Glossary of Lab Ventilation Terms

Included in the book “Laboratory Fume Hoods Explained”

Glossary of Terms

These are terms that are used in this book and are common when discussing fume hoods and laboratory ventilation systems:

Acceptable Indoor Air Quality: Air in which there are no known contaminants at harmful levels as determined by appropriate authorities and air which 80% or more of the people find acceptable.

Access Opening: The part of the fume hood through which work is performed; sash or face opening.

ACH, AC/H (air changes per hour), N: The number of times air is theoretically replaced in a space during an hour. An ACH rate for a room can be converted to volumetric airflow by multiplying the ACH number times the gross volume of the room.

Adjacent Roof Line: For the purposes of determining the laboratory chemical hood stack height, the adjacent roof will be within 6 feet horizontally of the nearest exhaust fan stack.

AHU, Air Handler Unit:  A device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU. Sometimes AHUs provide (supply) and or they can accept (exhaust).

Airflow Monitor: Device installed in a fume hood to monitor the average airflow through the sash opening.

Air Foil: Curved or angular member(s) at the fume hood entrance. The lower airfoil is a horizontal member across the lower part of the sash opening to provide a smooth air flow into the fume chamber across the work surface into the baffles.

Air Lock: An intermediate chamber between two dissimilar spaces with airtight doors or openings to each of the spaces. The doors are interlocked to ensure that at least one of them is always closed.

Air Volume: Quantity of air expressed in cubic feet (ft3) or cubic meters (m3).

Artificial Intelligence (AI) and Machine Learning:  Artificial intelligence (AI), machine learning and deep learning are three terms often used interchangeably to describe software that behaves intelligently. This software can evaluate vast amounts of data and make intelligent decisions.

Auto Sash: Is a feature where a fume hood sash has been motorized and can be controlled without using one’s hands.

Auxiliary Air: Supply or supplemental air delivered to a laboratory fume hood to reduce room air consumption.

Auxiliary Air Hood: A laboratory chemical hood with an external supply air plenum at the top of the laboratory chemical hood. The auxiliary air plenum provides a makeup airstream comprised of unconditioned or only minimally conditioned outside air to substantially reduce the amount of conditioned room air exhausted by the laboratory hood.

Baffle:  A panel located across the rear wall of the fume hood that directs the airflow through the fume chamber. The baffle creates a plenum at the rear of the hood to direct air to the exhaust outlet.

BAS, Building Automation System:  Is an intelligent system of both hardware and software, allowing the heating, venting and air conditioning system (HVAC), lighting, security, and other systems to communicate on a single platform.

Bench Hood: A fume hood that is located on a counter height work surface.

Biological Safety Cabinets, BSC: A fume hood like device that is intended for use with biological agents and not for chemical use.

Blower: see Fan.

Blower Exhaust Velocity: The speed at which air is exhausted from the fume hood exhaust stack. The minimum should be at least 3000 fpm (15.2 m/s).

Bypass: Compensating opening in a fume hood that functions to limit the maximum face velocity as the sash is lowered.

Bypass Hood (constant air volume bypass laboratory hood): A laboratory hood design that incorporates an opening (bypass area) in the upper portion of the laboratory hood structure. When the movable sash is fully open, the sash blocks off this bypass area and all of the airflow into the laboratory hood must pass through the open face area. However, as the sash is being closed to reduce the open face area, at a specific point an amount of bypass area is being uncovered. The increase in the bypass area opening offsets the decrease in the face area opening, thus providing an alternate path (the uncovered bypass area) for air to flow into the laboratory hood. When utilized with a constant air volume ventilation system, the bypass area keeps the laboratory hood face velocity relatively constant and from increasing to an objectionably high value as the sash is lowered. 

Canopy Hood: Ventilating enclosure suspended above work area to exhaust heat, vapor or odors. This device is not a laboratory fume hood.

Capture Velocity: Speed of air flowing past the face opening through a fume chamber at a speed necessary to capture generated fume vapors and/ or particulates and directed to the exhaust outlet. Measured in feet per minute (fpm) or meter per second (mps). 

CCD Chemical Containment Device: A new class of Exposure Control Device that represents the Next Generation of Laboratory Fume Hoods. These devices incorporate new technology not currently present in traditional fume hoods.

CFD (Computational Fluid Dynamics):  Is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve air flows. Computers are used to perform the calculations required to simulate and visualize the free-stream flow of the air within surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved.  A good tool for fume hood design, but generally lacking the definition to truly understand how the air is behaving.

Chemical hygiene officer: An employee who is designated by the employer and who is qualified by training or experience to provide technical guidance in the development and implementation of the provisions of the Chemical Hygiene Plan.

CHP (Chemical Hygiene Plan): Is a written program developed and implemented by the employer which sets forth procedures, equipment, personal protective equipment, and work practices that are capable of protecting employees from the health hazards presented by hazardous chemicals used in a particular laboratory.

Clean Air Hoods: These fume hood like devices aredesigned to provide product protection.  Often found in cleanrooms, they are not intended for user protection against exposure. 

Cobotic Hood: This is future generation of fume hood or chemical containment device that is driven by AI and actively participates and assists the user with safe operations.

Color Temperature: Color temperature is conventionally expressed in kelvins, using the symbol K, a unit of measure for absolute temperature. Color temperatures over 5000 K are called “cool colors” (bluish), while lower color temperatures (2700–3000 K) are called “warm colors” (yellowish).

Combination Hood: A fume hood assembly containing a bench hood section and a floor mounted section.

Combination Sash: A fume hood sash with a vertical framed member that moves vertically for setup and houses two or more horizontal sliding transparent viewing panels that can be opened for operation.

Constant air volume (CAV) ventilation system: A ventilation system designed to maintain a constant quantity of airflow within its ductwork.  Although relatively simple, a constant volume ventilation system typically requires the maximum ongoing energy usage since the system always operates at maximum capacity.

Counter Top: (See Work surface)

Cross Drafts: Air drafts or currents that flow parallel to or across the face opening of the fume hood.

Damper:  Device installed in a duct to control airflow volume.

Data acquisition, DAQ: Is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ system consists of sensors, DAQ measurement hardware, and a computer with programmable software.

Data Lake: Is a centralized repository that allows for storage of structured and unstructured data at any scale without having to first structure the data.  Using dashboards for visualization and real-time analytics it supports machine learning and is a guide to better decision making.

Demonstration Hood: A vented enclosure used for student demonstrations that has visibility on at least three sides, used primarily in schools. This device is not a laboratory fume hood.

Design Sash Position: The maximum open area of the hood that achieves the desired face velocity.

Differential Pressure (DP): The difference in pressure between two points of a system, such as between a room and the adjoining hallway, or the fume chamber and the room.

Dilution Ventilation: Ventilation airflow that dilutes contaminant concentrations by mixing with contaminated air, as distinguished from capturing the contaminated air.

Discharge Velocity: The speed of the exhaust air normally expressed in feet per minute (meters/second) at the point of discharge from a laboratory exhaust system. It is the air velocity as it leaves the last element of the exhaust system. The term “stack velocity” is sometimes used when referring to the speed of the exhaust airstream as it is discharged into the outside air.

Distillation Hood: A laboratory fume hood that provides a work surface approximately 18 inches (45.7 cm) above the room floor, to accommodate tall apparatus.

Diversity: Operating a system at less capacity than the sum of peak demand to reduce system size and energy usage.

Diversity Factor: A percentage factor that is applied to establish the theoretical maximum exhaust airflow quantity that is required at any point in time. For example, in an exhaust system consisting of six hoods, it could be assumed that no more than three hoods would be operated at one time. A diversity factor of 50% could be applied. Applying a diversity factor to the theoretical maximum required capacity of an HVAC system is often considered in the design of a VAV system. Incorporating a diversity factor enables downsizing HVAC system components and thus results in a smaller capacity ventilation system. The overall intention of applying a diversity factor when designing a VAV ventilation system is to achieve a lower life cycle cost (e.g., lower system first cost and/or lower system energy costs).

Dual Entry Hood: A bench type fume hood that has two sash openings, usually on opposite sides. These are often used in education and are not a laboratory fume hood.

Duct: Round, square or rectangular tube used to enclose moving air.

Duct Collar: Is the point where the fume hood connects to the duct.

Duct Transition: Is a section of duct that connects two dissimilar shapes or sizes of duct.  For example, a rectangular duct to a round duct.

Duct Velocity: Speed or air moving in a duct, usually expressed in feet per minute (fpm) or meters per seconds (mps).

Ductless Hood: A fume hood that is not connected to a laboratory ventilation system, rather, a ductless hood incorporates an exhaust fan and exhaust filters as an integral part of the design and discharges the exhaust directly back into the room. Ductless laboratory hoods are of limited size and capacity in comparison to conventional ducted laboratory hoods. These are for special applications and care must be taken when choosing the correct filter.

ECD, Exposure Control Device: This is any class of product that is designed to minimize the exposure of users to hazardous chemicals.

EHS, Environmental Health and Safety: This is an organizational function whose responsibility is to inspect and monitor environment, machineries and processes to ensure safety as per government rules and regulations and industry standards. They have knowledge of government rules and regulations and prepare standard operating procedures that helps users be safe and healthy.

Electrical Service Fixture: Outlet or other electrical device mounted directly to the face of the fume hood.

Exhaust Air: Air that is removed from an enclosed space and discharged to atmosphere

Exhaust Collar: Connection between duct and fume hood through which all exhaust air passes.

Exhaust Plume: This is the stream of contaminated air being discharged for the exhaust stack.

Exhaust Stack: Is the tube connected to the output of the exhaust fan.  It should have a minimal height of 10 feet or 3 meters above the roof line.

Exhaust Unit: Air moving device, sometimes called a fan or blower, consisting of a motor, impeller and housing.

Face:  Front access or sash opening of a laboratory fume hood.

Face Opening: measured in width and height.


Face Velocity: Average speed of air flowing perpendicular to the face opening and into the fume chamber of the fume hood and expressed in feet per minute (fpm) or meters per second (MPS), measured at the plane of the sash.

Face Velocity Monitor: This is a device mounted on the face of the fume hood and displays average face velocity. It often has an alarm when the velocity drops below the programmed minimum.

Facial Recognition: A biometric technology capable of identifying or verifying a person from a digital image.   

Fan: Air moving device, can be called an exhaust unit, or a blower, consisting of a motor, impeller and housing.

Fan Curve: A curve relating pressure vs. volume flow rate of a given fan at a fixed fan speed (rpm). This is necessary to properly size a fan.

Filter: Device to remove particles from air.

Friction Loss:  The static pressure loss in a system due to friction between moving air and the duct wall; expressed as inches w. g. 100 feet, or fractions of VP per 100 feet of duct.

Flame Resistance: The ability to withstand flame.

Floor-mounted hood (walk-in hood): A larger-size laboratory hood with sash and/or door arrangement that enables access from the floor to the top of the hood interior. The name unfortunately is a misnomer and although the design and height of these hoods may allow it, users should not walk into any hood that may represent a significant exposure hazard. Walk-in laboratory hoods enable larger equipment and apparatus (e.g., equipment on carts, gas cylinders, etc.) to be more readily put in and set up within the laboratory hood.

Fog machine: A machine that generates theatrical fog for airflow visualization in a fume hood. 

Fume Chamber: The interior of the fume hood measured in width, depth, and height.

Fume Cupboard: Europeanterm for laboratory fume hood.

Fume Hood Controller: Is a device that monitors different elements of hood performance and function.  It can control the sash, lights, interface display and VAV for exhaust and supply.

Fume Removal System or Laboratory Ventilation System:  Is engineered to effectively move air and fumes consistently through fume hood, duct and exhaust blower. Note: Room air, make-up air, auxiliary air (if used) and pollution-abating devices (if used) are integral parts of a properly functioning system and should be considered when designing a fume removal system.

Fume Removal System: A fume hood exhaust engineered to effectively move air and fumes consistently through fume hood, duct and exhaust unit.

Gauge Pressure: The difference between two absolute pressures, one of which is usually atmospheric pressure; mainly measured in inches water gauge (in. w. g.).

GEX, General Exhaust: General exhaust is used to keep the room in balance.  Based on the required ACH, if there is not enough air being exhausted through fume hoods and other exhaust devices, air is exhausted through the general exhaust.

Glove Box: A totally enclosed and controlled environment work area providing a primary barrier to confine and contain hazardous materials within.  This device is not a laboratory fume hood. 

Grille: A louvered or perforated face over an opening in an HVAC system.

Hazardous chemical: any chemical that is likely to be harmful to human health. Often defined by Government regulations and industry best practices. 

Heat Resistance: The ability to withstand heat without deteriorating.

Health Hazard: Any chemical that is classified as posing one of the following hazardous effects: acute toxicity (any route of exposure); skin corrosion or irritation; serious eye damage or eye irritation; respiratory or skin sensitization; germ cell mutagenicity; carcinogenicity; reproductive toxicity; specific target organ toxicity (single or repeated exposure); aspiration hazard or simple asphyxiant.

HEPA: High Efficiency Particulate Air (filter) Common standards require that a HEPA air filter must remove—from the air that passes through—at least 99.95% (European Standard) or 99.97% (ASME, U.S. DOE) of particles whose diameter is equal to 0.3 μm.

High Density Shielding: A barrier made of lead. Often used in Radioisotope Fume Hoods

High Heat Hoods: Heat loads in the fume chamber of a hood impacts both the airflow and containment. Some hoods are designed especially for working with high heat loads such as high temperature acid digestion.

Hood: A device which encloses, captures, or receives emitted contaminants. A hood and a fume hood are not the same thing. Often times a hood is vented enclosure.

Hood Entry Loss: The static pressure loss, stated in inches on a water gauge when air enters a duct through a hood. The majority of the loss is usually associated with a vena contracta formed in the duct.

Hood Static Pressure: The sum of the duct velocity pressure and the hood entry loss; it is the static pressure required to accelerate air at rest outside the hood into the duct at duct velocity.

HVAC: Heating Ventilating and Air Conditioning. Ventilation systems designed primarily for temperature, humidity, odor control, and air quality.

Imbalance: Condition in which ratio of quantities of supply air is greater or lesser than the exhaust air.

Inches of Water (inch w.g.): The pressure exerted by a column of water one inch in height at a defined reference condition such as 39°F or 4°C and the standard acceleration of gravity.

Indoor Air Quality, IAQ): indoor air quality related to temperature, humidity, CO2 and airborne contaminants.

IoT (Internet of Things): Is a system of interrelated computing devices, mechanical and digital machines provided with unique identifiers (UIDs) and the ability to transfer data over a network without requiring human-to-human or human-to-computer interaction. The ability to communicate with other devices.

IPA, Isopropyl Alcohol: Is being evaluated as a new way to do fume hood containment testing.  It may replace SF6 as the standard for containment testing. The IPA would be atomized inside the fume hood and its escape measured.

Isolator: A core component to the pharmaceutical industry, critical for a range of processes. These gas-tight enclosures provide a complete barrier to ensure aseptic conditions and containment. They are a class of ECD, exposure control devices.

Laboratory: A building, part of a building, or other place equipped to conduct scientific experiments, tests, investigations, etc., or to manufacture chemicals, medicines, or the like. A place to conduct experimentation, investigation, observation in support of research and discovery.  An area in which diverse mechanical services and special ventilation systems are available to control emissions and exposures from chemicals and harmful substances.

Laboratory Air Quality: Unlike normal indoor air quality, laboratory air is prone to have other chemical contaminants.  The nature of the work in the laboratory has the potential to release these contaminants into the air.  Without proper ventilation, the concentration of chemical contaminants can present health hazards.

Laboratory Fume Hood: A Laboratory Fume Hood shall be made primarily from flame resistant materials including the top, three fixed sides, and a single face opening. Face opening is equipped with a sash and sometimes an additional protective shield. Face opening will have a profiled entry and usually an airfoil designed to sweep and reduce reverse airflows on the lower surface. A Laboratory Fume Hood will be equipped with a baffle and, in most cases, a bypass system designed to control airflow patterns within the hood and manage the even distribution of air at the opening. The bypass system may be partially blocked to accommodate Variable Air Volume (VAV) Systems.

Laboratory Module:  Is the key unit in any lab facility. When designed correctly, a lab module fully coordinates all architectural and engineering systems.  Most laboratory modules are 10’6″ wide, but they vary in depth from 20′ to 33′, depending on the lab requirements and the cost-effectiveness of the structural system. The 10’6″ dimension is based on two rows of casework and equipment (each row 2’6″ deep) on each wall, a 5′ aisle, and 6″ for the wall thickness separating one lab from another

Laboratory Ventilation System, LVS: Air moving systems and equipment which serve laboratories. Includes all the mechanical components including fume hoods, exhaust systems, supply systems and all the necessary controls, valves and sensors.

Laboratory Ventilation Design Levels: This is a risk-based system defined in the ASHRAE Classification of Laboratory Ventilation Design Levels Standard.  It is similar to design levels used in biological labs (BSL1, BSL2, BSL3, BLS4) The chemical laboratory classifications are LVDL-0, LVDL-1, LVDL-2, LVDL-3, LVDL-4. 

Laminar Flow (Also Streamline Flow) – Airflow in which air molecules travel parallel to all other molecules; flow characterized by the absence of turbulence and a Re number less than 2000.

Laminar Flow/Clean Bench (Horizontal Laminar Flow and Vertical Laminar Flow):  A clean bench is a laminar flow work cabinet or similar enclosure that provides filtered air across the work surface to protect against product contamination. This device is not a laboratory fume hood.

Lazy Flow or Reverse Flow:  When doing smoke visualization, the smoke can be observed moving slowly without a direction.  Even when smoke does not escape from the hood, it could show points of weakness or instability in the hood.

LEL (Lower Explosion Limit): The minimum concentration of a particular combustible gas or vapor necessary to support its combustion in air is defined as the Lower Explosive Limit (LEL) for that gas. Below this level, the mixture is too “lean” to burn. The maximum concentration of a gas or vapor that will burn in air is defined as the Upper Explosive Limit (UEL). Above this level, the mixture is too “rich” to burn.

Liner: The liner defines the fume chamber of a fume hood. Usually includes the sides, back and roof along with exhaust plenum and baffle system of a laboratory fume hood.

Local Exhaust Ventilation: Devices that are used to capture and remove emitted contaminants before dilution into the workplace ambient air can occur. Can also be used to remove heat at the source of generation.

Loss of Containment: A condition where the fume hood fails to capture and contain contaminants allowing them to escape into the room air as fugitive contaminants.

Low Flow Laboratory Fume Hoods: Fume Hood designs that provide a reduction in the required exhaust air volume, when compared to the volume required for the same size fume hood to operate with a face velocity of 100 FPM through a fully opened vertical sash. A low flow hood is one that has had the exhaust volume reduced by operating through a smaller sash opening.

Low Velocity Laboratory Fume Hoods: Fume Hood designs that provide a reduction in the required exhaust air volume, when compared to the volume required for the same size fume hood to operate with a face velocity of 100 FPM.  Low Velocity Fume Hoods are also referred to as High Performance Fume Hoods and High Efficiency Fume Hoods.

Make-Up air (replacement air): Any combination of transfer air and air provided by a mechanical ventilation system to replace air being exhausted from a laboratory ventilation device. Air needed to replace or compensate for the air taken from the room by laboratory fume hood(s) and other air exhausting devices.

Manifold: A fitting or pipe with many outlets or connections relatively close together. In a fume hood application this is several hoods on a single exhaust duct.

Manometer: Device used to measure air pressure differential, often a u-shaped glass tube containing water or mercury.  There are also digital manometers.

Mechanical System: Is all the components that support the HVAC system, ductwork, exhaust and supply systems and all the controls.

Microorganism: A microscopic organism, usually a bacterium, fungus, or protozoan. All of these are to be handled in a BSC not a chemical fume hood.

Minimum Transport Velocity (MTV): The minimum velocity which will transport particles in a duct with little settling; the MTV varies with air density, particulate loading, and other factors.

Natural Ventilation: The movement of outdoor air into a space through intentionally provided openings, such as windows, doors, or other nonpowered ventilators, or by infiltration.  Fume hoods will not function safely with natural ventilation.  For safe fume hood operation, mechanical supply is required.

Negative Air Pressure: Air pressure lower than ambient.

Occupancy Sensor: This is a motion sensing sensor.  It detects human movement.  It can be used to detect a user working at a fume hood.

Occupied Zone: The region within an occupied space between 3” and 72” above the floor.

Odor: A quality of gases, vapors, or particles which stimulates the olfactory organs; typically, unpleasant or objectionable. In a laboratory if you can smell chemicals there are generally unhealthy concentrations of contaminants in the air.

Outdoor Air (OA): “Fresh” air mixed with return air (RA) to dilute contaminants in the supply air (SA).

Pa, Pascal: The unit for measuring vacuum or pressure in the International System or SI. One pascal equals one newton (SI unit of force) per square meter.

Particulate Matter: Small, light-weight particles that will be airborne in low velocity air [approximately 50 fpm (.25 m/s)].

Perchloric Acid Hood: A laboratory hood constructed and specifically intended for use with perchloric acid or other reagents that may form flammable or explosive compounds with organic materials of construction. A perchloric acid hood as well as its exhaust system must be constructed of all inorganic materials and be equipped with a water washdown system that is regularly used to remove all perchloric salts that may precipitate and collect in the laboratory hood and in the exhaust system. The exhaust fan must also be of a spark resistant design to ensure against ignition of any perchlorate deposits in the exhaust system.

Pitot Tube: A device used to measure total and static pressures in an air stream.

Plenum: A chamber used to distribute static pressure throughout its interior.

Plenum Chamber: Chamber used to equalize airflow.

Polyethylene: A plastic polymer of ethylene used chiefly for containers, fittings and sinks.

Polypropylene: Material is a polyolefin which is generally high in chemical resistance. This material is commonly used for acid waste piping as well as for deionized water.

Polyvinyl Chloride (PVC): A water insoluble, thermoplastic resin derived by the polymerization of vinyl chloride used chiefly for containers, fittings and piping.

Polyvinylidene Fluoride (PVDF): Material is a strong and abrasion resistant fluoropolymer. It is chemically resistant to most acids, bases and organic solvents, and is the preferred material for piping and faucets for ultra-pure water. Pure PVDF is an opaque white resin that is resistant to UV radiation, and is superior for non-contaminating applications.

Positive Air Pressure: Air pressure higher than ambient.

Posts (columns, facia, pillars): These are the vertical members of the fume hood face. Generally, they form an angular or radiused entry into the fume hood. They are also used for mounting the electrical and service fittings. They work with the upper and lower airfoils to direct air into the hood in a nonturbulent way.

Predictive Containment: Is a forecasting system that uses sensor output and processes it to predict the likelihood that the fume hood is safe and has no loss of containment.

Pressure Drop: The loss of static pressure between two points; for example, “The pressure drop across an orifice is 2.0 inches w.g.”

Radioisotope Hood:  Provides protection from applications requiring the use of radiochemicals. It has a fully sealed integral work surface reinforced to support lead shielding and coved interiors to facilitate decontamination.

Recirculation: Air removed or exhausted from a building area and ducted back to an air-handling system where it is mixed with outside fresh air. This air mixture is then conditioned and utilized for ventilation. Since air removed from a space is more often close to the temperature and humidity of the building interior than the outside air, the recirculation process enables achieving a greater reduction in heating and cooling energy than once used air where 100% outside air was utilized.

Reentrainment or reentry: The flow of contaminated air that has been exhausted from a building is drawn back into the building through air intakes or openings in the walls or doors and windows. 

Register: A combination grille and damper assembly. These supply air outlets can also be called diffusers or grilles.

Relative Humidity (RH): The ratio of water vapor in air to the amount of water vapor air can hold at saturation. A “RH” of 100% is about 2.5% water vapor in air, by volume.

Remote Control Valves: Valves usually installed in the walls of fume hoods with the control handles normally on the face of the hood which regulate and control the flow of the services to the outlets in the interior of the fume hood.

Replacement Air: (Also, compensating air, make-up air) Air supplied to a space to replace exhausted air and maintain balance.

Respirable Particles:  A collective group of fine solid particles, aerosols, mist, smoke, dust, fibers and fumes are called Respirable particulates.

Return Air: Air which is returned from the primary space to the AHU for recirculation.

Reynolds number (Re):  Is an important dimensionless quantity in fluid mechanics used to help predict flow patterns in different fluid flow situations. The Reynolds number is used to determine whether a fluid is in laminar or turbulent flow.

RFID, Radio-frequency identification: Uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID tag consists of a tiny radio transponder; a radio receiver and transmitter. When triggered by an electromagnetic interrogation pulse from a nearby RFID reader device, the tag transmits digital data, usually an identifying number, back to the reader.

Room Air: That portion of the exhaust air taken directly from the room.

Room Air Balance: A general term describing the requirement that a laboratory room have the proper relationship with respect to the total exhaust airflow from the room and the supply makeup airflow. The relationship of these airflows also establishes the pressure differential between the laboratory room and adjacent rooms and spaces.

Room Balance: In a laboratory we usually want the room to be under slight negative pressure.  That means we balance the room to have slightly less supply air coming into the room in relationship to the air being exhausted.

Room Controller: This is an electronic device that processes data from various inputs and then communicates with the Laboratory Ventilation System to manage the exhaust and supply air to maintain room balance.

Room Ventilation: The volumetric airflow through a room expressed in terms of cfm or L/sec. 

Sash:  A moveable panel or door set in the access opening of a fume hood to provide access, and to provide a protective shield.

SCFM (Standard Cubic Feet Per Minute): Airflow rate at standard conditions; dry air at 29.92 inches Hg gauge, 70 degrees F.

Scrubber, Fume: A device used to remove contaminants from fume hood exhaust, normally utilizing water.

SDS, Safety Data Sheet: Previously called a MSDS sheet. A safety data sheet is a standardized document that contains occupational safety and health data. The International Hazard Communication Standard (HCS) mandates that chemical manufacturers must communicate a chemical’s hazard information to chemical handlers by providing a Safety Data Sheet. SDS’s typically contain chemical properties, health and environmental hazards, protective measures, as well as safety precautions for storing, handling, and transporting chemicals.

Self-aware: Is an awareness of self and your relationship to the external world. A self-aware device is not only aware of what it is doing, but is also aware of its surroundings.  The programming not only looks at what the device is doing, but also considers what is happening around it.

Sensor Array: Is a group of sensors that are collectively taking multiple readings for a complex view of a situation.

Service Fixture: Item of laboratory plumbing mounted on or fastened to a laboratory fume hood

Setback: Is a setpoint below normal. For example, a system might have a night setback.  When it goes into the setback mode, it uses a different set of parameters than it does during day mode.

Shall: When used in a Standard indicates a mandatory feature.

Should: When used in a Standard indicates a recommendation, but is not a mandatory feature.

Slot Velocity: Speed of air moving through fume hood baffle openings.

Smart Hood: A smart hood is one that incorporates a digital controller to manage various functions and performance parameters of the fume hood. 

Smart Laboratories: A smart laboratory integrates a number of smart devices that can be controlled with a digital interface.

Smoke Candle: Smoke-producing device used to allow visual observation of airflow.

Smoke Machine: A machine thatcreates a non-toxic theatrical fog. Used for airflow visualization.

Spot Collector: A small, localized ventilation hood usually connected by a flexible duct to an exhaust fan. This device is not a laboratory fume hood.

Stack: The tube/device on the end of a ventilation system, which disperses exhaust contaminants for dilution by the atmosphere.

Standard Air:  Standard Conditions STP Dry air at 70 degrees F, 29.92 in Hg.

Standard Operating Procedure, SOP: This is a formal and written procedure for conducting activities.

Static Pressure: Air pressure in laboratory fume hood or duct, usually expressed in inches of water.

Static Pressure, SP:  The pressure developed in a duct by a fan; SP exerts influence in all directions; the force in inches of water measured perpendicular to flow at the wall of the duct; the difference in pressure between atmospheric pressure and the absolute pressure inside a duct, cleaner, or other equipment.

Static Pressure Loss: Measurement of resistance created when air moves through a duct or hood, usually expressed in inches of water.

Suction Pressure: See Static Pressure. 

Sulfur Hexafluoride (SF6): Tracer gas widely used for ASHRAE and EN testing. It can be detected with a leak detector to measure loss of containment.

Superstructure: The portion of a laboratory fume hood that is supported by the work surface.

Supplemental Air: Supply or auxiliary air delivered to a laboratory fume hood to reduce room air consumption.

Supply Diffuser: An outlet in the room for the supply air.  There are many types, but around fume hoods, a low velocity type is preferred.

Synchronized Supply Fume Hood: This is a new class of fume hood that brings supply air directly into the fume chamber and both the supply and the exhaust are controlled by independent VAV valves so that the proper balance with the fume chamber can be maintained.  As the exhaust volume goes up and down, the supply is synchronized to track it.

Synthetic Sensors: General-purpose sensing, wherein a single, highly capable sensor can indirectly monitor a large context, without direct instrumentation of objects. A super sensor that can indirectly monitor a number of activities and provide actionable output.

Table Top Hood: A small, spot ventilation hood for mounting on table tops. Used primarily in educational laboratories. This device is not a laboratory fume hood.

Temperature Gradient: A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location.

Fume Hood Test Room or Test Lab: This is a space designed and equipped based on criteria in the testing specifications such as ASHRAE110 and EN 14175.  This space is designed to have near perfect room conditions so what is being tested is the hood’s performance independent of the room. 

Thermal Anemometer: A device for measuring fume hood velocity utilizing the principle of thermal cooling of a heated element as the detection element.

Threshold Limit Valve-Time Weighted Average (TLV-TWA): The time-weighted average concentration for a normal 8-hour workday or 40-hour week, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect.

Titanium Tetrachloride: Chemical that generates white fumes used in testing laboratory fume hoods.

Total Pressure, TP: The pressure exerted in a duct as the Algebraic sum of the static pressure and the velocity pressure.

Total Suspended Particulate Matter: The mass of particles suspended in a unit volume of air (typically one cubic meter) when collected by a high-volume sampler.

Transfer air: Air that moves between spaces in a building, driven by the ventilation system.

Transport Velocity: Minimum speed of air required to support and carry particles in an air stream.

Turbulent Flow: Airflow characterized by transverse velocity components, as well as velocity in the primary direction of flow in a duct; mixing velocities.

Turndown: See setback

TWA, Time Weighted Average: The average exposure at the breathing zone.

Ultra-Low Flow Fume Hoods: This is a class of product that is designed to operate at very low average face velocities. For years the benchmark was 100fmp/0.5mps then the low flow hoods were operating at 60fmp/0.3mps.  Ultra-low flow goes below that.  Often times supplemental air is injected into the fume chamber in an effort to increase containment.

Variable Air Volume – two-position ventilation system: A constant air volume ventilation system (sometimes also referred to as a “two-position variable air volume system”) that is designed to provide two separate levels of airflow. The higher level of airflow is provided when a facility is normally occupied such as during regular work hours. The lower level of airflow is utilized during unoccupied times (e.g., nighttime, holidays, etc.) when ventilation needs and internal loads require less airflow.

Variable Air Volume (VAV): In a HVAC system, the supply air volume is varied by dampers or fan speed controls to maintain the temperature; in hoods, the exhaust air is varied to reduce the amount of air exhausted. If the fume hood is going to operate in VAV mode, then the room supply has to also be VAV to keep the room in balance.

Variable air volume (VAV) ventilation system: A type of HVAC system specifically designed to vary the amount of conditioned air supplied and exhausted from the spaces served. The amount of air supplied and intended to meet (but not exceed) the actual need of a space at any point in time. In general, the amount of air that is needed by a space is determined by the required rate and the amount of airflow necessary to maintain comfortable conditions (temperature and humidity). In laboratories, we also consider the required number of ACH to minimize the risk of exposure to the users.

Variable Frequency Drive (VFD): Is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed by varying motor input frequency. These are very common on fans used for exhaust and supply air.

Variable Volume Hood: A hood designed so the exhaust volume is varied in proportion to the opening of the hood face by changing the speed of the exhaust blower or by operating a damper or control valve in the exhaust duct.

Velocity, V: Magnitude of air motion perpendicular to the plane of the airflow cross section.

Velocity Pressure: Pressure caused by moving air in a laboratory fume hood or duct, usually expressed in inches of water.

Vena Contracta: is the point in a fluid stream where the diameter of the stream is the least, and fluid velocity is at its maximum, such as in the case of a stream issuing out of a nozzle (orifice). It is a place where the cross-section area is minimum.

Ventilated Enclosure: These are often fume hood like devices that are used for exhausting laboratory air but do not meet the definition or performance criteria of a laboratory fume hood.

Visualization of Airflow: Since airflow can’t be viewed directly, we have developed various methods to help visualize how the airflow is behaving. The most common is theatrical smoke or fog, also smoke bombs, smoke candles can be used.  Using a modified laser beam can further enhance the visualization by projecting it onto the smoke. This is helpful when trying to understand air patterns within the fume hood. 

Volume Flow Rate, Q: The quantity of air flowing in cubic feet per minute, cfm, scfm, acfm.

Volumetric airflow rate: The rate of airflow expressed in terms of volume (cubic feet or liters) per unit of time. These are commonly expressed as cubic feet per minute (cfm) in USCS units or liters per second (l/s) in SI units. (Also see room ventilation.)

Vortex: A mass of whirling air, a rotating air mass that often forms insides fume hoods.  The most common is a horizontal vortex that forms at the top of the fume chamber behind the sash.  But in combination sashed hoods, it is also common to see vertical vortex that form behind the closed section of sash.  The performance issue with vortex is that they can collect contaminants and spin them near the sash opening increasing the chance for loss of containment.

Walk-in hood: See floor-mounted hood.

Weather Cap: Device used at the top of an exhaust stack to prevent rain from entering the stack end. Because of the need for a high velocity discharge, only specially designed rain guards should be used. Rain caps that cover the stack are not acceptable.

Work Space: The part of the fume hood interior where apparatus is set up and fumes are generated. It is normally confined to a space extending from six inches (15.2 cm) behind the plane of the sash(es) to the face of the baffle, and extending from the work surface to a plane parallel with the top edge of the access opening.

Work Surface: The surface that a laboratory fume hood is located on and supported by a base cabinet. In the fume chamber, the surface is recessed to contain spills.