Chemical Fume Hoods
A chemical fume hood is local ventilation designed to limit a user’s exposure to hazardous substances. A chemical fume hood functions to capture, retain, and ultimately discharge out of the laboratory any noxious or hazardous vapors or fumes. A well-designed chemical fume hood, when properly installed and maintained, offers a substantial degree of protection to the user, provided that it is used correctly, and its limitations are understood.
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Work That Requires Use of Chemical Fume Hoods
Chemical Fume Hoods or other local ventilation devices are used when working with solvents that possess a high vapor pressure or a low flash point, and volatile toxic substances with a threshold limit value (TLV) less than 50 parts per million (ppm).
Limitation of Chemical Fume Hoods
A chemical fume hood is not designed to contain particulate matter, explosions, infectious materials, or gas releases from pressurized systems. A chemical fume hood is not a pollution control device. All contaminants that are removed by the ventilating system are released directly into the atmosphere. Apparatus used in hoods should be fitted with condensers, traps, or scrubbers to contain and collect waste solvents or toxic vapors or dusts. A chemical fume hood should not be used for waste disposal by way of evaporation. It is a violation of environmental regulations to intentionally evaporate hazardous chemicals in the chemical fume hood.
Check areas around the chemical fume hood for sources of cross drafts that may cause turbulence and result in leaks from the hood into the laboratory. Other personnel working in the laboratory should avoid walking behind individuals conducting work at the fume hood to reduce negative turbulence. Doors should remain closed reduce to maintain negative pressure in the laboratory relative to the hallway.
Proper Use of Chemical Fume Hoods
The air foil located at the front of the fume hood beneath the sash minimizes turbulence and creates smooth air flow for air entering the hood. For this reason, all apparatus and equipment should be located at least six inches away from the hood face; this distance is also indicated on each chemical fume hood with black and yellow tape. When a chemical fume hood is not in use, the sash should be closed completely, and the surface should be free of all materials and equipment. Chemicals should not be stored in a chemical fume hood but should alternatively be placed in appropriate chemical storage cabinets.
Required Face Velocity
The allowable face velocity for chemical fumes hoods at George Mason University ranges from 80-120 feet per minute (fpm). If the face velocity of a chemical fume hood is below 80 fpm or above 120 fpm or the hood is not operating properly, discontinue use and place a work order with Facilities Management, 3-2525, to have the chemical fume hood repaired. All non-functioning fume hoods must be reported to EHS, 703-993-8448.
Chemical Fume Testing
Chemical fume hood inspections are conducted by EHS:
- Annually (more frequently if required by funding agencies);
- Following the installation of a new chemical fume hood or renovation of a room where a chemical fume hood is located;
- When maintenance is performed on a chemical fume hood;
- Per request by the PI/LS if chemical fume hood performance is unsatisfactory; and
- As required due to specific laboratory operations conducted in a laboratory.
Each chemical fume hood should be labeled with an inspection sticker that displays the date that the hood was inspected, the measured face velocity, and the name of the inspector who conducted the test. Chemical fume hoods that fail inspection cannot be used until they are repaired by Facilities Management, 703-993-2525, and retested by EHS.
Proper Sash Height
During use, the sash must remain below the user’s breathing zone and no higher than a maximum height of 18” which is marked on the hood for easy reference. Working with the hood above 18” will result in an increase of surface area and reduction in the face velocity of the hood, this may result in splashes directly to the face of the user and loss of containment within the hood itself.
Why the Sash Should be Closed When Not in Use
Closing of chemical fume hood sashes when not in use reduces energy cost, helps to maintain comfortable conditions in the laboratory, and extends the functional life of the fume hood.
If there is a power outage, stop work and lower the sash of the fume hood to prevent the release of any fumes or vapors, contact Facilities Management 703-993-2525 to report the loss of power.
What can be Stored in the Chemical Fume Hood
Only chemicals necessary for work being performed in the hood may be stored within the fume hood for the duration of that procedure; excessive storage of chemicals and equipment in the fume hood will reduce or restrict the flow rate and safe working space.
What to do if the Chemical Fume Hood is in Alarm
The alarm may sound temporarily when raising the sash as the fan adjusts to increase the flow rate, this alarm should silence once the flow rate has stabilized, if not, immediately stop work and lower the sash to prevent the escape of any fumes or vapors and contact EHS 703-993-8448.
Use of Equipment and Apparatus in the Chemical Fume Hood
If large equipment must be placed in the chemical fume hood, use blocks to raise it approximately two inches above the surface so that air may pass beneath it. Ensure that all electrical devices are connected outside the chemical fume hood to avoid an electrical arc that can ignite a flammable or reactive chemical.
Special Requirements for Perchloric Acid
Do not use perchloric acid in a conventional chemical fume hood. Perchloric acid vapors accumulate in duct work and form perchlorate crystals that have the potential to explode, causing serious injury to personnel and damage to property. Contact EHS, 703-993-8448, if laboratory activities require the use of perchloric acid.
Special Requirements for Radioactive Materials
Work with certain radioactive materials requires the use of a dedicated radioisotope hood. Contact EHS 703-993-8448 for more information if you will be working with radioisotopes.
Spills in the Chemical Fume Hood
Provide secondary containment for containers that could break or spill. Clean all chemical residues from the chemical fume hood after each use, refer to the Chemicals Spills and Exposures page for more information on how to address the spill, and store waste in a sealed, labeled container placed n the satellite accumulation areas, SAA, until collected by EHS. Do not attempt to clean spills of unknown material. Lower the sash and contact EHS at 703-993-8448.