used for limited applications (e.g., used inside of an existing hood for a special application, such as odor control). Such applications must be reviewed and approved by EH&S on a case-by-case basis.

ANSI/AIHA Z9.5, 5.16

Portable hoods often do not meet the regulatory airflow requirements. Filters used with these units must be changed frequently and vary in filtration effectiveness from chemical to chemical. Experience has demonstrated that an OSHA compliance officer may require quarterly monitoring of hood exhaust to demonstrate the effectiveness of the filtration in the given application and the corresponding protection of the workers occupying the space. These hoods are often misused.

6.  Perchloric/Hot Acid Hoods: 

a)  Heated perchloric acid shall only be used in a laboratory hood specifically designed for its use and identified as “For Perchloric Acid Operations.” (Exception: Hoods not specifically designed for use with perchloric acid shall be permitted to be used where the vapors are trapped and scrubbed before they are released into the hood.)

NFPA 45, Chapter 6-11.1

Heated perchloric acid will give off vapors that can condense and form explosive perchlorates. Limited quantities of perchloric acid vapor can be kept from condensing in laboratory exhaust systems by trapping or scrubbing the vapors at the point of origin.

b) Perchloric acid hoods and exhaust duct work shall be constructed of materials that are acid resistant, nonreactive, and impervious to perchloric acid. 

8 CCR .1(e)(7)

NFPA 45, Chapter 6-11.2

ANSI/AIHA Z9.5 

c) The exhaust fan should be acid resistant and spark-resistant. The exhaust fan motor should not be located within the duct work. Drive belts should not be located within the duct work.

NFPA 45, Chapter 6-11.3 

d)  Ductwork for perchloric acid hoods and exhaust systems shall take the shortest and straightest path to the outside of the building and shall not be manifolded with other exhaust systems. Horizontal runs shall be as short as possible, with no sharp turns or bends. The duct work shall provide a positive drainage slope back into the hood. Duct shall consist of sealed sections. Flexible connectors shall not be used.

NFPA, Chapter 6-11.4

e)  Sealants, gaskets, and lubricants used with perchloric acid hoods, duct work, and exhaust systems shall be acid resistant and nonreactive with perchloric acid.

NFPA 45, Chapter 6-11.5

ANSI/AIHA Z9.5

f)  A water spray system shall be provided for washing down the hood interior behind the baffle and the entire exhaust system. The hood work surface shall be watertight with a minimum depression of 13 mm (1⁄2 inch) at the front and sides. An integral trough shall be provided at the rear of the hood to collect wash-down water.

8 CCR .1(e)(7)

NFPA 45, Chapter 6-11.6

ANSI/AIHA Z9.5

Perchloric acid is a widely used reagent know to produce flammable or explosive reaction products; hence, the need to have wash down capabilities after each use to remove residues. A watertight surface will contain any chemical spills or leaks from leaking to underneath hood.

g) Spray wash-down nozzles shall be installed in the ducts no more than 5 ft. apart. The ductwork shall provide a positive drainage slope back into the hood. Ductwork shall consist of sealed sections, and no flexible connectors shall be used.

NFPA 45, Chapter 6-11.4

h) The hood surface should have an all-welded construction and have accessible rounded corners for cleaning ease.

Good Practice per Stanford University EH&S

Access for cleaning is an important design feature.

i) The hood baffle shall be removable for inspection and cleaning.

NFPA 45, Chapter 6-11.7

j) Each perchloric acid hood must have an individually designated duct and exhaust system.

ANSI/AIHA Z9.5 

7.  Radioactive Material Use

a)  Laboratory hoods in which radioactive materials are handled shall be identified with the radiation hazard symbol.

NFPA, Chapter A-6-12.1

b)  Fume hoods intended for use with radioactive isotopes must be constructed of stainless steel or other materials that will not be corroded by the chemicals used in the hood.

NCRP Report # 8

NFPA 99, Chapter 5-4.3.3

DOHS

CRC Handbook of Laboratory Safety, 4th Ed.

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c)  The interior of all radioisotope hoods must have coved corners to facilitate decontamination.

NFPA 99, Chapter 5-4.3.3

DOHS

CRC Handbook of Laboratory Safety, 4th Ed.

IAEA, Safe Handling of Radionuclides

Cracks and crevices are difficult to decontaminate.

d) The hood exhaust may require filtration by HEPA or Charcoal HEPA filters. Where such is the likelihood, the hood must have a bag-out plenum for mounting such filters and fan capacity for proper operation of the hood with the filter installed. The most appropriate location for the plenum is near the exhaust port of the fume hood (i.e., proximal to the hood).

NFPA 99, Chapter 5-4.3.3

DOHS

CRC Handbook of Laboratory Safety, 4th Ed.

IAEA, Safe Handling of Radionuclides

e) Hoods used for radioactivity should have sashes with horizontal sliding glass panels mounted in a vertical sash. 

NFPA 99, Chapter 5-4.3.3

DOHS

10 CFR 20

CRC Handbook of Laboratory Safety, 4th Ed.

IAEA, Safe Handling of Radionuclides 

f) The cabinet on which the hood is installed shall be adequate to support shielding for the radioactive materials to be used therein.

NFPA 99, Chapter 5-4.3.3

DOHS

10 CFR 20

CRC Handbook of Laboratory Safety, 4th Ed.

IAEA, Safe Handling of Radionuclides

g) In general, glove boxes with HEPA filtered exhausts shall be provided for operations involving unsealed radioactive material that emit alpha particles. Consult with the Radiation Safety Program for specific requirements.

NFPA 99, Chapter 5-4.3.3

DOHS

10 CFR 20

CRC Handbook of Laboratory Safety, 4th Ed.

IAEA, Safe Handling of Radionuclides

8.  American with Disabilities Act (ADA) Hoods: Must consult with Stanford University’s ADA Compliance Office regarding the number lab hoods to install in facilities, which are accessible to and usable by individuals with disabilities – recommend minimally one ADA hood per laboratory floor. These hoods must provide appropriate worksurface heights, knee clearances, reach to controls, etc. to individuals in wheelchairs.

The CalDAG – California Disabled Accessibility Guidebook

The location of at least one ADA hood per floor will enable disabled individuals to conduct their research without having to transport chemicals, etc. in elevators.

9.  Glove Boxes: Glove boxes (positive and negative) must meet the type, design and construction of requirements ANSI/AIHA Z9.5-, 5.14.

ANSI/AIHA Z9.5

10.  Walk-in Fume Hoods: These hoods must meet the type, design and construction requirements of ANSI/AIHA Z9.5-, 5.13.

ANSI/AIHA Z9.5

11.  Special Purpose Hoods: These hoods include enclosures for operations for which other types of hoods are not suitable (e.g., enclosures for analytical balances, histology processing machines, special mixing stations, evaporation racks). These hoods must be designed per ANSI Z9.2 and the Industrial Ventilation manual.

ANSI/AIHA Z9.5

The fume hood is the best-known local exhaust device used in laboratories. Fume hoods remove hazardous dusts, fumes, gases, and vapors at their point of generation. They are one of the most reliable engineering controls in a laboratory. When properly installed and maintained, a well-designed hood can offer a substantial degree of protection to the user, provided that it is used appropriately and its limitations are understood.

Fume hoods are often regarded strictly as local exhaust ventilation devices to prevent toxic, hazardous, or offensive chemicals from entering the general laboratory atmosphere. However, hoods offer another significant type of protection by providing an effective containment device. When a chemical manipulation or reaction is performed within a hood (with the sash nearly or fully closed), a physical barrier is created between the workers in the laboratory and the operations inside the hood. Workers, therefore, are not only protected from exposure to harmful air contaminants, but injury will be minimized from splashes, spills, fires, and minor explosions that may occur inside the hood. 

The interior fume hood surface should be constructed of durable, corrosion-resistant, nonporous, noncombustible, and fire-resistant materials such as stainless steel, a unique composite, or polymer material. Corrosive materials can damage many types of materials, shortening fume hood life. In addition, some materials, when exposed to direct flame, emit noxious and toxic fumes. 

Fume hood performance

Fume hoods should be evaluated for performance when installed and before use to ensure adequate face velocities and the absence of excessive turbulence. Performance should be assessed against the design specifications for uniform airflow across the hood face and the total exhaust air volume. Equally important is the evaluation of operator exposure.

Successful hood performance depends on the speed (face velocity) of the air entering the fume hood’s front (sash opening). The hood’s face velocity can be significantly affected by cross-drafts created by the movement of people walking by or even the user’s presence in front of the hood and air currents from open windows and doors. Other factors affecting hood performance include hood design, thermal loading, and the amount and location of equipment in the hood.  

Fume hood face velocity

In most fume hood installations, the exhaust flow rate or quantity of air pulled through the hood is constant. Therefore, the sash can be adjusted to obtain an optimal flow rate for a particular operation. For example, when the sash is lowered and the cross-sectional area of the hood opening decreases, the hood face velocity increases proportionally.

Successful hood performance depends on the speed (face velocity) of the air entering the fume hood's front (sash opening).

Achieving a hood face velocity in the range of 60-120 ft/min is the basis for the successful design of a laboratory fume hood. The face velocity, also known as “control velocity,” is the speed of air through the hood face, which is necessary to contain the contaminants captured by the fume hood, thereby preventing their dispersion into the workplace. 

Too low a face velocity and the hood will not provide adequate exposure control. Too high a face velocity will likely increase the turbulence within the hood and cause contaminants to escape into the laboratory. Therefore, the most meaningful method of evaluating hood performance and obtaining the optimum airflow rate is to measure worker exposure while the hood is being used for its intended purpose.

Make-up air

All local exhaust ventilation systems must have air to exhaust. Since fume hoods exhaust air from the rooms in which they are installed, an adequate supply of air must replenish the exhaust air. Otherwise, the hoods will not be able to exhaust a sufficient volume of air to function efficiently as intended, causing contaminants to escape into the laboratory. To ensure that fume hoods operate correctly, an additional supply of air, known as “make-up air,” is required.  

Work practices

Adequate information and training must be provided at the time of a laboratory worker’s initial assignment so they can safely use fume hoods and ventilation equipment to minimize emissions and employee exposures.

The following is a partial list of guidelines for safe fume hood use and should be followed when using one:

• Do not store chemicals or equipment (which are not being used) or waste in the hood.
• Chemical waste should not be disposed of by evaporation in a hood.
• Keep your head outside the fume hood. Do not walk into a “walk-in” hood when it is operating.
• Use the fume hood with the sash as low as possible, at or below the indicated operating height.
• Periodically check the airflow through the hood face.
• Do not block the rear hood exhaust slots with equipment or materials. 
• Keep combustibles, such as paper towels, out of the hood. Paper items may also become drawn into the hood exhaust system, blocking or restricting airflow.
• Never use perchloric acid in a fume hood not explicitly designed for this purpose.

The OSHA Laboratory Standard, 29 CFR ., Appendix A, Section A. 4., expands on these guidelines. It lists general precautions and engineering controls for handling all laboratory chemicals in a fume hood, thus minimizing the risks from known and unknown hazardous substances.

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