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Environmental Health and Safety
The University of Mississippi

Chemical Fume Hoods

Chemical Fume Hood Guide:  Use, Design, Construction, and Maintenance

The University of Mississippi, Department of Health & Safety (DHS) has adopted the following policies and procedures for the use, design, installation, renovation, maintenance, and dismantling of chemical fume hoods on the UM campus and affiliated facilities.

Use of a Chemical Fume Hood

Questions and Answers

Some of the most common problems encountered in the use of fume hoods are included below. If you have any other problems concerning a fume hood, please call DHS (5433).

Who do I call if my fume hood does not appear to be working?

The Electric Shop of the University of Mississippi Physical Plant Department performs all fume hood maintenance. Your department needs to submit a notification with the Electric Shop to have the hood maintenance performed.

If your fume hood has an Airflow Indicator, check it to see if it operating in the normal range. If the Airflow Indicator shows that the hood is not working properly or you feel the indicator is incorrect, call DHS (5433). If your hood does not have an airflow indicator, call DHS (5433).

Can I use a strip of tissue paper to see if my hood is working?

The use of a strip of tissue paper will only give you a qualitative answer, that the fan is running, not a quantitative measure of the face velocity of the hood. A hood with an average linear flow rate of 75 linear fpm will still pull a tissue into the hood when attached to the sash edge, but it will be below the minimum average rate of 100 linear fpm required to work with most hazardous materials such as flammable liquids, carcinogens and some types of radioactive materials.

What is the proper flow rate of a hood where hazardous materials are used?  I thought that the faster the flow rate the safer it will be.

The proper flow rate is an average linear flow rate from 100 to 150 fpm. This average is taken from a minimum of 9 measurements. No measurement should be less than 75 fpm or more than 200 fpm. This is to assure adequate airflow to prevent vapors from escaping when you withdraw your arm from the hood and to prevent turbulence from excessive airflow around equipment or containers. Faster airflows can cause greater turbulence, which can actually blow vapors from inside the hood into the lab.

The alarm on my airflow monitor is constantly buzzing, can I just tape down the mute button?

The alarm is present to notify you that the hood is not working properly. If you disable the alarm, you may be exposing yourself and others in your lab to hazardous materials. Many times the airflow monitor alarm goes off even though the hood is working correctly. This can be due to an incorrectly calibrated airflow monitor, but how do you really know if the hood is working properly or not without having it checked?  Please do not disregard a hood alarm without knowing for sure what the problem is.

What is wrong with my hood if I can hear the motor running but the hood is not working?

Sometimes the motor can be heard, even though the fan belt is broken. Have your departmental office turn in a notification to the Physical Plant Electrical Shop for a belt replacement.

I can smell the halogenated solvent in the lab, whenever I use it in the hood. The hood was checked recently and it was fine. What is wrong with the hood?

Check the hood baffles. Most hoods have adjustable baffles to differentiate between heavier than air vapors and lighter than air vapors. If all of the hood suction is directed to the upper area of the hood, the higher density halogenated solvent vapors will collect at the bottom of the hood and spill over onto the floor. Most lab hoods should be set in the middle of the two extremes for general use. If you have any concerns regarding the adjustment of the hood baffles, please call DHS (5433).

My hood has recently failed a survey and I was told that the reason is because there is too much equipment and chemical containers in my hood. How can I do my work if I clean out my hood?

First, you must understand what hoods are designed to do. Hoods keep nasty things out of your face by trying to pull a uniform layer of air from the frontal area of the hood into the slots located in the back of the hood. If too much of the frontal area or back slots are blocked by bottles and equipment, it can either stop the movement of air into the hood or cause turbulence at the face of the hood. This can stop the hood from doing what it is designed to do, which is to prevent hazardous vapors from entering your lab. If necessary equipment is positioned to allow some airflow under and around the device, the hood will be able to do its job. Remember to only keep the chemicals you are currently using in the hood, store the remaining chemicals in an approved safety cabinet. Also remember to place all objects at least 6 inches back from the front sash. If you need help regarding the placement of equipment in hoods, please call DHS (5433).

My hood is near a door that is always open. Sometimes I have had colleagues tell me that they can smell chemicals in the hallway near my lab. Is my hood not working properly?

Your hood may not be working properly, but it is more likely that air is being drawn from the lab into the hallway past your hood face. This flow of air across the face of the hood can actually pull vapors into the lab and into the hall. The same thing can happen when people walk past the face of a hood or a nearby air conditioner or fan is blowing across the face of a hood. The best solution is to close the door, to prevent the cross flow of air from pulling solvent vapors into the room and hall.

I work with some pretty smelly chemicals and I need to keep them in a hood or the whole building will stink. Is this a problem?

No. Some hoods can be used for storage only, but the containers need to be positioned so that they do not completely block the frontal area of the hood or the slots in the back of the hood. The containers should also be positioned to allow airflow around the bottom and sides to prevent turbulence. For further advice regarding the long-term storage of chemicals in hoods, please call DHS (5433).

Special Uses

I. Perchloric Acid

Perchloric Acid is a very strong oxidizer, which in contact with organic materials can form an explosive reaction product. For this reason, special construction materials are required for chemical fume hoods in which substantial quantities of perchloric acid are frequently used. For additional information or consultation, contact the DHS.

A. Chemical Fume hoods designated for use with perchloric acid should be clearly identified with a warning sign.

I. The fan motor must be located outside of the air stream.

II. Radioactive Materials

The Radiation Safety Officer (RSO) recommends that all laboratory processes, using dispersible radioactive materials, be performed in a certified fume hood. Any use of Tritiated water in amounts > 1 Curie (3.7 x 10E10 Bq) or use of tritiated nucleotide precursors in amounts > 100 mCi (3.7 x 10E9 Bq) must use a hood or glovebox. If you are unsure of whether a radioactive material needs to be used in a hood, please call DHS (5433).

A. Iodination mini hoods must be located within an already operative chemical fume hood.

NOTE: If you are unsure of the suitability or performance of a chemical fume hood anytime prior to or during the use of radioactive materials, please call DHS (5433).

III. Carcinogens

The use of carcinogens requires a working chemical fume hood. Before anyone can work with or obtain a carcinogen from a stockroom, they must be approved to use carcinogens, which requires submitting an “Application to Use Carcinogens” Form. Please call the DHS (5433) or check the DHS webpage to obtain the form.

NOTE: If you have any question regarding the safety of a chemical fume hood in which carcinogens are to be used, please call DHS (5433).

Construction, Installation, and Renovation

I. Laboratory Design

A. Laboratories with fume hoods must be designed to have no recirculation of air to that lab or any other spaces. Recirculation of laboratory air will result in indoor air quality problems.

E. Windows in labs containing fume hoods must be fixed closed. Breezes coming in through open lab windows can adversely affect the proper functioning of a hood. Turbulence caused by these wind currents can easily bring the contaminated air inside the hood out into the lab.

II. Supply Air

A. Before a new fume hood is put into operation an adequate supply of make-up air must be provided to the lab. A fume hood exhausts a substantial amount of air. For this reason additional make-up air must be brought into the room to maintain a proper air balance.

F. Supply air intake locations must be located a minimum of 50 feet from the discharge point of any fume hood. Intakes should also not be located in the vicinity of loading docks, generators or other devices generating harmful emissions. Re-entrainment of emissions will cause obvious indoor air quality problems.

III. Fume Hood Construction and Installation

A. Chemical storage cabinets directly under a newly installed or renovated fume hood shall be ventilated to the fume hood exhaust. Cabinets shall be vented using stainless steel flexible ducting connected above the hood at the exhaust duct collar.   All penetrations must be sealed.

P. Noise from fans, ductwork and air velocities shall not exceed 65 dBA inside the lab area.

IV. Ductwork

A. Gang ducting of fume hoods is not recommended unless required by special conditions or in new buildings properly designed for this purpose. These must be properly designed with final approval from the DHS. It is difficult to keep each fume hood on a common duct balanced and working properly. If the fume hoods are in different ventilation zones within the building, air balancing becomes virtually impossible. Additionally, if the fan serving a gang-ducted system shuts down, chemical contaminants can  “backflow” from one lab to another. The entire system must be shut down during servicing and repair work, causing considerable user inconvenience.

Minimum 18 gauge, Type 316 stainless steel. Coated galvanized steel may be considered for non-hospital installations under special conditions. However, special attention to quality control is required.

  • Heliarc inert gas with Type 316 welded seams.
  • Follow Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) Round Industrial Duct Construction Standards for duct supports and reinforcement using stainless steel material.
  • Follow SMACNA 1985 HVAC Duct Construction Standards using Type 316 stainless steel for exhaust stack on roof.

E. New duct installations must be tested at negative pressure, 1 1/2 times its operating pressure (per SMACNA). Test should show zero leakage.

H. Duct velocities should be maintained below 1,600 feet per minute (fpm) to minimize noise.

V. Exhaust Fan

A. New exhaust fans should be oriented to exhaust in an upward direction. Any other type of fan orientation increases the workload required from the fan.

C. Fans shall be constructed of materials compatible with the chemicals being transported in the air through the fan. If flammable gas, vapor or combustible dust is present in concentrations above 20% of the Lower Explosive Limit (LEL), the fan construction shall be as recommended by AMCA’s Classification for Spark Resistant Construction.

VI. Exhaust Stack

A. Fume hood exhaust stacks must extend at least seven feet, (10 ft. is preferred) above the roof or at least two feet above the top of a parapet wall, whichever is greater. Discharge must be directed vertically upward.

E. Exhaust stacks should be secured with earthquake restraints, where applicable.

VII. Plumbing

A. All plumbing utilities must have a shut-off valve or cock adjacent to the hood.

D. Waste outlet should be connected into the building’s acid resistant laboratory waste system, if present.

VIII. Electrical

A. Electrical outlets must be outside the hood. The atmosphere inside a fume hood may contain flammable gases or vapors, which can ignite, resulting in a fire or explosion. For this reason, any activity—including plugging into and unplugging from an electrical outlet—which may produce a spark, must be performed outside the hood.

B. Lighting fixtures should be of the fluorescent type. Fluorescent bulbs give off less heat than conventional bulbs. They help maintain a safe and comfortable work area inside the hood.

C. Light fixtures should be sealed and vapor tight, UL-listed, and protected by a transparent impact resistant shield. The potential for flammable or combustible atmospheres requires explosion-proof electrical equipment.

IX. Sashes

A. Sashes may be horizontal, vertical, or a combination, and should have the capability to completely close off the hood face.

Laminated safety glass, when internal temperature is anticipated to be less than 160°F.

  • Tempered safety glass when high internal temperatures are anticipated that will result in sash surface temperatures greater than 160°F.

NOTE: Polycarbonate (Lexan) is not suitable for fume hood sashes due to poor chemical resistance to some commonly used reagents. In addition, it is a combustible plastic, which is not intended for use when safety glass is required as a fire stop. Existing Lexan sashes will be replaced as necessary.

Servicing and Dismantling

During routine servicing and repair or dismantling of a chemical fume hood the potential exists for exposure to hazardous substances that had been used or stored in the hood. To guard against this, certain protective measures, appropriate to the specific situation, should be implemented before the work begins.

I. Before a fume hood is serviced or dismantled Lab personnel must:

A. Remove all equipment in the hood, which may impede or impair access.

B. Remove all chemicals and radioactive materials in the hood, which may pose a hazard.

C. If necessary, decontaminate the interior of the hood as appropriate.

D. If necessary, don PPE (Personal Protective Equipment:  such as goggles, respirator, gloves, coveralls or arm guards).

E. If the hood fume needs to be turned off, notify laboratory workers and post an “Attention, This Hood is Being Repaired” Notice on the hood.

Note : The iodination mini hood housed within a fume hood should be shut down and covered with plastic whenever the fume hood requires servicing, this includes servicing at the roof.

A designated person from the laboratory is responsible for ensuring that the procedures mentioned above have been done. Upon completion of the necessary decontamination procedures, the responsible party must notify the person listed at the bottom of the Hood Repair Notice so that repair work may begin.

The DHS may be consulted regarding appropriate decontamination procedure.

II. Fume Hood Service Procedure

The following procedures are to be followed by anyone who must service any part of a fume hood system at the University. (Service includes mechanical work, sheet metal work, painting, and electrical work.)

A. Locate the appropriate fan or motor on the roof that needs service and the room in which the fume hood is housed. Electricians should have hood-to-fan information. A specific fume hood and the room in which it is housed can be identified by either a fan number marked on the fume hood structure or a room number marked on the fan housing.

B. Communicate to lab personnel the need to service the fan or hood and obtain permission to shut down the hood. If lab personnel are not available, contact the building manager or Department Chair to obtain permission to shut down the hood.

Do Not Turn Off Fan Without Permission From An Authorized Person and after all hazardous materials have been removed from the fume hood.

C. Fill out a Hood Repair Notice and fix it to the hood sash. Then shut down the fan. Note: Information on the tag should include:

  • Date of shutdown
  • Building location
  • Room number
  • Estimated time period when hood will be off
  • Your name or your supervisor’s name and phone number

D. If the fan to be worked on is located near hood exhaust stacks, which do not have a 7 to 10 ft. extension, those fans must also be turned off. If this is not possible, an appropriate respirator and safety glasses supplied by your supervisor are to be used (consult DHS at 5433).

E. After service is completed, restart the fan and remove the Hood Repair Notice from the fume hoods.

Testing and Certification

The requirements for testing and initial certification of new campus hoods are described above. Existing hoods are required to have their performance tested and certified on an annual basis.

A. DHS is responsible for performing the annual testing and certification of campus fume hood performance to meet campus requirements. DHS is responsible for maintaining testing and certification records.

B. Once a hood has been certified, DHS will attach a small inspection tag. This tag will list the class, average face velocity and date of testing. Each hood will also be provided with a green sticker, which contains helpful reminders concerning the use of your hood and who to contact in an emergency.

C. If hood performance is significantly outside the limits of acceptable performance, DHS will post the hood as Out of Service for continued use and inform lab personnel. The Department in charge of the lab submits the work order for the repair of a hood. After Physical Plant has performed repairs or adjustments to the hood, DHS will retest and certify the hood.

D. If you have a fume hood that has not been inspected in the past year, or still has an Out of Service notice and has not been repaired, please contact the DHS office (5433).

References

Air Movement and Control Association, Inc. (AMCA)

American Industrial Hygiene Association, American National Standard for Laboratory Ventilation, 1992

American Conference of Governmental Industrial Hygienists (ACGIH), Industrial Ventilation Manual, 1989 edition.

American National Standard Institute, Laboratory Ventilation, ANSI Z9.5-1992.

American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE), Fundamental Handbook.

American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE), ASHRAE 110, Methods of Testing Performance of Laboratory Fume Hoods.

National Fire Protection Association (NFPA), Blower and Exhaust Systems, NFPA No. 91-1973.

National Fire Protection Association (NFPA), Fire Protection for Laboratories Using Chemicals, NFPA 45.

Sheet Metal and Air Conditioning Contractors National Association, Inc. (SMACNA)

Hood Information Sticker

The following reminder label should be affixed to your fume hood. It highlights the most frequently asked questions. If your sticker is missing or damaged, contact Health and Safety (5433) for a replacement.

USE YOUR HOOD WISELY! Help Prevent Air Pollution – Minimize use of volatile chemicals. Keep containers properly labeled and closed. Do not evaporate hazardous waste or other volatile chemicals. Obey Fume Hood Repair signs.

For your safety : Use this hood to reduce your exposure to airborne hazardous chemicals. Avoid storing chemicals here. Do not place items in the hood that will obstruct air flow.

To report a spill that you can not clean up yourself, call Health and Safety 915-5433.

For hazardous waste disposal, call Health and Safety 915-5433.

For fume hood maintenance, have your department put in a notification to the electric shop.