Laboratory Standard & Design Guidelines – Stanford Environmental Health & Safety

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

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

number of users/hood
frequency of use
% of time working at hood
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 5154.1(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 5154.1(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}

_DOHS2010