SECTION 6B: Fume Hoods and Laboratory Ventilation

• Fume Hoods and Laboratory Ventilation
• How a Fume Hood Works
• Using Chemical Fume Hoods
• Good Work Practices
• Common Misuses and Limitations
• Hood Performance Indicators
• Evaluation and Maintenance
• Other Laboratory Exhaust Systems

 

Fume Hoods and Laboratory Ventilation (top)

One of the primary safety devices in a laboratory is a chemical fume hood. A well-designed hood, when properly installed and maintained, can offer a substantial degree of protection to the user, provided that it is used appropriately and its limitations are understood.

This section covers a number of topics aimed at helping laboratory workers understand the limitations and proper work practices for using fume hoods and other local ventilation devices safely.

There are basically two types of fume hoods at Princeton, they are:
Constant volume – where the exhaust flowrate or quantity of air pulled through the hood 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 hood increases proportionally. Thus, higher face velocities can be obtained by lowering the sash.

And variable air volume (VAV) - where the exhaust flowrate or quantity of air pulled through the hood varies as the sash is adjust in order to maintain a set face velocity. Therefore, when the sash is lowered and the cross-sectional area of the hood opening decreases, the velocity of airflow (face velocity) through the hood stays the same while less total air volume is exhausted.

 

How a Fume Hood Works (top)

 

 

 

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 in the hood through ductwork connected to the hood 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 direct the air being exhausted. In many hoods, they may be adjusted to allow the most even flow. It is important that the baffles are not closed or blocked since this blocks the exhaust path.

The airfoil or beveled frame around the hood face allows more even airflow into the hood by avoiding sharp curves that can create turbulence.

In most hood installations, the exhaust flowrate or quantity of air pulled through the hood 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 hood increases proportionally. Thus, higher face velocities can be obtained by lowering the sash.

Using Chemical Fume Hoods (top)

A fume hood is used to control exposure of the hood user and lab occupants to hazardous or odorous chemicals and prevent their release into the laboratory. A secondary purpose is to limit the effects of a spill by partially enclosing the work area and drawing air into the enclosure by means of an exhaust fan. This inward flow of air creates a dynamic barrier that minimizes the movement of material out of the hood and into the lab.

In a well-designed, properly functioning fume hood, only about 0.0001% to 0.001% of the material released into the air within the hood actually escapes from the hood and enters the laboratory.

When is a Fume Hood Necessary?

The determination that a fume hood is necessary for a particular experiment should be based on a hazard analysis of the planned work. Such an analysis should include:

• A review of the physical characteristics, quantity and toxicity of the materials to be used;
• The experimental procedure;
• The volatility of the materials present during the experiment;
• The probability of their release;
• The number and sophistication of manipulations; and
• The skill and expertise of the individual performing the work.

 

Good Work Practices (top)

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:

1. Adequate planning and preparation are key.The hood user should know the Standard Operating Configuration (SOC) of the hood and should design experiments so that the SOC can be maintained whenever hazardous materials might be released. The SOC refers to the position of the sash. A schematic drawing of the SOC is displayed on the front of each chemical fume hood.
2. 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.
3. 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 (15% or more for a magnehelic gauge) from that on the sticker, the hood may not be operating properly.

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, obtain 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.

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.

Images from Kewaunee Fume Hoods

Bad placement of materials. Good placement of materials. Best placement of materials.

• • Provide catch basins for containers that could break or spill, to minimize the spread of spilled liquids.
  • Keep the sash completely lowered any time an experiment is in progress and the hood is unattended. Note: Lowering the sash not only provides additional personal protection, but it also results in significant energy conservation.
  • Never use a hood to control exposure to hazardous substances without first verifying that it is 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.  See figure below for suggested baffle positions.

 

 

Normal baffle position - all slots are open.	Slot position for high temperature equipment, such as hot plates.  Lower slot is minimized since heated vapors tend to rise.	Slot position for heavy gases and vapors.  Upper slot is minimized.
Images from Kewaunee Fume Hoods

• 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.

Images from Kewaunee Fume Hoods

Poor placement of large equipment Good placement of large equipment

• • 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. See Use of Hood as a Storage Device for more information.
  • 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.
  • 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.
  • Clean all chemical residues from the hood chamber after each use.
  • All electrical devices should be connected outside the hood to avoid electrical arcing that can 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. See Common Misuses of Fume Hoods for more information.

 

Common Misuses and Limitations (top)

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

• Particulates: 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 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. 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 washdown system must be used for such work. 
• Air Foil Sills: Many fume hoods are equipped with flat or rounded sills or air foils which 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.
• Horizontal Sliding Sashes: The hood user should never remove sliding sashes. Horizontal sash hoods are designed and balanced with no more than half the face open at any time. Removal of sashes may reduce the face velocity below acceptable levels.
• 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. 
• 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. 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. See Flammable Materials for more information about proper storage of flammable, toxic, or odorous chemicals.

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 the containment provided. 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.

 

Hood Performance Indicators (top)

All fume hoods at Princeton University are equipped with some type of continuous airflow monitoring device, either in the form of a magnehelic gauge, a color coded flow indicator or a face velocity monitor. Some are equipped with alarms.

Each hood also has a survey sticker with important information to help determine whether the particular hood is functioning properly and is appropriate for the work to be performed.

Continuous Monitoring Devices

Static Pressure Gauge (Magnehelic)

 

 

 

The gauge is a flow rate indicator with a scale that reads in units of pressure, 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 evaluation sticker. A difference of 15% 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 a re-survey of the hood.

Color Coded Flow Indicators

Some hoods are equipped with FlowSafe devices, rather than magnehelic gauges. This device constantly measures the face velocity of the hood and, using a needle that either points to green (for good) or red, indicates whether or not the hood is functioning properly.

 

 

Face Velocity Monitors

Some of the newer hoods have constant face velocity measuring devices. An LED readout of the face velocity is found on the device on the top corner of the hood opening. The readout indicates the actual face velocity of the hood, and should be a negative number, to reflect that the direction of flow is negative, into the hood, rather than positive, out of the hood.

 

 

Alarm-Equipped Hoods

Many hoods in areas of Frick, Moffett and E-Quad are equipped with sash position alarms. These hoods are designed to operate with the hood sash lowered to approximately 20 inches above the base of the hood, in order to conserve energy by exhausting air at a lower flow rate than would otherwise be necessary.

When the sash is raised above 20 inches, a buzzer will sound and a red light will begin flashing, alerting the hood user and other laboratory occupants that the hood face velocity is now likely to be below 100 feet per minute. In the event that the sash must be raised above 20 inches, such as when large equipment must be installed or removed, the buzzer can be turned off manually, but the light will continue flashing until the sash is lowered below the 20 inch mark.

All chemical manipulations performed in an alarm equipped hood should be done with the sash opening at 20 inches or less.

Hood Survey Sticker

Every chemical fume hood on campus should have a survey sticker affixed to the front of the hood in a conspicuous location. The sticker contains basic information about hood performance as of the most recent survey 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 discussing problems with a particular hood.

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

The Flow Monitor Reading is the reading of the magnehelic gauge or other continuous monitoring device at the time of the survey. Where the hood has two possible exhaust rates, as is the case for some hoods in Frick, the reading corresponding to each rate may be indicated as, for example, 0.31/0.42.

The Inspected on date is the date of the last hood survey. Hoods that have not been surveyed within the past year should not be used until tested by EHS.

The By line gives the name of the EHS technician who surveyed the hood.

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.

 

Do not use a hood that has no survey sticker. If a survey is needed, call EHS at 258-5294.

 

Evaluation and Maintenance Program (top)

Hood Surveys

EHS surveys each fume hood annually. The face velocity of the fume hood is measured with the sash in the Standard Operating Configuration (SOC). The inspection sticker is positioned on the hood so the arrow is in the proper location for the maximum safe sash position. The reading of the continuous monitoring device is recorded on the hood sticker.

After each performance survey, a written report of the results is furnished to the individual responsible for the hood (e.g., the Principal Investigator or laboratory manager), the Chemical Hygiene Officer for the department, and the Special Facilities staff for the laboratory building.

When Problems are Noted

There are several factors that can affect the performance of the hood, resulting in low face velocity or turbulent airflow. These include mechanical problems or exhaust slots blocked by large objects or excessive storage.

If a problem is found during the hood survey, a written notice will be provided on-site to the laboratory or taped to the sash of the fume hood. If the problem requires the need for work practice changes (e.g., blocked exhaust slots or excessive storage), the laboratory worker should make the recommended changes and call EHS at 258-5294 to have the hood resurveyed.

If maintenance is necessary, the laboratory worker may send a copy of the written notice to the building Special Facilities staff to request maintenance. EHS does not initiate maintenance or ensure that it is completed. Special Facilities will contact EHS when the work is complete to have the hood resurveyed.

Requesting Maintenance

Providing maintenance for fume hoods is a function of the Facilities Department, and is performed by Special Facilities personnel and the MacMillan shops. Since the hood user is the person most aware of how a hood is being used on a day to day basis, it is the responsibility of the hood user to determine that maintenance is necessary and to request that it be performed.

If a hood user believes that the hood is not performing adequately, the following steps should be taken:

1. An inadequate face velocity may result from obstructions to the airflow in the hood. These may be caused by large quantities of equipment in the hood or by paper or other material drawn into the exhaust slots. The user should first check for such obstructions and remove or modify them.
2. The user may obtain initial maintenance through Special Facilities. If Special Facilities is unable to correct the problem, they will seek assistance from the MacMillan maintenance shops.
3. The hood sash should be lowered until repairs are complete. Place a sign on the hood reminding users not to use the hood.
4. If maintenance efforts are not sufficient to correct the deficiency, engineering changes may be necessary. When notified of such a situation, the user or a department representative should request an evaluation of the problem by the Facilities Engineering Department.

 

Other Laboratory Exhaust Systems (top)

Many laboratories use equipment and apparatus that can generate airborne contaminants, but cannot be used within a fume hood. Examples include gas chromatographs, ovens, and vacuum pumps.

Other types of local exhaust ventilation systems may be required to control contaminants generated by these operations. Such systems must not be installed without explicit approval of the building facility manager, Facilities Engineering and/or maintenance personnel. See Common Misuses of a Fume Hood for more information.

Elephant Trunks

An elephant trunk is a flexible duct or hose connected to an exhaust system. It can only capture contaminants that are very close to the inlet of the hose, typically less than a distance equal to one half of the diameter of the duct.

Elephant trunks can be effective for capturing discharges from gas chromatographs, pipe nipples or the end of tubing. However, the effectiveness of the elephant trunk should be carefully evaluated before they are used to control releases of hazardous substances.

Canopy Hoods

A canopy hood in a laboratory is constructed in a similar fashion to the overhead canopy hoods seen in kitchens. In order for the canopy hood to be able to capture contaminants, the hood requires a relatively large volume of air movement, making them somewhat costly to operate. The canopy hood works best when the thermal or buoyant forces exist to move the contaminant up to the hood capture zone.

 

 

One of the biggest problems with canopy hoods is that, in most cases, they are designed such that the contaminated air passes through the individual's breathing zone. The airflow is easily disrupted by cross currents of air.

For the most part, canopy hoods should only be used for exhaust of non-hazardous substances.

Slot Hoods

There are many types of slot hoods, each suited for different types of operations. In general, a slot hood requires less airflow than a canopy hood and is much more effective than an elephant trunk or canopy hood, when installed properly.

 

 

Slot hoods are best used for operations that require more working room than a fume hood and where a limited number of low toxicity chemicals are used. The placement of the opening(s) and the velocity of airflow are based on the application, particularly dependent upon the vapor density of the chemical(s).

Examples of good uses for slot hoods are darkrooms and acid dipping operations.

Downdraft Hoods

Downdraft hoods or necropsy tables are specially designed work areas with ventilation slots on the sides of the work area. This type of system is useful for animal perfusions and other uses of chemicals with vapor densities heavier than air.

 

 

Toxic Gas Cabinets

Highly toxic or odorous gases should be used and stored in gas cabinets.In the event of aleak or rupture, a gas cabinet will prevent the gas from contaminating the laboratory.

Gas cabinets should be connected to laboratory exhaust ventilation using hard duct, ratherthan elephant tubing, since such tubing is more likely to develop leaks. Coaxial tubing should be used for delivering gas from the cylinder to the apparatus. Coaxial tubing consists of an internal tube containing the toxic gas, inside another tube. In between the two sets of tubing is nitrogen, which is maintained at a pressure higher than the delivery pressure of the toxic gas. This ensures that, in the event of a leak in the inner tubing, the gas will not leak into the room.

 

 

Glove Box

There are two general types of glove boxes, one operating under negative pressure, the other operating under positive pressure. Glove boxes consist of a small chamber with sealed openings fitted with arm-length gloves. The materials are placed inside the chamber and manipulated using the gloves.

 

 

A glove box operating under negative pressure is used for highly toxic gases, when a fume hood might not offer adequate protection. A rule of thumb is that a fume hood will offer protection for up to 10,000 times the immediately hazardous concentration of a chemical. The airflow through the box is relatively low, and the exhaust usually must be filtered or scrubbed before release into the exhaust system.

A glove box operating under positive pressure may be used for experiments that require protection from moisture or oxygen. If this type of glove box is to be used with hazardous chemicals, the glove box must be tested for leaks before each use. A pressure gauge should be installed to be able to check the integrity of the system.

Biosafety Cabinets

A conventional fume hood should not be used for work with viable biological agents. A biosafety cabinet is specially designed and constructed to offer protection to both the worker and the biological materials.

 

 

 

Similarly, a biosafety cabinet should generally not be used for work with hazardous chemicals. Most biosafety cabinets exhaust the contaminated air through high efficiency particulate air (HEPA) filters back into the laboratory. This type of filter will not contain most hazardous materials, particularly gases, fumes or vapors. Even when connected to the building exhaust system, a ducted biosafety cabinet may not achieve a face velocity of 95 - 125 feet per minute, making it inappropriate for use with hazardous chemicals.

Ductless Fume Hoods

Use of a "ductless fume hood" is strongly discouraged. These devices work by using a fan to draw air into a chamber, through one or more filters, and back into the laboratory. EHS and several professional safety and engineering organizations do not recommend the use of ductless fume hoods for several reasons. First, it is difficult to determine whether the filters are functioning adequately or need to be changed; thus, the potential for recirculating toxic materials into the laboratory is significant. In the event of a chemical spill, the hood is usually not able to contain the spilled material or the potentially high concentrations of chemical vapors.

Second, the face velocity of the hood is normally below 80 feet per minute. The hood is normally designed such that the air does not flow smoothly and evenly through the hood. Both of these characteristics make it likely for disruption of airflow or turbulence, causing unfiltered air to leak into the laboratory.

Clean Benches

Clean benches are similar to appearance as a fume hood however do not exhaust air from the laboratory. A clean bench is a device that draws air from the lab through a HEPA filter and vents the filtered air downwards onto a work surface to keep the materials within free from particulate contamination. These devices are not to be used with hazardous materials as they provide no personal protection. Do not store materials on top of this hood as this will block the filter, overload the motor, and provide poor product protection.