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.
Chemical fume hoods have a movable front sash that acts as a protective barrier and an interior baffle that directs the air being exhausted. Baffles may be adjusted in order to capture the maximum amount of aerosols, fumes, and vapors. Consult the manufacturer’s manual on how to use baffle adjustments, or contact the Safety Officer for guidance.
The exhaust flow rate or quantity of air pulled through most chemical fume hoods is constant. Therefore, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of airflow (face velocity) through the chemical fume hood increases proportionately. Thus, higher face velocities can be obtained by lowering the sash. An operating sash height of 40-44 cm (16-18 inches) must not be exceeded.
For work with hazardous chemicals, the chemical fume hood should have a face velocity of 0.41m/sec (80 feet per minute, fpm), 0.61m/sec (100 (fpm). Higher face velocities, 0.61m/sec (100-120 fpm) are recommended for work with acutely toxic chemicals. All chemical fume hoods are required to be equipped with a device that is capable of continuously monitoring face velocity. In the event that the face velocity of a chemical fume hood is below 0.41m/sec (80 fpm) or above 0.61m/sec (120 fpm) or the hood is not operating properly, contact the Safety Officer. The chemical fume hood may not be used until it is repaired and evaluated.
Chemical Fume Hood Limitations
A chemical fume hood is not designed to contain high velocity releases of 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. It is a violation of environmental regulations to intentionally evaporate hazardous chemicals in the chemical fume hood.
Chemical Fume Hood Evaluations
Chemical fume hoods are tested at least annually (more frequently if required by funding agencies) to verify that the hoods are working properly and providing adequate protection. 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 and retested.
Using Chemical Fume Hoods
Good laboratory practices must be employed while performing work in a chemical fume hood to facilitate adequate protection. A list of recommended practices follows:
- Use a chemical fume hood or other local ventilation device when working with volatile substances with a threshold limit value (TLV) less than 50 parts per million (ppm).
- Design experiments in consideration of chemical fume hood space and air flow.
- Before beginning work, verify that the type of chemical fume hood to be used and the face velocity are appropriate for the chemicals involved and the procedure to be performed.
- Do not use perchloric acid in a conventional chemical fume hood. Perchloric acid vapors accumulate in ductwork and form perchlorate crystals that have the potential to explode, causing serious injury to personnel and damage to property.
- Check area around the chemical fume hood for sources of cross drafts that may cause turbulence and result in leaks from the hood into the laboratory.
- Ensure that the inspection sticker is current (within one year).
- Verify that the reading from the continuous air flow monitoring device is no less than 0.41m/sec (80 fpm), no greater than 0.61m/sec (120 fpm), and within 15% of the face velocity value listed on the inspection sticker. If the reading differs significantly from that on the sticker, the chemical fume hood may not be operating properly.
- Visually inspect the baffles (openings at the top and rear of the hood) to be sure that the slots are open and unobstructed. For optimum performance, adjust the baffles when working with high temperature equipment and/or heavy gases or vapors. Review the manufacturer’s information on baffle adjustments and optimal performance settings.
- Conduct all experiments 20 cm from the front of the chemical fume hood.
- Provide secondary containment for containers that could break or spill.
- 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 electrical arcing that can ignite a flammable or reactive chemical.
- Clean all chemical residues from the chemical fume hood chamber after each use.
- Keep the sash completely lowered when the chemical fume hood is not in use or when an experiment in the hood is left unattended. Lowering the sash not only provides additional personal protection, but also results in significant energy conservation.
- Absolutely avoid storing chemicals and equipment in the chemical fume hood.
- Never turn off the chemical fume hood.