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When to Use a Fume Hood

Fume hood use should be based on a risk assessment of the procedure. This assessment should consider characteristics of the materials used, experimental procedure, and skill and expertise of the user.

How Fume Hoods Work

A fume hood is a ventilated enclosure in which gases, vapors and fumes are contained. An exhaust fan situated on the top of the laboratory building pulls air and airborne contaminants through connected ductwork and exhausts them to the atmosphere. There are two types of fume hoods at °ÄÃÅÁùºÏ²ÊÔ¤²â: Constant volume (CV): where the quantity of air pulled through the hood is constant. Therefore, when the sash is lowered, the velocity of airflow through the hood increases proportionally. Thus, higher face velocities can be obtained by lowering the sash. Variable air volume (VAV): where the quantity of air pulled through the hood varies as the sash is adjusted in order to maintain a set face velocity. Therefore, when the sash is lowered, the velocity of airflow through the hood stays the same.

Performance Indicators

All fume hoods will be certified yearly. Upon certification, a hood survey sticker will be attached to the face of the fume hood. This sticker indicates the unique fume hood identification number, the measured face velocity, and date of certification, and the initials of the person performing the certification. If you have questions about certification parameters, or if your fume hood has NOT been certified in the past year, please contact EHS.

The fume hoods are also outfitted with a "Maximum Sash Height" sticker. This is the maximum height your sash can be raised to and still meet the minimum face velocity standards. As a general rule, user's should always work at the lowest feasible height to ensure maximum protection.

Proper Work Practices

The level of protection provided by a fume hood is affected by the manner in which the fume hood is used. No fume hood, however well designed, can provide adequate containment unless good laboratory practices are used, as follows:

• Check the hood survey sticker to ensure that the hood has been certified within the last year.
• Check the hood flow indicator and compare its reading to the reading indicated on the hood survey sticker. If the reading differs significantly from that on the sticker, the hood may not be operating properly.
• Before use, open the fume hood sash and allow the hood to run for at least five minutes in order to reach an appropriate flow rate.
• Mark a line with tape 6 inches behind the sash and keep all chemicals and equipment behind that line during experiments. This will help to keep materials from escaping the hood when disturbances like air currents from people walking past the hood interfere with airflow at the face of the hood.
• Keep the sash at the level of the MAXIMUM SAFE SASH HEIGHT sticker (usually 18 inches) any time an experiment is in progress.
• Keep the sash completely lowered any time an experiment is in progress AND the hood is unattended.
• Extend only hands and arms into the hood and avoid leaning against it.
• Do not block baffle slots. If large equipment must be placed in the hood, put it on blocks to raise it approximately 2 inches above the surface so that air may pass beneath it.
• Do not use the hood as a storage device. Keep only the materials necessary for the experiment inside of the hood. If chemicals must be stored in the hood for a period of time, install shelves on the sides of the hood, away from the baffles.
• The fume hood alarm will sound when the flow rate dips to unsafe levels. This can happen when the sash is lifted above the maximum safe sash height.  Instead of silencing the alarm, try lowering the sash. If the alarm sounds when the sash is at a safe level, contact EHS.
• Emergency Purge button

Common Misuses and Limitations

Used appropriately, a fume hood can be a very effective device for containment of hazardous materials, as well as providing some protection from splashes and minor over pressurizations. Even so, the average fume hood does have several limitations.

• Pressurized systems: Gases or vapors escaping from pressurized systems may move at sufficient velocity to escape from the fume hood.
• Explosions: The hood is not capable of containing explosions, even when the sash is fully closed. If an explosion hazard exists, the user should provide barriers of sufficient strength to deflect or contain it.
• Perchloric Acid: A conventional fume hood must not be used for perchloric acid. Perchloric acid vapors can settle on ductwork, resulting in the deposition of perchlorate crystals. Perchlorates can accumulate on surfaces and have been known to detonate on contact, causing serious injury to researchers and maintenance personnel. Specialized perchloric acid hoods, made of stainless steel and equipped with a wash-down system must be used for such work.
• Highly Hazardous Substances: A well designed fume hood will contain 99.999 – 99.9999% of the contaminants released within it when used properly. When working with highly dangerous substances needing more containment than a fume hood offers, consider using a glove box.
• Waste Disposal: A fume hood should not be used for waste disposal. It is a violation of environmental regulations to intentionally send waste up the hood stack.

Campus Infrastructure

The majority of the fume hoods on campus are integrated as part of the campus building management systems. These systems are variable flow systems and ensure that the fume hoods are performing properly. An example of the Siemens control panel can be seen below:

• The hoods are designed to flow at 100fpm when the sash is opened up to 18 inches. If the sash is opened any further, the hood may go into alarm, meaning there is not sufficient flow and the sash should be closed back below 18 inches as soon as possible.
• For temporary work, the panel does have the ability to silence the alarm with the mute button
• The control panel also has an emergency purge button for those cases where a temporary increase in flow may be warranted (e.g. a spill or other unexpected release).
• For more information on the functionality of the Siemens control panel please see the video below (courtesy of Siemens):

 

Fume Hood vs. Biosafety Cabinet

Occasionally, both fume hoods and biosafety cabinets are referred to as "hoods." This can be misleading, as these two pieces of equipment serve different purposes. Make sure you are using the proper containment equipment for your work:

Chemical Fume Hood	Biological Safety Cabinet
Used for hazardous chemicals	Used for infectious biological agents
Protects the user	Protects the user, environment, and material
No HEPA filter	Exhaust is HEPA filtered
Exhausts air outside the building	(Usually) exhausts the air back into the room

 

Ductless Fume Hoods

Occasionally, a ductless fume hood may be installed in a laboratory. Unlike regular fume hoods, these do NOT exhaust the air outside of the building. Instead, ductless fume hoods rely on filters and other absorbent materials. It is very important that the manufacturer's recommendations regarding filter type and change-out schedule be followed to ensure proper protection.