NOTE This standard governs the specification, selection, construction, installation, and performance verification of laboratory exhaust and chemical fume hood systems serving spaces that generate hazardous chemical, biological, or radiological vapors. (1.1)

NOTE The following are within scope of this standard. (1.2)

NOTE The following are outside the scope of this standard and are governed elsewhere. (1.3)

NOTE A chemical fume hood draws contaminated air away from the operator's breathing zone and discharges it — it protects the operator from chemical vapors but provides no product protection and no biological filtration; a biological safety cabinet (BSC) protects the sample and operator from biological agents through HEPA filtration but does not reliably contain chemical vapors. The two are not interchangeable: specifying one where the other is required is a recurring source of safety failures and re-procurement. Where biological agents are present, refer the cabinet selection to NSF/ANSI 49. (1.4)

NOTE ANSI/ASSP Z9.5-2022 supersedes the former ANSI/AIHA Z9.5-2012 designation; the sponsoring organization changed from AIHA to ASSP, and references to the older designation shall be read as the current edition. (2.3)

NOTE NSF/ANSI 49 is listed for disambiguation only; it governs biological safety cabinets, which are outside this standard's scope. (2.4)

NOTE Face velocity traversal alone shall not be accepted as proof of containment; the tracer-gas (SF6) containment test shall be performed and shall meet the specified control level — a hood can meet its average face velocity target and still fail to contain because a well-formed face velocity profile says nothing about whether vortices at the sash carry tracer back into the operator's breathing zone; the ASHRAE 110 protocol exists precisely because face velocity and containment diverge in practice. (4.3)

NOTE Hood body aerodynamic performance shall be established by the ASHRAE 110 AM rating, not by face velocity setpoint alone — an aerodynamically optimized hood body achieves containment at a lower face velocity than a poorly formed body, and selecting hoods on a single fpm target without regard to the demonstrated AM rating both wastes exhaust energy on good hoods and under-protects on bad ones; the AM designation is the governing performance metric, and the face velocity setpoint is a means, not the criterion. (4.5)

NOTE The design face velocity setpoint shall be selected for the specific hood and hazard within the Z9.5 performance range — ANSI/ASSP Z9.5-2022 establishes 80 to 120 fpm (0.4 to 0.6 m/s) average at the sash opening as the working range; 100 fpm is the common default for general chemistry research; lower velocities reduce energy and can perform well on aerodynamically superior hoods; higher velocities do not necessarily improve containment and can induce sash-edge turbulence; the ±20% point-to-point uniformity limit guards against a misleading average produced by hot and dead spots. (5.2)

NOTE Room background air change rate is independent of hood exhaust and is set by the room hazard, not the hood count — ANSI/ASSP Z9.5-2022 §5 ranges from 4 ACH for low-hazard and ASHRAE 170 clinical spaces to 10 ACH for high-hazard chemical research; 6 ACH is the common default for general chemistry teaching and research labs; this background rate dilutes fugitive emissions in the room and is in addition to, not satisfied by, the air drawn through the hoods. (5.4)

NOTE Make-up air shall track exhaust closely enough to preserve the design pressure relationship under all sash and VAV conditions — if exhaust exceeds supply the laboratory goes excessively negative, slamming doors and drawing contaminants in from corridors; if supply leads exhaust the room can go positive and push vapors into adjacent spaces; on VAV systems the supply must respond fast enough to follow hood exhaust swings without inverting the pressure relationship; make-up air coordination is a shared boundary with Dedicated Outdoor Air Systems and, where energy recovery is applied, with Energy Recovery Ventilators. (5.6)

NOTE Hood type selection trades energy, flexibility, and hazard suitability — CAV hoods exhaust a fixed volume regardless of sash position and are simplest and lowest first cost; VAV hoods modulate exhaust to hold face velocity as the sash moves, saving substantial energy but requiring responsive controls and supply coordination; specialty hoods (perchloric acid, radioisotope, high-heat/digestion, distillation column) address hazards that ordinary hoods cannot; a CAV hood cannot be readily rebalanced to VAV later, so default to VAV in any research lab where future flexibility matters. (6.2)

NOTE Sash type governs both ergonomics and exhaust demand — vertical-rising sashes give an unobstructed full-width opening and are the default for general bench work; horizontal-sliding sashes reduce open area and lower exhaust on VAV systems; combination sashes offer both at higher cost and complexity; on VAV systems the chosen configuration directly sets the maximum open area and therefore the maximum exhaust the system must be sized to deliver. (6.5)

NOTE Liner selection is a chemical-compatibility decision, not an aesthetic one — epoxy resin is the general-purpose default, suitable for the majority of organic and dilute-acid chemistry; polypropylene resists strong acids and bases used in trace-metal and digestion work; Type 304 stainless steel gives a coved, easily decontaminated surface for radioisotope work; Type 316 stainless steel is required for perchloric acid because of its oxidizing service; FRP is used where specific corrosion resistance is needed; the liner must be matched to the worst-case chemistry because a mismatched liner degrades and becomes a containment and contamination problem. (6.7)

NOTE Perchloric acid service demands oxidizer-resistant materials and routine wash-down because perchlorate salts are shock-sensitive — perchloric acid vapor condenses in the hood and duct and deposits perchlorate crystals that become friction- and shock-sensitive explosives when dry; the wash-down system flushes these deposits before they accumulate; a hot-water flush at roughly 60 to 70 °C dissolves them more effectively than cold; the entire wetted path must be Type 316 stainless steel since FRP and ordinary stainless are incompatible with the oxidizing service; the wash-down drain must connect to the acid waste system and that connection must appear on the plumbing and structural drawings early, because it is frequently missed until the hood arrives. (7.4)

NOTE Radioisotope hood interiors must be fully decontaminable — coved corners and a seamless stainless liner leave no crevice where contamination can lodge, so the interior can be wiped down and surveyed to clearance; a dedicated monitoring port allows continuous air sampling of the exhaust for released activity; these features distinguish a radioisotope hood from a general chemical hood and are not optional retrofits. (7.6)

NOTE High-heat work needs vertical clearance and heat-tolerant materials — digestion blocks, hot plates, and distillation columns produce both tall apparatus and hot, often corrosive, vapor; interior clearance is typically increased to 36 to 42 in. versus the standard 30 in. to accommodate columns and glassware; the liner top and ductwork are rated for the elevated temperature (ceramic or stainless rather than epoxy); standard hoods used for this duty deform, discolor, and lose containment over the apparatus. (7.8)

NOTE Ductless hoods are filtration devices and are inappropriate for chemicals that break through carbon — a ductless hood draws air through activated carbon and HEPA media and returns it to the room; carbon does not capture formaldehyde, strong acids, strong bases, or many common solvents; a saturated filter releases captured vapor back into the room with no warning; filter compatibility must be confirmed against the specific chemicals in use before a ductless hood is selected, and ductless hoods are never a substitute for a ducted hood on hazardous chemistry. (7.10)

NOTE A VAV hood must never be allowed to fall to zero exhaust at a closed sash — even with the sash down a hood must continue to capture fugitive emission from spills and apparatus left inside; ANSI/ASSP Z9.5-2022 requires a minimum exhaust that preserves a safe face velocity at the minimum sash opening; a minimum near 150 to 250 cfm is typical for a 4-ft hood but must be set explicitly; omitting the minimum cfm setpoint is a common VAV controls error that leaves the hood unsafe at rest. (8.2)

NOTE Maximum exhaust scales with hood width and open face area — at a given face velocity the exhaust a fully open hood draws is set by its open area; typical maxima are 400 to 600 cfm for a 4-ft hood, 600 to 900 cfm for a 6-ft hood, and 800 to 1,200 cfm for an 8-ft hood; these figures size the branch duct and the connected load on a manifold; the actual value follows directly from the chosen face velocity and sash configuration. (8.4)

NOTE Sizing a manifold fan for 100% of connected hoods is wasteful and is an avoidable RFI — not every hood on a manifold runs at full open simultaneously; ANSI/ASSP Z9.5 and ASHRAE Chapter 16 permit a diversity factor, typically 0.50 to 0.75 of the summed connected maximum, with 0.65 a common default; sizing the fan to the full undiversified total oversizes the fan, the VFD, and the make-up air, and is regularly flagged in design review; the factor selected must be justified by the expected simultaneous-use pattern of the served labs. (8.6)

NOTE Dedicated versus manifolded topology drives fan selection — a dedicated single-hood fan isolates one hood (appropriate for incompatible or especially hazardous service) but multiplies roof penetrations and fans; a manifolded system collects several hoods on one riser to a single or redundant high-plume fan, which is more economical and allows dilution of any one hood's effluent in the combined stream; fan construction (FRP or stainless centrifugal, vane-axial, or high-plume dilution) follows from the corrosivity of the effluent and the dispersion requirement; high-plume dilution fans induce ambient air to raise stack exit velocity and lift the plume clear of the building. (9.2)

NOTE Redundancy is a continuity-of-containment decision — an N arrangement has no spare; a failed fan stops exhaust and the hoods become unsafe; N+1 provides a standby fan that takes over on failure or for maintenance, which is the common choice for research manifolds; 2N provides full duplication for the most critical facilities; automatic switchover and its sequencing with the bypass damper and VFD must be specified, not left to the controls trade to infer. (9.4)

NOTE Stack height and intake separation must be resolved in early design, not after the roof is set — a stack discharging too low or too close to an intake re-entrains its own effluent into the building; 10 ft above the roofline is a practical minimum; ASHRAE Chapter 16 dispersion modeling is warranted whenever a stack falls within 50 ft of any intake; discovering a re-entrainment problem after the roof is built forces expensive remediation or a high-plume fan upgrade; the stack location relative to intakes is an arrangement that belongs on the drawings: exhaust stack and intake locations. (9.8)

NOTE The airflow monitor is the operator's only real-time indication that the hood is safe — without a monitor an operator cannot know that exhaust has failed or fallen below a safe face velocity; ANSI/ASSP Z9.5-2022 requires a monitor with an audible and visual alarm; a setpoint roughly 20% below design (for example, an 80 fpm alarm against a 100 fpm design) warns before containment is actually lost; the sensing technology (paddle/differential-pressure, hot-wire anemometer, or averaging pitot array) is selected for accuracy and maintainability. (10.4)

NOTE VAV controls hold face velocity as the sash moves; supply must follow — a VAV hood controller reads sash position and modulates the exhaust valve to keep face velocity constant; the sash sensor may be ultrasonic, infrared, or mechanical; whether the controller is stand-alone or BAS/DDC-integrated, the room supply must be able to respond fast enough to follow the exhaust swing or the room pressure inverts; specifying VAV hoods without confirming the supply system's response speed is a common coordination failure. (10.6)

NOTE Independent specification of the bypass damper and VFD produces controls conflicts — on a modulating high-plume system the bypass damper admits induction air while the VFD sets fan speed; the two together hold both stack exit velocity and system static pressure; if the damper and VFD are specified by different trades without a unified sequence, they fight each other and generate RFIs and unstable operation. (10.8)

NOTE Duct material is a chemical-service decision — PVC-lined galvanized duct suits many general chemical exhausts; FRP resists a broad range of corrosives; stainless steel handles high temperature and is mandatory for perchloric acid; FRP shall never be used for perchloric acid service, where its organic resin is incompatible with the oxidizing condition and Type 316 stainless steel is required; round duct is generally preferred over rectangular for the negative pressures and cleanability of lab exhaust. (11.2)

NOTE Leakage class scales with the toxicity of the effluent — because lab exhaust runs at negative pressure, leakage draws room air inward and is less hazardous than positive-pressure leakage, but it still degrades capture and balance; SMACNA Leakage Class 3 (3 cfm/100 sf at 1 in. w.g.) is the minimum for standard lab exhaust; perchloric acid and highly toxic chemical service require a tighter sealed/pressure-tested construction (Leakage Class 6 or pressure-tested) to ensure nothing escapes the duct path. (11.4)

NOTE Lab exhaust duct fire-rating is governed by NFPA 90A and NFPA 91, not by ordinary HVAC practice — hazardous-vapor exhaust duct carries flammable effluent, so its penetrations of fire barriers must preserve the rating without compromising the exhaust function; NFPA 90A governs the rated assembly and damper requirements at penetrations and NFPA 91 governs the construction and installation of the vapor-conveying duct itself; penetration locations and ratings are an arrangement to coordinate with the fire protection engineer: rated penetrations and firestopping. (11.7)

NOTE Make-up air strategy trades energy against capture stability — untempered transfer air from adjacent spaces is the lowest energy option but introduces uncontrolled temperature and humidity at the hood; room DOAS supply tempers and distributes make-up through the room (the common modern approach), coordinated with Dedicated Outdoor Air Systems; a tempered auxiliary (compensating) air device delivers make-up directly at the hood face but is energy-intensive and rarely specified for new work; whatever the strategy, exhaust and supply must be balanced or the lab pressure relationship fails. (12.2)

NOTE Bench area per hood is limited by NFPA 45 fire hazard class — NFPA 45-2019 limits the maximum bench area served per hood as a function of the laboratory's flammable-liquid quantity classification (for example, 50 ft² per hood for the highest-quantity Class A labs); the limit constrains how much hazardous bench work a single hood may serve and must be coordinated with the fire protection engineer during layout, because it can drive the number of hoods required. (13.2)

NOTE The tracer-gas control level is the contractual acceptance criterion — the ASHRAE 110 designation reports the tracer-gas concentration at the breathing zone at a stated release rate (4 AM 0.05 means under 0.05 ppm SF6, the criterion for standard labs; some owners accept 4 AM 0.10 for lower-hazard applications); the field AI test must meet the specified level; without this language in the specification, commissioning agents frequently omit the tracer-gas test, leaving the owner with no performance baseline. (14.3)

NOTE Annual testing and inspection labeling must be contractually required, not assumed — NFPA 45-2019 requires an inspection label on each hood recording the date, average face velocity, and next inspection date; annual ASHRAE 110 field testing maintains the containment baseline established at commissioning; these recurring requirements belong in the specification so the owner inherits a maintainable, documented system rather than an untested one. (14.6)

NOTE Cross-drafts defeat containment regardless of exhaust volume — a draft across the hood face from a nearby supply diffuser, an opening door, or someone walking past can exceed the face velocity locally and peel contaminant out of the hood; diffuser throw and door and traffic paths must be kept clear of the sash plane, which is why hood location is coordinated against the room airflow layout rather than chosen for convenience alone. (15.3)

NOTE Service connections must be located before the hood arrives — the hood's service fixtures, electrical, exhaust collar, and (for perchloric hoods) the wash-down acid waste drain all rely on rough-in placed during construction; the acid waste drain in particular is frequently missed because it must appear on the plumbing and structural drawings and be installed before the hood is set; coordinating these connections late forces wall and floor demolition. (15.6)