effectively. Thus, a canopy hood works best when thermal or buoyant forces exist that move the contaminant up to the hood capture zone (a few inches below the opening). However, because canopy hoods are generally placed well above a contaminant source so that laboratory personnel can operate underneath them, they draw contaminants past the breathing zone and into the exhaust system. If a canopy hood exists in a laboratory, use it only for nonhazardous service, such as capturing heated air or water vapor from ovens or autoclaves. For design advice, consult the American Conference of Governmental Industrial Hygienists ventilation manual (ACGIH, 2004) and ANSI Z9.2.

9.C.3.4 Downdraft Hoods

Downdraft hoods or necropsy tables are specially designed work areas with ventilation slots on the sides of the work area. This type of system is useful for animal perfusions, gross anatomy laboratories, and other uses of chemicals where there is a need to have full access over and around the materials (which would be obstructed by the three sides of a chemical hood) and the chemicals in use have vapor densities that are heavier than air.

9.C.3.5 Clean Benches or Laminar Flow Hoods

A clean bench or laminar flow hood resembles a chemical hood but is not intended to provide protection to the user. A clean bench is generally closed on three sides and either is fully open in the front or has a partial opening. Some have hinged or sliding sashes. On the top or back of the clean bench, HEPA filters pull room air through the filters and pass that air across the work surface, providing clean air. The clean bench is for product protection, not personal protection, and is not connected to the ventilation system. Mark such equipment “not for use with hazardous materials” to remind laboratory personnel not to use anything in it that they would not use on the benchtop.

9.C.3.6 Ventilated Balance Enclosures

Ventilated balance enclosures are commonly used in laboratories to weigh toxic particulates. These devices are installed with different specifications for face velocity than the standard laboratory chemical hood and are well suited for locating sensitive balances that might be disturbed if placed in a laboratory chemical hood. The average face velocity is specified at 75 fpm plus or minus 10 fpm (0.40 ± 0.05 m/s). Individual face velocity at each grid point should be within a tolerance of plus or minus 20 fpm (0.10 m/s). Ventilated balance enclosures are typically equipped with HEPA filters to remove hazardous particulates captured within the device prior to exhaust. They can be either the recirculating type or 100% exhausted to the exterior.

Housings for ventilated balance enclosures are generally constructed of minimum 3/8-in.-thick (10-mm-thick) clear acrylic. Edges of the vertical sides are beveled, rounded, or otherwise aerodynamically designed to reduce turbulence at the perimeter of the face. Ventilated balance enclosures consist of an integrated dished base that facilitates cleaning at the interface of the vertical and horizontal surfaces. Airfoil sills have an ergonomic radius on the front edge. Sash configuration consists of a hinged single sash pane for cabinet widths and provides a full, clear, and unobstructed side-to-side view of the entire cabinet interior. Sash openings are usually at a fixed height of 8 to 12 in. (200 to 300 mm) above the work surface.

9.C.3.7 Gas Cabinets

Whenever possible, minimize use of highly toxic or hazardous gases and restrict them to lecture bottles that are placed on stands and used within the confines of a chemical hood.

Use and store containers for highly toxic or hazardous gases, such as diborane, phosgene, or arsine, that are too large to be used within a chemical hood in ventilated gas cabinets. In the event of a leak or rupture, a gas cabinet prevents the gas from contaminating the laboratory. Consult the standards developed by SEMI for specific, recommended exhaust rates for gas cabinets.

Connect gas cabinets to laboratory exhaust ventilation using metal ductwork, rather than flexible tubing, because such tubing is more apt to develop leaks. Use coaxial tubing for delivering gas from the cylinder to the apparatus. Coaxial tubing consists of an internal tube containing the toxic gas, inside another tube. Nitrogen, which is maintained at a pressure higher than the delivery pressure of the toxic gas, is between the two sets of tubing, ensuring that, in the event of a leak in the inner tubing, the gas will not leak into the room.

9.C.3.8 Flammable-Liquid Storage Cabinets

Store flammable and combustible liquids only in approved flammable-liquid storage cabinets, not in a chemical hood, on the bench, or in an unapproved storage cabinet. These cabinets are designed to prevent the temperature inside the cabinet from rising quickly in the event of a fire directly outside of the cabinet. These cabinets may be ventilated or unventi-



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