A laboratory chemical fume hood is a partially enclosed workspace that is exhausted to the outside of the building. When used properly, hazardous gases and vapors generated inside the fume hood are captured before they enter the breathing zone. This serves to minimize your exposure to airborne contaminants.

Terminology

General-purpose fume hoods, the most common use type, are used to prevent exposure to toxic, irritating, or noxious chemical vapors and gases. A face velocity of 100 feet per minute (fpm) provides efficient vapor capture while reducing fume hood turbulence.

Types of Chemical Fume Hoods

There are three basic types of general purpose fume hoods: (1) standard, (2) bypass, and (3) auxiliary air. In addition, at Texas State University, there are three classes of fume hoods: A, B, and C.

Classifications

The following guidelines are recommendations for face velocities of three classes of chemical fume hoods.

Chemical fume hoods are approved for three general types of uses: General Purpose, Radioisotope, and Perchloric Acid. Hoods Fume hoods approved for each of these uses will appear alike but require different functional and operating parameters.

Speciality Fume Hoods

Radioisotope Fume Hoods
Radioisotope fume hood systems are ideally made from welded stainless steel to ensure against absorption of radioactive materials. In order to comply with licensing requirements, iodinations using radioactive sodium iodide (I-125) must be performed in a fume hood with a minimum face velocity of 125 fpm. The yellow label on the fume hood will indicate the hood is "Approved for Iodine-125" with a minimum face velocity of 125 fpm. This higher face velocity is not required for use of RIA kits with low activity

Walk In Hoods
These fume hoods have single vertical sashes or double vertical sashes and an opening that extends to the floor. These hoods are typically used to accommodate large pieces of equipment.

Canopy Hoods
These hoods capture upward moving contaminants and are good for heat-producing operations. Workers may be exposed to contaminants if they work under the hood, however.

Safe Operating Procedures for Chemical Fume Hoods

• Confirm that the fume hood is operational.
  • If fitted with a local on/off switch, make sure the switch is in the "on" position; check the airflow gauge if so equipped. In the absence of a gauge, observe the plastic "flow check ribbon" taped to the lower corner of the sash. Airflow can be visually assessed by noting that the ribbon is pulled gently into the fume hood. The most recent fume hood test data and optimum sash height are indicated on the yellow label affixed to the fume hood face. Never work with a malfunctioning fume hood, report problem hoods to Facilities via a Facilities Focus request.
• Maintain operations at least 6" inside the fume hood face.
  • Vinyl tape can be attached to the work surface to serve as a visual reminder.
• Lower sash to optimum height.
  • Optimum height is the sash height at which airflow is maximized without creating turbulence, generally 100 feet per minute. A yellow label placed on the fume hood face indicates the most recently recommended sash height. [This does not apply to variable volume chemical hoods] . With unattended or potentially explosive processes, conduct the operation behind a lowered sash or safety shield.
• Keep head out of fume hood except when installing and dismantling equipment.
• Keep fume hood storage to an absolute minimum.
  • Keep only items needed for the ongoing operation inside the fume hood. Keep the back bottom slot clear at all times as it serves as an exhaust port for chemicals generated near the work surface. Raise large objects at least two inches off the fume hood surface to minimize air flow disruption.
• Minimize foot traffic around the chemical fume hood.
  • A person walking past a chemical fume hood can create competing currents at the fume hood face, causing vapors to flow out. Other sources of competing air currents such as open windows and fans must also be avoided while using a chemical fume hood.
• Use extreme caution with ignition sources inside a chemical fume hood.
  • Ignition sources such as electrical connections, Variac controllers and open flame can be used inside a chemical fume hood as long as there are no operations involving flammable or explosive vapors. If possible, ignition sources should remain outside the fume hood at all times.
• Replace fume hood components prior to use.
  • Every component of a chemical fume hood, whether airfoil, baffle, or sash, plays a vital role in preventing the escape of hazardous materials from the fume hood. Any fume hood components removed to conduct maintenance or repair activities, or to set up experimental apparatus must be replaced prior to using the fume hood for contaminant control.

Monitoring Fume Hood Function

The ultimate goal of chemical fume hood use is to contain the contaminants generated within the fume hood. While direct measurement of containment is difficult and costly, there exists a surrogate measurement for fume hood performance that is simple and straightforward -- – fume hood face velocity. A face velocity of 100 fpm with a minimum of turbulence is generally considered a good compromise between competing air forces and the creation of turbulent eddies. With the exception of VAV fume hoods, face velocity is strongly dependent on sash height. It is therefore important when using the fume hood to position the sash at the height indicated by the arrow on the yellow fume hood certification label. When not in use the sash should be closed completely. This will reduce noise levels and ease the load on the heating or air conditioning system.

The face velocity of each chemical fume hood should be tested annually. Contact the Risk Management & Safety office at 245-3616 if your fume hood has not been calibrated within the past year.

Fume hood function can change from one moment to the next due to system irregularities such as fume hood storage and use, broken belts, electrical malfunctions, or maintenance activities. For this reason it is important that you always verify air flow prior to conducting procedures inside the fume hood. This can be done by checking the airflow monitor on the fume hood.

Myth - When working with highly hazardous materials, the higher the face velocity the better.
While it is important to have a face velocity between 100 and 125 feet per minute (fpm), velocities higher than this are actually harmful. When face velocity exceeds 125 fpm, eddy currents are created which allow contaminants to be drawn out of the fume hood, increasing worker exposures.

Myth - A chemical fume hood can be used for storage of volatile, flammable, or odiferous materials when an appropriate storage cabinet is not available.
While it is appropriate to keep chemicals that are being used during a particular experiment inside the chemical fume hood, fume hoods are not designed for permanent chemical storage. Each item placed on the work surface interferes with the directional airflow, causing turbulence and eddy currents that allow contaminants to be drawn out of the fume hood. Even with highly volatile materials, as long as a container is properly capped , evaporation will not add significantly to worker exposures. Unlike a chemical fume hood, flammable materials storage cabinets provide additional protection in the event of a fire.

Myth - The airfoil on the front of a fume hood is of minor importance. It can safely be removed if it interferes with my experimental apparatus.
Airfoils are critical to efficient operation of a chemical fume hood. With the sash open , an airfoil smoothes flow over the fume hood edges. Without an airfoil , eddy currents form, causing contaminates to be drawn out of the hood. With the sash closed, the opening beneath the bottom airfoil provides for a source of exhaust air.

1. Confirm that the fume hood is operational. If a fume hood is equipped with a local on/off switch, make sure the switch is in the on position. Check the air flow gauge if so equipped. Check the telltale (a green, one inch by six inch piece of crepe paper attached to the bottom of the sash). The telltale should be noticeably pulled toward the back of the fume hood.
   
2. Set up work at least six inches from the face opening. This will avoid turbulence at the sash edge and provide greater protection.
   
3. Separate and elevate each instrument. Use blocks or racks to elevate equipment one to two inches off the fume hood deck surface so that air can easily flow around all apparatus with no disruption.
   
4. Lower sash to the optimum height. The sash will then act as a physical barrier in the event of an unplanned incident in the fume hood.
   
5. Keep fume hood storage to an absolute minimum. Keep only items needed for ongoing operation inside the fume hood. Excess materials in the fume hood disrupt airflow and can act as a barrier or cause airflow to bounce back across the face of the fume hood. Keep the back bottom slot clear at all times as it serves as an exhaust port for fumes and heat generated near the surface.
   
6. Minimize foot traffic near and around the fume hood. A person walking past the fume hood can create competing air currents. Other cross drafts should be eliminated such as open doors or fans.
   
7. Use extreme caution with ignition sources inside a fume hood. Ignition sources such as electrical connections and equipment, hot plates, controllers, and open flame will ignite flammable vapors or explosive particles from materials being used in the fume hood. All electrical equipment used inside a fume hood must be designed or certified as intrinsically safe unless it can be absolutely established (and enforced) that flammable or explosive materials will not be used in a particular fume hood.
   
8. Never put your head inside a fume hood while operations are in progress. The plane of the sash is the imaginary boundary that should not be crossed except to set up or dismantle equipment.
   
9. Clean up spills as soon as possible.
   
10. Do not dismantle or modify the physical structure of the fume hood or exhaust system in any way without first consulting facilities. Any component removed to conduct maintenance, repair, or to set up an experiment must be replaced prior to using the fume hood.
    
11. Report airflow problems and problems with the physical structure of the fume hood to Buildings and Grounds as soon as possible.
    
12. Lower the sash completely when you are not physically working in the fume hood.

¡IMPORTANT!
If a power failure or other emergency occurs (e.g., building fire or fire within the fume hood), close the fume hood sash and call for emergency assistance.