How a Fume Hood Works

A fume hood is a ventilated enclosure in which gases, vapors and fumes are captured and removed from the work area. 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.

The typical fume hood found in Princeton University laboratories is equipped with a movable front sash and an interior baffle. Depending on its design, the sash may move vertically, horizontally or a combination of the two and provides some protection to the hood user by acting as a barrier between the worker and the experiment.

The slots and baffles within the hood direct the air and, in many hoods, can be adjusted to allow the most even flow. It is important to prevent the baffles from becoming blocked, by excessive material storage or equipment, since this significantly affects the exhaust path within the hood and as a result, the efficiency of hood capture. 

The beveled frame around the hood face, called the airfoil, allows for even air flow into the hood by eliminating sharp curves to reduce turbulence.

There are two basic types of fume hoods. They are:

Constant volume – where the exhaust flowrate or quantity of air pulled through the hood is constant. In this configuration, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of airflow (face velocity) through the hood increases proportionally. Thus, the velocity of air at the hood face is increased with the lowering of the sash.

Variable air volume (VAV) - where the exhaust flowrate or quantity of air pulled through the hood varies as the sash is raised or lowered in order to maintain a constant face velocity. Therefore, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of air flow (face velocity) through the hood remains constant, reducing the total air volume exhausted.

This section pertains to all buildings except Frick Chemistry building – see separate section below.

Hood Survey Sticker

Every chemical fume hood on campus is performance tested annually by EHS as indicated by a survey sticker affixed to the front of the hood. Do not use a hood that has not been tested within a year; e.g., is not labeled or the survey date exceeds one year. The sticker contains basic information about hood performance as of the survey date and should be consulted each time the hood is used.

The EHS Hood Number is a unique identifier for the particular hood. Refer to this number when communicating issues/problems about a particular hood.

The Inspection Sticker is aligned on the hood so the arrow indicates the proper location for the maximum safe sash position.

Static Pressure Gauge (Magnehelic)

Most fume hoods are equipped with static pressure gauges that measure the difference in static pressure across an orifice in the duct, or between the laboratory and the fume hood exhaust duct. Most of the devices are aneroid pressure gauges, such as magnehelics, that are typically mounted on the front of the hood above the sash.

The gauge reads in units of pressure (e.g. mm Hg), rather than velocity. Changes in the magnehelic reading are not linearly proportional to changes in face velocity; therefore it should only be used as an index of hood performance.

The magnehelic gauge reading at the time of the most recent hood survey is shown on each fume hood survey sticker. A difference of 20% or more in the magnehelic reading from that shown on the sticker is an indication that the flow rate in the duct, and thus the face velocity, may have changed significantly since the last survey. If the user notices such a change, or has any other reason to suspect that the hood is not operating properly, contact EHS at 258-5294 for an evaluation of the hood. Please refer to the hood number when calling.

Color Coded and Digital Flow Indicators

Some hoods are equipped with color indicating devices, or digital flow rate displays rather than or in addition to magnehelic gauges. These devices constantly measure the face velocity of the hood and point to green (for good) or red, or give the actual digital flow rate value.
 

Frick Chemistry Laboratory

Waldner hoods are measured at the upper sash stop (though this higher opening should only be used for loading and unloading). A PASS/FAIL sticker with date is affixed near the alarm lights. The magnehelic is marked to indicate proper reading.

Fisher hoods in the teaching labs are measured at the normal operating sash height stop and a PASS/FAIL sticker with date is affixed.

The number to use when referencing a hood in Frick is the Maximo number on the yellow sticker in the upper left hand corner of the hood sash.

If hood performance is judged to be unsuitable for use with hazardous chemicals, a sticker with this information is placed on the hood instead of the survey sticker.

Hoods are routinely inspected at least annually. If a hood fails during a routine annual survey EHS will arrange for its repair. If it has been more than a year since the last inspection, contact EHS.

 

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 follow:

The hood user should know the normal operating configuration (NOC) of the hood and should design experiments so that this configuration can be maintained whenever hazardous materials are at risk. The NOC refers to the position of the sash established when the hood was installed and certified (i.e. how far open is the maximum safe sash position). This is where the survey sticker with arrow is located.

Before using the hood, the user should check the hood survey sticker to determine where the sash should be positioned for optimum containment for that particular unit.

The hood user should also check the magnehelic gauge or other hood performance indicator and compare its reading to the reading indicated on the hood survey sticker. If the reading differs significantly (20% or more for a magnehelic gauge) from that on the sticker the hood may not be operating properly. Contact EHS with hood location and contact information.

Never use a hood to control exposure to hazardous substances without first verifying that it is operating properly.

To optimize the performance of the fume hood, follow the practices listed below:

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, etc., interfere with airflow at the face of the hood.

Poor	Good	Best
Poor placement of materials.	Good placement of materials.	Best placement of materials.
Images from Kewaunee Fume Hoods

Keep the sash completely lowered any time an experiment is in progress and the hood is unattended. Note: Lowering the sash provides additional personal protection from projectiles and can also result in significant energy conservation. Keep the hood sash clean and clear.

Check area around the hood for sources of cross drafts, such as open windows, supply air grilles, fans and doors. Cross drafts may cause turbulence that can allow leaks from the hood into the lab. Also, avoid placing fans or equipment with fans in the hood in an orientation that causes the fan to blow out of the hood as the inward flow is unlikely to be strong enough to keep air and possible contaminants from escaping out of the hood. 

Extend only hands and arms into the hood and avoid leaning against it. If the hood user stands up against the face of the hood, air currents produced by turbulent airflow may transport contaminants into the experimenter's breathing zone.

Visually inspect the baffles (openings at the top and rear of the hood) to be sure that the slots are open and unobstructed. Keep baffles and other ventilation openings clean, free of accumulating dust, and unobstructed by collections of numerous containers. The work surface of a fume hood is not the place to store materials. It is meant to keep hazardous materials from being inhaled during manipulation.

For optimum performance, adjust the baffles when working with high temperature equipment and/or heavy gases or vapors. See figure below for suggested baffle positions.

Normal	Hot Work	Heavy Gases
Normal baffle positioning	Baffle position to use for hot work	Baffle position for heavy gases

                                                                                                   

Normal baffle position all slots are evenly opened.

High temperature work such as using hot plates; lower slots are minimized since heated vapors tend to rise

Heavy gasses and vapors are better captured when upper slots are minimized

Do not block 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. See figure below.

Poor Placement	Good Placement
Poor large equipment placement.	Good large equipment placement.

 

Place large or bulky equipment near the rear of the fume hood. Large items near the face of the hood may cause excessive air turbulence and variations in face velocity.

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.

Provide secondary containment for containers that could break or spill, to minimize the spread of spilled liquids. Dishpan type containers are provided by EHS upon request. Contact EHS or use the Safety Supply Order Form (login required).

Items contaminated with odorous or hazardous materials should be removed from the hood only after decontamination or if placed in a closed outer container to avoid releasing contaminants into the laboratory air.

When using cylinders containing highly toxic or extremely odorous gases, purchase only the minimal practical quantity. Consider using a flow-restricting orifice to limit the rate of release in the event of equipment failure. In some circumstances, exhaust system control devices or emission monitoring in the exhaust stack may be appropriate.

All electrical devices should be connected to receptacles located outside the hood to eliminate the potential for electrical arcing that could ignite a flammable or reactive chemical.

DO NOT USE A HOOD FOR ANY FUNCTION FOR WHICH IT WAS NOT INTENDED. Certain chemicals or reactions require specially constructed hoods. Examples are perchloric acid or high-pressure reactions. Most special use hoods are labeled with the uses for which they are designed.

Common Misuses & 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 when used as designed, the average fume hood does have several limitations.

• Projectiles: A fume hood is not designed to contain high velocity releases of particulate contaminants unless the sash is fully closed.
• Pressurized systems: Gases or vapors escaping from pressurized systems may move at sufficient velocity to overcome the capture velocity and 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 anchored barriers, shields or enclosures of sufficient strength to deflect or contain it. Such barriers can significantly affect the airflow in the hood.
• 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. Shock-sensitive 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.
• Air Foil Sills: Many fume hoods are equipped with flat or rounded sills or air foils which have been specifically designed to direct the flow of air smoothly across the work surface. Sills should not be removed or modified by the hood user. Objects should never be placed on these sills. Materials released from containers placed on the sills may not be adequately captured. In addition, an object on the sill may prevent the quick and complete closure of the sash in an emergency.
• Spill Containment Lips: Most modern fume hoods have recessed work surfaces or spill containment lips to help contain minor liquid spills. In many cases, these lips are several inches wide. Containers of liquids should not be placed on the hood lip.
• Tubing for Exhaust: Tubing is frequently used to channel exhaust to the hood from equipment located some distance away. This is not an effective control method.
• Connections to the Exhaust System: Occasionally, a researcher may need local exhaust ventilation other than that provided by an existing fume hood. A new device may not be connected to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device.
• Microorganisms: Work involving harmful microorganisms should be done in a biosafety cabinet, rather than a chemical fume hood. See the Biosafety Manual for more information.
• 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.
• Pollution Control: An unfiltered 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 as appropriate.
• Waste Disposal: A fume hood should not be used for waste disposal. It is a violation of environmental regulations to intentionally allow volatile waste to evaporate and escape up the hood stack. As described above, the hood is not a pollution control device.

The Fume Hood as a Storage Device

Fume hoods are designed specifically to provide ventilation for the protection of lab occupants during chemical manipulations. The airflow they provide is greatly in excess of that needed for storage of closed containers of even the most toxic of volatile materials. Storing materials in this way is, therefore, a misuse of an expensive piece of equipment.

In general, the storage of chemicals in fume hoods is strongly discouraged. The realities of available space and equipment in some laboratories may make it difficult or impossible to completely prohibit the use of hood workspaces for storage. In such a case, the following general policy is recommended:

Hoods Actively in Use for Experimentation

Storage of materials should be minimized or eliminated altogether. Materials stored in the hood can adversely affect containment. In addition, the hood is frequently the focus of the most hazardous activities conducted in the laboratory. The presence of stored flammable or volatile, highly toxic materials can only exacerbate the problems resulting from an explosion or fire in the hood. Even if they are not directly involved in such an event, attempts to control or extinguish a fire may result in the spilling of stored materials.

Hoods Not in Active Use

Materials requiring ventilated storage (e.g., volatile and highly toxic, or odorous substances) may be stored in a hood if they are properly segregated and the hood is posted to prohibit its use for experimental work.

Alterations to Fume Hood

Changes or additions to an existing fume hood without the explicit approval of the department's facilities manager or Special Facilities supervisor is prohibited. Adding devices to even the simplest exhaust system without adequate evaluation and adjustment will usually result in decreased performance of the existing hood and/or inadequate performance of the additional device. Additionally, some components of older hoods may contain asbestos and therefore should not be damaged.

All fume hoods are assigned to a principal investigator and lab or shop group in SHIELD. They can be found under the ‘Equipment’ tab on the main lab/shop page.

If you have questions or do not see your hood listed, contact [email protected]. 

Click on the “View” link to open the equipment detail page to review important information corresponding to the hood. The current and all previous annual performance check reports for the hood are available to be reviewed here. Documents or other service records can also be uploaded and shared/viewed as you wish.