Best Practices for Selecting and Using a Fume Hood
Ensure clean air in the lab with the right fume hood materials, specifications, and usage
The fume hood is the best-known local exhaust device used in laboratories. Fume hoods remove hazardous dusts, fumes, gases, and vapors at their point of generation. They are one of the most reliable engineering controls in a laboratory. When properly installed and maintained, a well-designed hood can offer a substantial degree of protection to the user, provided that it is used appropriately and its limitations are understood.
Fume hoods are often regarded strictly as local exhaust ventilation devices to prevent toxic, hazardous, or offensive chemicals from entering the general laboratory atmosphere. However, hoods offer another significant type of protection by providing an effective containment device. When a chemical manipulation or reaction is performed within a hood (with the sash nearly or fully closed), a physical barrier is created between the workers in the laboratory and the operations inside the hood. Workers, therefore, are not only protected from exposure to harmful air contaminants, but injury will be minimized from splashes, spills, fires, and minor explosions that may occur inside the hood.
The interior fume hood surface should be constructed of durable, corrosion-resistant, nonporous, noncombustible, and fire-resistant materials such as stainless steel, a unique composite, or polymer material. Corrosive materials can damage many types of materials, shortening fume hood life. In addition, some materials, when exposed to direct flame, emit noxious and toxic fumes.
Fume hood performance
Fume hoods should be evaluated for performance when installed and before use to ensure adequate face velocities and the absence of excessive turbulence. Performance should be assessed against the design specifications for uniform airflow across the hood face and the total exhaust air volume. Equally important is the evaluation of operator exposure.
Successful hood performance depends on the speed (face velocity) of the air entering the fume hood’s front (sash opening). The hood’s face velocity can be significantly affected by cross-drafts created by the movement of people walking by or even the user’s presence in front of the hood and air currents from open windows and doors. Other factors affecting hood performance include hood design, thermal loading, and the amount and location of equipment in the hood.
Fume hood face velocity
In most fume hood installations, the exhaust flow rate or quantity of air pulled through the hood is constant. Therefore, the sash can be adjusted to obtain an optimal flow rate for a particular operation. For example, when the sash is lowered and the cross-sectional area of the hood opening decreases, the hood face velocity increases proportionally.
Successful hood performance depends on the speed (face velocity) of the air entering the fume hood's front (sash opening).
Achieving a hood face velocity in the range of 60-120 ft/min is the basis for the successful design of a laboratory fume hood. The face velocity, also known as “control velocity,” is the speed of air through the hood face, which is necessary to contain the contaminants captured by the fume hood, thereby preventing their dispersion into the workplace.
Too low a face velocity and the hood will not provide adequate exposure control. Too high a face velocity will likely increase the turbulence within the hood and cause contaminants to escape into the laboratory. Therefore, the most meaningful method of evaluating hood performance and obtaining the optimum airflow rate is to measure worker exposure while the hood is being used for its intended purpose.
Make-up air
All local exhaust ventilation systems must have air to exhaust. Since fume hoods exhaust air from the rooms in which they are installed, an adequate supply of air must replenish the exhaust air. Otherwise, the hoods will not be able to exhaust a sufficient volume of air to function efficiently as intended, causing contaminants to escape into the laboratory. To ensure that fume hoods operate correctly, an additional supply of air, known as “make-up air,” is required.
Work practices
Adequate information and training must be provided at the time of a laboratory worker’s initial assignment so they can safely use fume hoods and ventilation equipment to minimize emissions and employee exposures.
The following is a partial list of guidelines for safe fume hood use and should be followed when using one:
- Do not store chemicals or equipment (which are not being used) or waste in the hood.
- Chemical waste should not be disposed of by evaporation in a hood.
- Keep your head outside the fume hood. Do not walk into a “walk-in” hood when it is operating.
- Use the fume hood with the sash as low as possible, at or below the indicated operating height.
- Periodically check the airflow through the hood face.
- Do not block the rear hood exhaust slots with equipment or materials.
- Keep combustibles, such as paper towels, out of the hood. Paper items may also become drawn into the hood exhaust system, blocking or restricting airflow.
- Never use perchloric acid in a fume hood not explicitly designed for this purpose.
The OSHA Laboratory Standard, 29 CFR 1910.1045, Appendix A, Section A. 4., expands on these guidelines. It lists general precautions and engineering controls for handling all laboratory chemicals in a fume hood, thus minimizing the risks from known and unknown hazardous substances.