Laboratory fume hood and exhaust ventilation field guide for HVAC

Laboratory exhaust ventilation, and why it fails quietly

Laboratory exhaust ventilation is the system that keeps the air a chemist breathes free of what they are working with. The fume hood pulls air in across its opening and carries the fumes away in a dedicated duct, a fan on the roof drives the flow, a stack throws the exhaust high enough to disperse, and the room is held negative so anything that escapes stays in the lab. Take any one of those away and the others cannot cover for it.

Here is the part that gets people hurt: it fails quietly. A hood pulling 60 fpm instead of 100 looks identical to one that is working. A sash left high, an exhaust fan fighting a starved makeup-air system, a stack dumping into a nearby intake, a lab that has gone positive to the corridor, none of it makes a sound or shows a symptom until someone is exposed. There is no nuisance trip, no puddle, no smell that arrives in time.

That is why this is engineered and tested work, not set-and-forget equipment. The job is to hold the face velocity, keep the exhaust dedicated and corrosion-resistant, disperse the plume away from the building, and keep the lab negative with enough tempered makeup air to feed every hood at once. The commercial exhaust-fan guide and the building-pressurization guide cover the fan and the air balance in depth. This guide is the lab-specific version, where the contaminant is hazardous and the margin for error is gone.

The hood is the primary containment

The fume hood is the worker's primary protection, ahead of gloves, a coat, or a respirator. It is engineered control at the source: it captures the fume where it is generated and exhausts it before it reaches the person standing in front of it. Personal protective equipment is the last line, the thing that catches what the hood missed. The hood is the line that is supposed to mean the others never get tested.

Treat the hood as a containment device and the priorities fall into order. The face opening is the boundary that has to stay swept inward at all times. The interior is contaminated space the worker reaches into, not a place to put their head. The exhaust path behind it has to stay under negative pressure all the way to the outside, so a leak pulls room air in rather than pushing fume out.

Containment is the whole point, and containment is what gets tested. Face velocity is a proxy for it, a number that is easy to measure and correlates with capture. The thing that actually matters is whether tracer gas released inside the hood reaches the breathing zone, which is what the ASHRAE 110 test measures and what the industrial hygienist signs off on.

What is fume hood face velocity?

Face velocity is the speed of air moving inward through the hood opening, and it is the number the trade carries for whether a hood is working. It is commonly held around 100 fpm, with practical designs landing in the 80 to 120 fpm band. ANSI/AIHA Z9.5, the laboratory ventilation standard now published as ANSI/ASSP Z9.5, takes a performance approach rather than mandating a single figure, and recent editions suggest roughly 80 to 100 fpm as a starting point. OSHA's non-mandatory guidance describes adequate flow in the 60 to 100 fpm range. The containment goal set by the engineer and the industrial hygienist controls the actual target, not the rule of thumb.

Both directions hurt you. Too low and the inward sweep cannot overcome the eddies a person makes standing at the face, so fume rolls back out into the breathing zone. Too high, above roughly 120 to 150 fpm, and the fast air tears across the airfoil and around the worker's body, creating turbulence that pulls contaminant out of the hood instead of holding it in. The instinct that more is safer is wrong here.

Measure it with a calibrated vane or thermal anemometer in a grid across the opening, not a single center reading. Z9.5 commonly looks for the average to hold the target with no individual point deviating more than about 20 percent, because an even face is what proves the whole opening is capturing, not just the middle.

The sash, and why you keep it low

The sash is the movable glass front of the hood, and it does two jobs that a new tech rarely connects. It is a physical shield between the worker's face and a splash or a flask that lets go, and it is the thing that sets the face velocity. Lower the sash and you shrink the opening, which on a constant-volume hood raises the velocity through the smaller area, and on a variable-volume hood lets the controls throttle the exhaust down while holding the same velocity.

Work with the sash as low as the task allows, and keep your body and your face behind it. Many hoods carry a sash stop or a marked working height, often around 18 in, that gives the design face velocity and the rated protection. Reaching over the sash or sliding it wide open because it is convenient breaks both jobs at once: the shield is gone and the velocity drops on a constant-volume hood.

The blunt version is this. The single most common unsafe act at a fume hood is working with the sash high. It feels like nothing is wrong because nothing looks wrong, and the hood keeps spilling fume past the person the whole time.

How the hood captures: airfoil, baffles, and the smoke test

Capture is not just suction at the opening. A working hood shapes the airflow with a few features that have to stay clear. The airfoil at the bottom front lifts the incoming air off the work surface and breaks up the dead zone where fume would otherwise pool at the sill. The baffles at the rear, a set of slots top and bottom, split the exhaust so the hood pulls evenly from the whole interior instead of short-circuiting out the top.

Where the work sits inside the hood matters as much as the airflow. Keep equipment at least 6 in back from the face, because the first few inches inside the opening are a turbulent zone where containment is weakest. Block the rear baffle slots with stored bottles and you starve the lower pull and create a pocket that does not clear. The hood is an engineered airflow geometry, and clutter is what defeats it.

Prove the capture with smoke, not faith. A smoke pencil or a fog source run across the face and around the interior shows whether the air is actually sweeping inward at every point or curling back out at a corner. It is the fastest field check there is, it is part of the ASHRAE 110 procedure, and it finds the reverse-flow problem a face-velocity average will hide.

What is a VAV fume hood?

A variable-air-volume (VAV) fume hood is one whose exhaust airflow changes with the sash so the face velocity stays constant. A sash sensor reads the opening, a controller drives a fast-acting venturi valve in the exhaust, and the flow rises when the sash opens and falls when it closes, holding the target velocity at every position. Good systems respond within a second or two, fast enough that the velocity does not dip while a hand lifts the sash.

A constant-air-volume (CAV) hood pulls the same total airflow no matter where the sash sits. It holds the design velocity only at full sash, and as the sash lowers, the velocity climbs because the same air squeezes through a smaller opening. A bypass grille above the sash bleeds in air as the sash closes to keep the velocity from running away, which is why a CAV bypass hood is the older standard answer. It is simpler and has no controls to fail, but it exhausts full airflow around the clock whether anyone is using it or not.

VAV is where the energy savings live, because conditioned makeup air is the real cost of a lab and a closed-sash VAV hood can drop to a fraction of its open flow. The savings are largest where users actually close sashes, so a VAV system is only as good as the habit and the sash alarm that reinforces it. The control approach and the minimum-flow setting belong to the project engineer and the containment requirement, not to a default.

The dedicated, corrosion-resistant exhaust

Lab exhaust runs on its own dedicated system and never mixes with the building's comfort return. The air leaving a hood is contaminated by definition, so it goes straight outside and is never recirculated to other spaces or run back through an air handler. Tying a hood into general return, or sharing a duct with a comfort exhaust, is the kind of mistake that quietly distributes lab fumes through occupied floors. Keep it separate, single-purpose, and exhaust-only.

The duct has to survive what the hood carries. Standard galvanized corrodes fast in acid and solvent service, so lab exhaust commonly uses corrosion-resistant material chosen for the chemistry: fiberglass-reinforced plastic (FRP), coated steel, polypropylene or PVC for many acids, and stainless grades such as 304 or 316 where temperature or specific corrosives demand it. The material selection follows the chemicals in use and the engineer's call, not a single default.

One detail separates lab exhaust from comfort ductwork and it is a safety feature, not an accident. The entire run inside the building stays under negative pressure, achieved by putting the fan at the discharge end on the roof and pulling the duct rather than pushing it. A leak in a negative duct draws room air in. A leak in a positive duct would push fume out into the ceiling plenum the duct passes through, which is exactly what you cannot allow.

Manifolded vs individual exhaust

There are two ways to connect the hoods to the outside. An individual system gives each hood its own fan and stack. A manifolded system combines many hoods into a common duct served by a central bank of fans. Most larger labs built today are manifolded, and the reasons are dilution, redundancy, and control.

Manifolding dilutes. Combining the streams mixes the output of many hoods so no single chemical reaches a high concentration in the discharge, and it lets one tall, high-velocity stack do the dispersion work for the whole building instead of a forest of small ones poking through the roof. It also enables redundancy: a central bank can be built with a standby fan, commonly an N+1 arrangement, so a fan failure does not drop containment on every hood it served. And it pairs naturally with VAV, because the combined flow varies smoothly as sashes move.

Individual systems are simpler and isolate one hazard to one stack, which has a place for a perchloric acid hood or a radioisotope hood that must be kept separate and wash-down clean. The tradeoff is more roof penetrations, more small stacks to disperse, no shared redundancy, and a fan failure that takes a hood fully offline. The choice is the engineer's, driven by the chemistry, the count of hoods, and what has to stay segregated.

The exhaust fan

The lab exhaust fan lives at the discharge end, usually on the roof, so the duct it serves stays under negative pressure all the way through the building. That placement is the whole reason a duct leak pulls in instead of blowing out. The fan is sized for the total exhaust at design conditions, with the static pressure of a long corrosion-resistant run and the energy to throw the stack plume at high velocity both in the number.

Two features set a lab fan apart from a comfort fan. First, redundancy: lab containment is a life-safety function, so a central manifold is commonly built with a standby fan and automatic changeover, and the system is designed so a single failure does not collapse the negative pressure on the hoods. Second, ignition control where flammables are handled. A fan moving flammable vapors should be spark-resistant construction, built so the wheel cannot strike the housing and throw a spark, specified to the AMCA spark-resistant classifications and coordinated with NFPA 45.

Wetted parts and wheels are chosen for the chemistry the same way the duct is, often FRP, coated steel, or a higher alloy. Belt-driven fans need the belts and bearings on the maintenance list, because a fan that slows or stops is a hood that has quietly lost containment.

The stack and the dispersing discharge

The stack is the safety device at the end of the system, and its job is dispersion: throw the exhaust high enough and fast enough that it dilutes in the atmosphere and never comes back into the building. Two levers do the work, height and velocity. ANSI/AIHA Z9.5 commonly points to a discharge velocity on the order of 3000 fpm and a stack reaching at least 10 ft above the adjacent roof line, extending above any rooftop screen, so the plume clears the turbulent air that wraps around the building.

Velocity matters as much as height because of stack wake downwash. A slow plume gets caught in the low-pressure pocket on the lee side of the stack and curls straight back down onto the roof, undoing the height. The high exit speed launches the plume up through that pocket and gives it momentum to rise and dilute. This is why a lab stack discharges straight up at speed and why you never put a rain cap on it that deflects the plume sideways.

The 3000 fpm and 10 ft figures are common reference values, not the whole answer. Z9.5 allows a design to depart from them when a site-specific dispersion analysis demonstrates the dilution criteria are met at every intake and receptor. On a tight site, near a taller neighbor, or with an especially toxic discharge, that modeling is the responsible path, and the engineer and the AHJ control what is acceptable.

What is exhaust re-entrainment?

Re-entrainment is the lab plume coming back into the building, usually by being drawn into a fresh-air intake after the stack failed to disperse it. It is the design failure the stack exists to prevent, and it is the reason hood exhaust cannot be treated like a bathroom fan that just needs to get air outside the wall. Outside the wall is not far enough when the air is toxic.

Several things drive it. A stack too short or too slow lets the plume downwash onto the roof. An intake too close to the stack, or downwind of it on the prevailing wind, pulls the plume straight back in. A rooftop unit, a taller adjacent building, or a parapet can create recirculation zones that trap and return the exhaust. Wind is the variable that ties it together, because the worst case is rarely a calm day.

The defenses are separation, height, and velocity working together: keep intakes well away from and ideally upwind of stacks, raise and speed up the discharge, and where the geometry is tight, model it. Z9.5 sets the framework and a dispersion analysis settles the hard cases. Re-entrainment that shows up as headaches or odors at the intakes after a building opens is expensive to fix, because the cure is usually a taller stack or a relocated intake on a finished roof.

Keeping the lab negative to the corridor

A chemical lab is held at negative pressure relative to the corridor and the spaces around it, so air flows into the lab and fume cannot drift out to occupied areas. That direction is set by an offset: the lab exhausts more air than it supplies, and the deficit is made up by transfer air pulled in from the corridor through the door undercut and gaps. Lose the offset and the lab can go positive and push contaminated air into the building.

The offset is a deliberate number, commonly framed as exhaust exceeding supply by around 10 percent or by a fixed airflow such as a couple hundred CFM, sized so the door consistently pulls inward without slamming. It builds into a cascade across the building: the labs sit negative to the corridors, the corridors and offices sit neutral to slightly positive, and the dirtiest rooms sit most negative of all, so air always moves from clean toward dirty and out the stack.

Holding that offset while the hoods modulate is the hard part on a VAV lab, because the supply has to track the exhaust in real time to keep the room from swinging positive when sashes close. The building-pressurization guide covers the air balance and the controls that hold it. The lab-specific point is simply that negative is not optional here, it is containment, and it has to survive every hood position.

Makeup air for the hoods

Every cubic foot a hood exhausts is a cubic foot the lab has to bring back in, and on a lab full of hoods that is a large, deliberate makeup-air load. The makeup air is supplied, tempered, and delivered low and gently so it does not blow across the hood faces and disturb capture. Starve the makeup and the whole system collapses: the fans cannot pull the design exhaust, the face velocity drops, the lab fights to find air through every crack, and containment goes with it.

Tempering is part of safety, not just comfort. Dumping raw winter air at a hood face creates cold drafts that cross the opening and break capture, and it freezes coils and makes the space unusable. The makeup unit conditions the air to a neutral supply temperature so it can be introduced at volume without disturbing the hoods. This is also where a lab's energy cost concentrates, which is the real argument for VAV hoods that cut the makeup load when sashes are closed.

The supply has to be able to feed every hood drawing at once, the true simultaneous demand, not an averaged guess. Size the makeup short and the hoods quietly compete for air and none of them holds velocity. The makeup-air and pressurization guides work this balance in detail. For the lab it reduces to one rule: the makeup air has to keep up with the exhaust, or the hood is not safe.

The dangerous interactions

The pieces of a lab ventilation system are coupled, and the failures come from the couplings, not from one box breaking. This is the system view, and it is what separates lab work from comfort HVAC. The same change that is harmless in an office can spill fume in a lab.

Turn off the exhaust, or let a fan fail without a standby, and the lab loses its negative pressure and the hoods stop containing at the same instant. Shut down or undersize the makeup air and the exhaust fans cannot pull their design flow, so face velocity drops across every hood at once even though nothing visibly changed. Run too many VAV hoods open together past what the system was balanced for and they fight each other for a fixed pool of exhaust and supply, and the weakest hood loses velocity. Push the building positive by over-supplying or by an exhaust trip and the lab vents its fumes outward into the corridor.

The lesson the trade learns the hard way: never touch one part of a lab system in isolation. Disabling an exhaust fan for service, closing a makeup damper, or rebalancing a floor without accounting for the hoods can put a worker in front of a hood that looks normal and is spilling. Interlocks, standby fans, and a commissioning that tested the failure modes are what keep the couplings from becoming the exposure.

Lab air change rate vs hood exhaust

The room air change rate is a separate job from the hood. It is dilution ventilation for the whole lab, flushing the general air to handle a small spill, off-gassing, or fugitive vapor that never made it into a hood. The hood protects the worker at a point source. The air changes protect the room. Designs carry both, and confusing them is how a lab ends up under-ventilated despite plenty of hood exhaust.

Common practice has landed around 6 to 12 air changes per hour for an occupied chemical lab, with a setback to a lower rate, often something like 4 ACH, when the lab is unoccupied to save energy. Recent ANSI/ASSP Z9.5 has put numbers around this, in the range of roughly 4 to 10 ACH, and NFPA 45 frames minimums for labs using chemicals, commonly cited near 4 ACH unoccupied and 8 ACH occupied. The adopted edition and the project documents control the figure.

One caution from the standard itself. Z9.5 has noted that air changes per hour is not really the right concept for contaminant control, because dilution depends on where the contaminant is and where the air moves, not just on a volumetric rate. Treat the ACH number as a floor for general dilution and the hood as the actual protection, and do not let a high room air change rate convince anyone a weak hood is acceptable.

Snorkels, canopy hoods, and biosafety cabinets

The fume hood is not the only capture device, and matching the device to the hazard is part of the design. A snorkel, or elephant trunk, is a small articulated local-exhaust arm you position right at a point source, good for a bench instrument or a soldering station where a full hood is overkill. It only captures within a few inches of its mouth, so placement is everything.

A canopy hood hangs over a process and relies on rising air to carry contaminant up into it. That makes it useful only for genuinely hot processes, an autoclave or a steam bath, where the thermal lift does the work. Used over a cold chemical source it does almost nothing, because there is no plume to catch and the worker stands in the path of whatever drifts up.

A biosafety cabinet is a different animal that looks similar and is not interchangeable. A chemical fume hood protects only the worker and exhausts everything outside. A biosafety cabinet uses HEPA filtration to protect the worker, the sample, and the environment, and many types recirculate filtered air back into the room. HEPA traps particles and biological agents but does nothing for chemical vapor, so a standard biosafety cabinet is wrong for volatile chemistry, and a fume hood is wrong for sterile or infectious work. The hazard, biological or chemical, decides which one you are allowed to use.

What is an ASHRAE 110 test?

ANSI/ASHRAE 110 is the standard performance test that proves a fume hood actually contains, rather than just measuring how fast air moves at the face. It has three parts run together: a face-velocity traverse across the opening, a flow-visualization step using smoke to watch the air sweep inward and around the interior, and the tracer-gas test that is the heart of it. A known rate of tracer is released from an ejector inside the hood while a mannequin with a sensor at its breathing zone stands at the face, and an analyzer reads how much tracer reaches that breathing zone.

The tracer has historically been sulfur hexafluoride (SF6), measured at very low concentrations. SF6 is a strong greenhouse gas, and some jurisdictions, California among them, have restricted its use for field tests, so alternative tracers are coming into use. The result is reported as a containment level in parts per million at the breathing zone, and lower is better.

The test runs in three conditions, and the distinction matters. As Manufactured (AM) is the factory test of the bare hood. As Installed (AI) is the test in the real lab with the real exhaust, makeup air, and room currents, which can be very different from the factory. As Used (AU) is the test with real apparatus and a real user in place. A hood can pass AM in a clean factory and fail AI because of cross-drafts from a door or a diffuser blowing across its face. The acceptance level and which conditions are required come from the project specification, the industrial hygienist, and the standard's edition, not from a single universal number.

The face-velocity monitor and alarm

Every working hood should have a flow monitor at the user's eye level that tells them, before they start, that the hood is pulling. It continuously reads airflow or face velocity and alarms, visibly and audibly, when the flow falls below the safe setpoint. Without it, a worker has no way to know a fan tripped or a duct clogged, because a dead hood looks exactly like a live one.

The monitor is a safety device and gets treated like one. It needs a clear normal indication so the user is not trained to ignore a constant nuisance light, a real low-flow alarm tied to the actual airflow, and periodic calibration so the number it shows matches what an anemometer measures at the face. A monitor reading green while the real face velocity has dropped is worse than no monitor, because it manufactures false confidence.

A sash-position alarm earns its place too, especially on VAV hoods. It warns when the sash is raised above the safe working height or left open when the hood is idle, which is the most common unsafe condition and the one that also wastes the most makeup air. The alarm is what turns the rule about keeping the sash low into a habit.

Commissioning the lab ventilation

Commissioning is where you prove the system contains before anyone works in it, and on a lab it is not optional polish. The whole point of the previous sections is that the failures are invisible, so the only way to know the lab is safe is to test it as a system at the conditions it will actually run.

The scope is broad because the parts are coupled. Verify face velocity at every hood with a calibrated grid traverse. Run the ASHRAE 110 test, at minimum as-installed, to confirm real containment with the real exhaust and makeup in service. Confirm the room pressure offset holds the lab negative to the corridor, and that it stays negative through the full range of sash positions on VAV hoods. Check that the makeup air can feed every hood at simultaneous full flow without the velocities sagging. Exercise the controls: open and close sashes and watch the VAV valves track and the velocity hold. And run smoke at each face to see the capture with your own eyes.

Test the failure modes, not just the happy path. Trip a fan and confirm the standby starts and the negative pressure holds. Simulate a makeup-air loss and watch what the hoods do. A commissioning that only verifies the design intent on a calm afternoon has not tested the conditions that actually expose people.

Maintenance and recertification

A lab ventilation system drifts out of safety quietly, so it lives on a maintenance schedule rather than a complaint-driven one. The fan is first: belts stretch and slip, bearings wear, and a fan turning slower than design pulls less air and drops face velocity across every hood it serves. Direct-drive and standby fans need their own exercise and verification so the redundancy is real on the day it is needed.

The duct is the slow failure. Corrosion-resistant material buys years, not forever, and acid service eats joints, dampers, and the low points where condensate collects. Inspect for thinning, leaks, and blockage, because a duct leak on a negative system is at least pulling in rather than out, but a holed or restricted duct still robs the hoods of flow. Keep the monitor calibrated, because a drifted monitor is a safety device lying to the user.

Recertify the hoods on a cadence, commonly annually and after any change to the hood, the exhaust, or the room airflow that could affect it. Recertification re-measures face velocity and reruns the containment check, and it catches the slow drift before it becomes an exposure. The sash mechanism is on the list too: stops, counterbalances, and tracks have to keep the sash moving freely and stopping where it should, because a sash that is hard to lower is a sash that gets left up.

The industrial hygienist and safe use

The hardest part of lab ventilation is the part the HVAC scope does not own: how the hood gets used. A hood balanced perfectly to 100 fpm protects nobody if the worker stores reagent bottles across the back baffle, runs with the sash wide open, or puts their head inside to read a gauge. The engineered control and the human practice have to meet, and the industrial hygienist is the person who owns that meeting.

The industrial hygienist defines the containment goal from the actual chemistry, sets the face-velocity target and the test acceptance, and writes the safe-use practices into the chemical hygiene plan that OSHA's laboratory standard requires. The HVAC and commissioning teams deliver and prove the airflow. The hygienist confirms it matches the hazard and trains the users on the practices that keep it working.

The blunt field truths belong here. A fume hood is not a storage cabinet, and a hood crowded with bottles is a hood that cannot capture. Keep work at least 6 in back from the face, keep the sash low, keep your face out of the opening, and do not assume a quiet hood is a working hood without the monitor confirming it. The best ductwork in the building cannot fix a user who treats the hood as a shelf.

What to document

A lab hood that nobody can show is safe is a liability waiting on an inspection or an exposure report. The record is what proves containment was verified and what gives the next certifier a baseline to compare against. Capture the measured face velocity and the grid points, the ASHRAE 110 result and which conditions were tested, the room pressure offset, the makeup-air verification, the controls check, the monitor calibration, and the date and certifier for each.

Tie those records to the specific hood and keep them where the certification cadence and the alarm history travel with the asset. A field tool such as FieldOS keeps the face-velocity readings, the test reports, the photos of the smoke check, and the recertification dates attached to the hood and timestamped, so the annual comparison is a lookup instead of a search through binders. The point is the same as on any safety system: if it was not recorded, it cannot be defended, and the slow drift that maintenance exists to catch is only visible against last year's number.

Common mistakes

• Low or uneven face velocity that passes on the center reading but spills at a corner.
• Lab exhaust tied into the building comfort return or recirculated instead of run dedicated to outside.
• Makeup air undersized for simultaneous hood demand, so face velocity sags when several hoods run at once.
• A stack too short or too slow, or an intake too close, that re-entrains the plume into the building.
• The lab going positive to the corridor when sashes close, pushing fume out to occupied space.
• No ASHRAE 110 containment test or no working flow monitor, so containment was never actually proven.
• Working with the sash high, storing bottles across the rear baffle, or putting the head inside the hood.

Field checklist

Standards and references

The governing document for lab ventilation is ANSI/AIHA Z9.5, now published as ANSI/ASSP Z9.5, Laboratory Ventilation. It is the source for the performance approach to face velocity, the air change guidance, and the stack discharge values such as the common 3000 fpm velocity and the 10 ft height above the roof, while allowing a dispersion analysis to justify a departure. It is a consensus standard, so what it sets becomes enforceable when a project specification or the AHJ adopts it.

Hood performance is tested to ANSI/ASHRAE Standard 110, the tracer-gas containment method with its As Manufactured, As Installed, and As Used conditions. Fire and explosion protection for labs using chemicals comes from NFPA 45, which drives the flammable-vapor limits and the spark-resistant fan considerations, alongside the ignition-control framing engineers coordinate with it. OSHA's laboratory standard, the Occupational Exposure to Hazardous Chemicals in Laboratories rule, requires the chemical hygiene plan that ties the engineering controls to safe practice. ASHRAE provides the broader design framework for the air systems that feed it.

Hedge the design numbers to the people who own them. The face velocity, the dedicated exhaust and its materials, and the stack height and velocity are set by ANSI/AIHA Z9.5, the project engineer, and the industrial hygienist, and confirmed by the AHJ for the adopted edition and any local amendments. Verify the current edition and the specific requirement before citing any figure on a submittal, because these standards revise on a cycle and a jurisdiction can amend them.

Units, terms, and definitions

Lab ventilation borrows terms from several trades, and the same idea reads differently on a hood schedule, a test report, and a chemical hygiene plan.

Face velocity is in feet per minute (fpm), with metric reports in meters per second (about 0.5 m/s near 100 fpm). Exhaust and makeup airflow are in cubic feet per minute (CFM), or liters per second in metric. Room ventilation is in air changes per hour (ACH). Pressure offset is in inches of water column (in. w.c. or in. wg) or pascals. Stack discharge velocity is in fpm or m/s. Tracer-gas containment is reported in parts per million (ppm) at the mannequin breathing zone.

Fume hood

An enclosure that protects a worker by drawing air in across its opening and exhausting the contaminated air outside, the primary containment for chemical work

Face velocity

The inward air speed across the hood opening, in fpm, commonly near 100, the working proxy for capture

Sash

The movable front of the hood that shields the worker and sets the face velocity by changing the opening size

CAV vs VAV hood

Constant-air-volume pulls fixed airflow at all sash positions; variable-air-volume tracks the sash to hold a constant face velocity and save energy

Dedicated exhaust

A corrosion-resistant, exhaust-only duct system that serves the hoods and never mixes with or recirculates to the building comfort air

Stack re-entrainment

The exhaust plume coming back into the building through an intake because the stack failed to disperse it

ASHRAE 110

The tracer-gas test that proves hood containment by measuring how much tracer reaches a mannequin's breathing zone, run as-manufactured, as-installed, and as-used

Lab pressurization

Holding the lab negative to the corridor by exhausting more air than is supplied, so fumes stay in the lab

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