LABORATORY FUME HOODS

Laboratory ventilation should not be less than 6 air changes per hour.  This flow is not necessarily sufficient to prevent accumulation of chemical vapors.  Many common laboratory procedures can result in the generation of hazardous fumes.  Therefore, fume hoods are the most important components used to protect laboratory workers from exposure to hazardous chemicals and agents. 

Laboratory fume hoods are the most common local exhaust ventilation devices found in the laboratory.  Fume hoods are used to prevent hazardous, offensive, or flammable gases and vapors from mixing with the general room air and to prevent the breathing in of these vapors.  In addition, a hood will provide a physical barrier (with the sash down) from spills, explosions, and fires.

Fume hoods should be regarded as backup safety devices that contain and exhaust toxic, offensive or flammable materials when the containment of an experiment fails. Therefore, apparatus used in hoods should be fitted with condensers, traps, or scrubbers to contain and collect waste solvents or toxic vapors or dusts.

Check the hood prior to use to ensure it is operational. Hold a tissue at the face of the hood to see if the tissue is pulled toward the hood. Never let go of the tissue.

To be effective, laboratory fume hoods must be used correctly.  The National Research Council in "Prudent Practices for Handling Hazardous Chemicals in the Laboratory" recommends that the following factors should be remembered in the daily use of hoods:

1. Keep fume hood exhaust fans on at all times.
2. Hoods should be evaluated before use to ensure adequate face velocities and the absence of excessive turbulence.  Face velocities that are too low or too high will reduce the containment efficiency of a fume hood.  In most cases, the recommended face velocity is between 80 and 100 feet per minute (fpm).  Face velocities between 100 and 120 fpm may be used for substances of very high toxicity or where outside influences adversely affect hood performance.  Face velocities approaching or exceeding 150 fpm should not be used, because they may cause turbulence around the peripheral of the sash opening and actually reduce the capture efficiency of the fume hood. Call Facilities Services to report inoperable hoods.  
3. If possible, position the fume hood sash so that work is performed by extending the arms under or around the sash, placing the head in front of the sash, and keeping the glass between the worker and the chemical source.  The worker views the procedure through the glass, which will act as a primary barrier if a spill, splash, or explosion should occur.
4. Avoid opening and closing the fume hood sash rapidly, and avoid swift arm and body movements in front of or inside the hood.  These actions may increase turbulence and reduce the effectiveness of fume hood containment.
5. Hoods are not intended primarily for storage of chemicals.  Materials stored in them should be kept to a minimum.  Stored chemicals should not block vents or alter airflow patterns.  Whenever practical, chemicals should be moved from hoods into cabinets for storage.
6. Place chemical sources and apparatus at least 6 inches behind the face of the hood.  In some laboratories, a colored stripe is painted on, or tape applied to, the hood work surface 6 inches back from the face to serve as a reminder.   Quantitative fume hood containment tests reveal that the concentration of the contaminant in the breathing zone can be 300 times higher from a source located at the front of the hood face than from a source placed at least 6 inches back.  This concentration declines further as the source is moved farther toward the back of the hood.
7. Place equipment as far to the back of the hood as practical without blocking the bottom baffle.
8. Separate and elevate each instrument by using blocks or racks so that air can flow easily around all apparatus.
9. Do not use large pieces of equipment in a hood, because they tend to cause dead spaces in the airflow and reduce the efficiency of the hood.
10. If a large piece of equipment emits fumes or heat outside a fume hood, then have a special purpose hood designed and installed to ventilate that particular device.  This method of ventilation is much more efficient than placing the equipment in a fume hood, and it will consume much less air.
11. Do not modify fume hoods in any way that adversely affects the hood performance.  This includes adding, removing, or changing any of the fume hood components, such as baffles, sashes, airfoils, liners, and exhaust connections.
12. Solid objects and materials (such as filter paper) should not be permitted to enter the exhaust ducts of hoods as they can lodge in the ducts or fans and adversely affect their operation.
13. An emergency plan should always be prepared for the event of ventilation failure (power failure, for example) or other unexpected occurrence such as fire or explosion in the hood.  (See Fire and Evacuation Procedures)
14. Persistent problems with fume hoods should be reported to Facilities Services or to the EHRS Department (X8925).

University of the Sciences in Philadelphia • 600 South Forty-third Street • Philadelphia, PA 19104-4495 • phone: 215-596-8800 • email: safety@usp.edu