A fume hood is a ventilated enclosure used to contain gases, vapors, and fumes. It is one of the most effective devices used to protect laboratory professionals from air contaminants and hazardous chemicals. Proper performance and use of the fume hood is essential in providing safety to the user and other laboratory personnel.
There are multiple types of fume hoods found in modern research labs:
Ductless Fume Hoods
A Ductless fume hood uses carbon filters to remove fumes and vapors from a laboratory. Instead of exhausting the fumes outside, the fume hood uses carbon filters to capture the harmful fumes and then return the clean air back into the laboratory.
Constant Volume Fume Hoods
A constant volume fume hood uses a built-in exhaust air bypass. These fume hoods exhaust the same amount of air all the time, regardless of sash position. As the sash is lowered and raised, the velocity at the face of the hood changes. This change in face velocity can result in less-than-optimal hood performance.
Variable Volume Fume Hoods
A variable air volume fume hood has control over the air that is exhausted from the unit and reduces the costs of the device by maintaining a constant velocity. This fume hood, also known as a constant velocity hood, is a hood that has been fitted with a face velocity control, which varies the amount of air exhausted from the fume hood in response to the sash opening to maintain a constant face velocity.
Distillation Fume Hood
A distillation hood is characterized by a low worktop height which results in a large working height for the operator. This allows tall distillation equipment to be installed and mounted in the work chamber. Otherwise, it has similar features to that of a standard fume hood.
Perchloric Acid Fume Hood
Perchloric acid reacts violently with organic materials. Dried perchloric acid is also highly explosive. Therefore, perchloric fume hoods require built-in water wash down systems in order to prevent perchlorate salt deposits. Interior liners are made of acid resistant materials like stainless steel. Interior corners are coved to aid in cleaning. All procedures that use perchloric acid must be confined to a perchloric fume hood, to prevent dangerous reactions with other chemicals.
Radioisotope Fume Hood
Radioisotope fume hoods are constructed specifically to protect users from radioactive materials. They have specially constructed worktops to withstand the weight of lead shielding plates and may also have lead laced sashes. Interiors are made of stainless steel with coved corners to aid in decontamination.
Acid Digestion Fume Hood Acid digestion fume hoods have special liners manufactured of acid resistant materials such as unplasticized PVC. For acid digestion applications involving high service temperatures, other materials such as PVDF may be used. Sashes may be made of polycarbonate to resist hydrofluoric acid etching.
Floor Mounted or “Walk-In” Fume Hood Floor mounted fume hoods are used for applications which require large apparatus. As the name implies, these hoods are floor mounted without any work surface. This facilitates the transfer of equipment and materials into, and out from the hood.
Figure 1: Typical Fume Hood Components
Misconceptions Associated with Fume Hoods
The higher the face velocity the better.
While it is generally important to have a face velocity of 100 fpm, velocities higher than this are actually harmful. When face velocity exceeds 150 fpm eddy currents are created which allow contaminants to be drawn out of the hood, increasing worker exposures. Check with local safety regulations on the maximum face velocity before using the hood.
The airfoil on the front of a hood is of minor importance and can be removed.
Airfoils are critical to efficient operation of a chemical hood. With the sash open an airfoil smoothes flow over the hood edges. Without an airfoil eddy currents form, causing contaminates to be drawn out of the hood. With the sash closed, the opening beneath the bottom airfoil provides for a source of exhaust air.
My fume hood monitor tells me the FPM so I don’t need to test it.
Per code, all hoods and exposure control devices shall be equipped with a flow indicator, flow alarm, or face velocity alarm indicator as applicable to alert users to improper exhaust flow. Some are “low tech” but many modern DDC systems incorporate a digital fume hood airflow monitor with a FPM/CFM display.
The issue is the accuracy or the readings. The DDC system is converting the measured CFM value through the air valve to FPM based on the sash position feedback. However, neither the sash position nor the CFM is 100% accurate, therefore the FPM as displayed on the monitor may not accurately represent the face airflow velocity.
100 FPM +/-10% is acceptable
As discussed below, if you are in California, Cal/OSHA requires the exhaust system to “provide an average face velocity of at least 100 feet per minute”. There is no +/-10%. An average reading of 90 FPM does not meet the letter of the code.
Codes, Standards and Recommended Practices
The following are the primary organizations and standards regarding fume hoods:
–OSHA Part 1910.1450. The agency regulations regarding fume hood operation are listed in the Code of Federal Regulations Volume 29 Part 1910.1450. This code addresses several aspects of laboratory design and operation. Regarding hoods it is primarily concerned with airflow at the face of the hood, monitoring, maintenance and exhaust.
-Cal/OSHA. When laboratory-type hoods, also known as laboratory fume hoods, as defined below are used to prevent harmful exposure to hazardous substances, such hoods shall conform to all applicable provisions of Article 107, and shall conform to provisions of Section 5154.1.
–ANSI/ASHRAE 110-1995. Method of Testing Performance of Laboratory Fume Hoods. This standard is published by the American National Standards Institute and the American Society of heating, Refrigerating and Air Conditioning Engineers, Inc. It concerns itself primarily with methods of testing fume hoods to check their operation.
–ANSI/AIHA Z9.5. Titled “The American National Standard for Laboratory Ventilation” this standard is published by ANSI and the American Industrial Hygiene Association. It covers a variety of lab ventilation issues including hood monitoring, face velocities and exhaust.
–NFPA 45. This standard is prepared by the National Fire Protection Association. It recommends hood construction, location, fire protection, specialty hoods, identification, inspection, testing and maintenance, and exhaust.
–SEFA 1.2-1996. SEFA is the Scientific Equipment & Furniture Association. Its publication “Laboratory Fume Hoods Recommended Practices” covers design requirements of hoods, face velocities and testing.
1) Airflow
Proper air flow at the face of the hood is probably the most common cause of confusion regarding fume hood operation. Here are what the codes and standards say:
OSHA: “General air flow should not be turbulent and should be relatively uniform throughout the laboratory, with no high velocity or static areas; air flow into and within the hood should not be excessively turbulent; hood face velocity should be adequate. (Typically, 60-110 fpm.)”
CAL/OSHA
“(1) Laboratory-type hood face velocities shall be sufficient to maintain an inward flow of air at all openings into the hood under operating conditions. The hood shall provide confinement of the possible hazards and protection of the employees for the work that is performed. The exhaust system shall provide an average face velocity of at least 100 feet per minute with a minimum of 70 fpm at any point, except where more stringent special requirements are prescribed in other sections of the General Industry Safety Orders, such as Section 5209. The minimum velocity requirement excludes those measurements made within 1 inch of the perimeter of the work opening.
(2) When a laboratory-type hood is in use to contain airborne hazardous substances and no employee is in the immediate area of the hood opening, the ventilation rate may be reduced from the minimum average face velocity of at least 100 feet per minute to a minimum average face velocity of 60 feet per minute if the following conditions are met:
(A) The reduction in face velocity is controlled by an automatic system which does not require manual intervention. The automatic system shall increase the airflow to the flow required by (c)(1) when the hood is accessed.
(B) The laboratory-type hood has been tested at the reduced flow rate according to the tracer gas method specified in Section 7, Tracer Gas Test Procedure, of ANSI/ASHRAE 110-1995, Method of Testing Performance of Laboratory Fume Hoods, which is hereby incorporated by reference, and has a hood performance rating of 4.0 AU 0.1 or less. The test may be performed with or without the mannequin described in the ANSI/ASHRAE 110-1995 tracer gas method.
The tracer gas test need only be performed once per hood. However, if employers have chosen to perform the tracer gas test on subsequent occasions, it is the most recent record of test results and test configuration that shall be maintained pursuant to subsection (c)(2)(C).
(C) The record of the most recent tracer gas test results and the "as used" test configuration shall be maintained as long as the automatic system is operable and thereafter for five years. “
ANSI/AIHA Z9.5: “Each hood shall maintain an average face velocity of 80-120 fpm with no face velocity measurement more than plus or minus 20% of average.”
SEFA: “Face velocities of laboratory fume hoods may be established on the basis of the toxicity or hazard of the materials used or the operations conducted within the fume hood. Note: Governmental codes rules and regulation may require specific face velocities. A fume hood face velocity of 100 fpm is considered acceptable in standard practice. In certain situations, face velocity of up to 125 fpm or as low as 75 fpm may be acceptable to meet required capture velocities of the fume hood.”
2. Monitoring/Alarms
Many older labs are equipped with fume hoods that do not have air flow monitoring devices. The type of device is not specified, but according to the following codes and standards if you’re putting in a hood or remodeling an older one they are now a requirement.
OSHA: “...each hood should have a continuous monitoring device to allow convenient confirmation of adequate hood performance before use. If this is not possible, work with substances of unknown toxicity should be avoided or other types of local ventilation devices should be provided.”
ANSI/AIHA Z9.5: “New and remodeled hoods shall be equipped with a flow-measuring device.”
NFPA 45: “New and remodeled hoods shall be equipped with a flow-measuring device.
3. Maintenance/Inspection
As with all equipment maintenance is important to proper operation.
OSHA: “Quality and quantity of ventilation should be evaluated on installation, regularly monitored (at least every 3 months), and re-evaluated whenever a change in local ventilation devices is made.”
ANSI/AIHA Z9.5: “A routing performance test shall be conducted on every fume hood at least annually or whenever a significant change has been made to the operational characteristics of the system”
NFPA 45: “When installed or modified and as at least annually thereafter, laboratory hoods, laboratory hood exhaust systems, and laboratory special exhaust systems shall be inspected and tested.”
NFPA 45: “Special use laboratory hoods and special use local exhaust systems shall be identified to indicate their intended use.” “A sign shall be affixed to each hood containing the following information from the last inspection: Inspection interval, Last inspection date, Average face velocity, location of fan that serves hood, Inspectors name. Exception: In lieu of a sign, a properly maintained log of all hoods giving the above information shall be deemed acceptable.”
4. Exhaust
ANSI/AIHA Z9.5: “Discharged in manner and location to avoid re-entry into the laboratory building or adjacent buildings at concentrations above 20% of the allowable concentrations inside the laboratory under any wind or atmospheric conditions.” Exhaust stack: “Be in a vertical up direction at a minimum of 10 feet above the adjacent roof line as so located with respect to opening and air intakes of the laboratory or adjacent buildings to avoid re-entry.”
NFPA 45: “Air exhausted from laboratory hoods and other special local exhaust systems shall not be re-circulated.” “Air from laboratory units and laboratory work areas in which chemicals are present shall be continuously discharged throughout systems maintained at a negative pressure relative to the pressure of normally occupied areas of the building.
Fume Hood Testing (FHT) and Certification
ASHRAE Standard 110, or simply ASHRAE 110, explains a method for testing the performance of laboratory fume hoods. Essentially, it outlines a quantitative test procedure for determining the operating capabilities, such as containing and exhausting fumes, of a fume hood.
There are three test types, As Manufactured (AM), As Installed (AI) and As Used (AU). AM is done once the hood is built and is generally done in a testing lab where environmental factors are removed as much as possible. AI testing is done once the hood is installed in the lab but before anything is placed inside. Finally, AU testing is done in the lab with materials (beakers, equipment, etc.) inside the hood.
The standard fume hood certification includes face velocity testing to document and obtain an acceptable airflow and calculated total CFM volume, through the front opening of the hood with the sliding sash at a height that allows functional use.
Typically, the owner establishes a program to inspect and certify their laboratory fume hoods. The certification program may be based on OSHA regulations (Standard 29 CFR 1910.1450) and the AIHA Laboratory Ventilation (ANSI/AIHA Standard Z9.5). Such a program may include the allowable average and maximum exposure levels, allowable fume hood face velocities and required speed of response for variable volume operations. The owner should also determine the agency responsible for conducting the testing; this may be an in-house department or an outside agency.
The National Environmental Balancing Bureau (NEBB’s) Fume Hood Performance Testing program provides certification of firms and individuals that meet the criteria established by NEBB. As more facilities are using fume hoods in their research laboratories, there is a demand for certified, competent firms to make sure a facility’s fume hoods are operating effectively.
Certification in Fume Hood Testing (FHT) offer hospitals, laboratories, and educational facilities proof of a firm and individual’s technical knowledge, skills, and instrumentation to test fume hoods for effectiveness.
Fume hoods are installed in laboratories to protect workers from hazardous vapors generated by laboratory experiments. The level of protection provided by a fume hood is affected by the manner in which the fume hood is designed, constructed, operated and used. No fume hood, however well designed, can provide adequate containment unless operating face airflow velocity is verified.
GMC Cx has NEBB Fume Hood Testing Certified Professionals on staff and can perform onsite testing and certifications for As-Installed fume hoods and As-Used fume hoods.