Fume HoodInformationCenter

A fume hood is a ventilated enclosure used in laboratories to protect users from inhaling hazardous fumes, vapors, and dust. It creates a barrier between the user and the chemicals by drawing air inward through the sash opening and exhausting it safely outside the building. It is the primary engineering control for chemical hazard containment and is required by OSHA (29 CFR 1910.1450) whenever workers may be exposed to hazardous chemicals above action levels.

Most regulatory guidelines (ANSI/AIHA Z9.5) recommend a face velocity of 80–120 feet per minute (fpm) at the sash opening, with 100 fpm as the most common institutional target. High-performance low-flow hoods validated by ASHRAE 110 tracer gas testing may safely operate at 60–80 fpm. Higher is not always better — velocities above 150 fpm create turbulence at the sash face that can paradoxically reduce containment by generating outward-directed eddy currents.

No. A standard chemical fume hood does NOT protect samples from biological contamination, does not have HEPA filtration, and is not rated for biohazardous materials. For biological work (cell cultures, bacteria, viruses), you must use a Biosafety Cabinet (BSC). Class II BSCs recirculate HEPA-filtered air over the work surface, protecting both the user and the sample. Using a chemical fume hood for BSL-2 or higher biological work is a serious safety violation.

Personal protective equipment complements—but does not replace—fume hood protection. At minimum, wear: safety glasses or chemical splash goggles (depending on hazard), a laboratory coat, and appropriate gloves (nitrile for most chemicals; check chemical compatibility). For highly toxic, corrosive, or volatile chemicals, a face shield may be needed. Never rely on a respirator as a substitute for a properly functioning fume hood — hoods are the primary engineering control.

Several chemical types require specialized hoods. Perchloric acid (HClO₄) requires a dedicated perchloric acid hood with stainless steel construction and an integrated wash-down system — its vapors form explosive perchlorate salts in standard ductwork. Radioactive materials require HEPA-filtered radioisotope hoods. Biological materials require biosafety cabinets, not fume hoods. Hydrofluoric acid requires specific compatible liner materials. Always consult your institution's EH&S office before using highly toxic, reactive, or unusual chemicals.

Chemical odors outside a running fume hood are a warning sign of hood failure or spillage. Immediately stop work, cover open containers, and close the sash. If the odor is strong or the chemical is acutely hazardous, evacuate the area and call emergency services or EH&S. Do not resume work until the cause is identified and corrected, and the hood has been retested. For ductless hoods, chemical odors indicate filter breakthrough — the hood must not be used until filters are replaced.

The simplest field test is a tissue paper or smoke tube test: hold a tissue just inside the sash opening and observe that it is drawn inward. Formal testing includes face velocity measurement with an anemometer (ASHRAE 110 FM test) and a smoke visualization test (SM test). The most sensitive test is the ASHRAE 110 tracer gas (AS test) using SF6 gas — a result ≤ 0.05 ppm confirms containment. Annual professional certification is required; do not rely only on informal tests.

The sash is the sliding glass panel at the front of the fume hood that serves as both a physical barrier and an airflow controller. It must be kept at or below the certified safe working height (marked by a sticker on the hood — typically 18 inches) during all chemical work. Raising the sash above this mark reduces face velocity proportionally, compromising containment. Close the sash fully whenever you step away, even briefly — this protects both safety and energy efficiency.

Keep all equipment and open containers at least 6 inches back from the sash opening — materials at the sash face are in the turbulent zone and can release vapors outward. Elevate equipment on blocks or lab jacks to allow airflow beneath (improves vapor capture). Never block the rear baffle slots — leave the back 3–4 inches clear. Distribute equipment laterally across the hood width rather than clustering everything in the center. Avoid placing equipment near side walls where recirculation vortices can trap vapors.

No — fume hoods are not chemical storage cabinets. Stored containers add clutter that disrupts airflow patterns and reduce containment performance. NFPA 45 limits flammable liquid quantities inside a hood to the minimum amount needed for current work (generally ≤ 1 gallon). Sealed containers gain no benefit from hood ventilation. Store chemicals in designated flammable cabinets or chemical storage areas. Only open containers actively in use belong inside the hood.

Stand slightly to the side of the sash opening rather than directly in front when possible. Avoid leaning deeply into the hood and withdrawing rapidly — the body's wake creates a puff of air that can draw vapors outward. When you must work inside the hood, lean in slowly and withdraw gradually. Keep your arms inside the hood while actively manipulating chemicals, rather than reaching in and out repeatedly. Never work with your head inside the hood above the sash level.

Fume hoods must be certified at minimum annually per ANSI/AIHA Z9.5 and OSHA requirements. Certification involves face velocity measurement, smoke visualization, and alarm function testing — performed by a qualified technician who attaches a dated sticker to the hood. Re-certification is also required after: any hood repair or relocation, HVAC system modifications, room renovations, after any known or suspected performance failure, or following filter replacement on ductless hoods.

The most impactful performance-reducing mistakes: raising the sash above the certified working height (cuts face velocity proportionally), blocking the rear exhaust baffles with equipment or containers, placing equipment at the sash face (turbulent zone), storing large quantities of chemicals inside (disrupts airflow), rapid arm movements at the sash plane (generates outward vapor pulses), and allowing lab doors to remain open (cross-drafts). All of these are user-controlled and immediately correctable.

Treat an airflow alarm as a real safety event: immediately stop all work involving open hazardous materials, cover open containers if safe to do so, and close the sash fully. Do not attempt to continue working through an active alarm. If the alarm clears within 1–2 minutes (transient HVAC fluctuation), verify face velocity with a tissue test before resuming. If the alarm persists, contact facilities and EH&S — do not use the hood until the cause is identified and corrected, and the hood is retested.

Ductless fume hood activated carbon filters should be replaced per the manufacturer's schedule (typically every 6–12 months), but this is a minimum — actual service life depends on chemical types and quantities used. Replace immediately if: chemical odors are detected in the room (filter breakthrough), the filter has reached its maximum rated weight gain, or colorimetric indicators signal saturation. Never operate a ductless hood with an overdue filter. Log all filter replacements with date, chemical usage history, and filter weight.

Daily user responsibilities: visually check the airflow alarm indicator (green/normal), promptly clean any chemical spills from the work surface, and close the sash when not actively working. Weekly: clean the work surface with an appropriate cleaner, check sash glass for cracks. Monthly maintenance by lab manager or facilities: inspect baffles for buildup, verify bypass slot is unobstructed, check sash operation and counterweight mechanism. Report any unusual noises, reduced airflow, or damaged components to facilities immediately.

A ducted fume hood exhausts air through ductwork to the outside of the building — it handles all chemical types and is the gold standard for safety. A ductless (recirculating) hood filters exhaust air through activated carbon and returns it to the room. Ductless hoods require no ductwork (easy installation), but are limited to specific chemicals that carbon filters can capture; they cannot be used for carcinogens, reproductive toxins, highly toxic compounds, or unknown chemical mixtures. When in doubt, use a ducted hood.

VAV (Variable Air Volume) fume hoods use sash position sensors to modulate exhaust volume: when the sash is down, exhaust drops to a minimum setpoint (30–50% of maximum); at full sash height, exhaust reaches design maximum. This contrasts with conventional CAV (Constant Air Volume) hoods that exhaust at full volume regardless of sash position. VAV systems typically reduce fume hood energy consumption by 30–60%, and are eligible for LEED credits. They require integration with VAV room supply air controls to function safely.

A biosafety cabinet (BSC) protects both the user and the sample from biological contamination using HEPA filtration. Class II BSCs (the most common) recirculate HEPA-filtered air over the work surface and exhaust through HEPA filters. A standard chemical fume hood does NOT protect samples from contamination and has no HEPA filtration — it is not suitable for biological work. Use a BSC for biohazardous materials, a chemical fume hood for chemical hazards. Never use one as a substitute for the other.

A perchloric acid hood is a specialized fume hood required for any work involving hot perchloric acid (HClO₄). Perchloric acid vapors react with organic materials in standard ductwork to form explosive perchlorate salts. Perchloric acid hoods have stainless steel interiors and ductwork (non-reactive), a dedicated independent exhaust system, and an integrated water wash-down system to flush accumulated perchlorates from ductwork and the fan. This wash-down must be activated after every use and tested periodically. Never use a standard fume hood for perchloric acid digestions.

Standard fume hood widths are 4 feet, 5 feet, and 6 feet (interior measurement). Choose based on the size of your largest apparatus and the number of items you need inside simultaneously. Remember that overcrowding a hood is a safety hazard — it is better to have a larger hood used properly than a small hood crammed with equipment. Standard interior height is 30–36 inches; walk-in hoods are available for tall apparatus. For multiple simultaneous users or large-scale work, consider multiple hoods rather than one very large hood.

OSHA 29 CFR 1910.1450 (Occupational Exposure to Hazardous Chemicals in Laboratories) is the primary rule. It requires a written Chemical Hygiene Plan (CHP) describing engineering controls including fume hoods, use of engineering controls as the primary exposure reduction method, employee training, and medical surveillance. The standard does not specify exact face velocity numbers but requires that exposures remain below Permissible Exposure Limits (PELs). ANSI/AIHA Z9.5 is the recognized technical standard for demonstrating compliance.

ASHRAE Standard 110 (Method of Testing Performance of Laboratory Fume Hoods) defines the accepted test methods for fume hood performance. It includes three tests: the Face Velocity (FM) test using an anemometer, the Smoke Visualization (SM) test using a smoke tube, and the Tracer Gas Containment (AS) test using SF6 gas. The AS test is the most sensitive — a pass requires ≤ 0.05 ppm SF6 at a mannequin's breathing zone during a 4 L/min SF6 challenge. Testing can be performed 'as manufactured' (AM), 'as installed' (AI), or 'as used' (AU) with actual equipment in place.

ANSI/AIHA Z9.5 (Laboratory Ventilation) is the comprehensive American standard for laboratory ventilation systems. Published by the American Industrial Hygiene Association, it specifies face velocity requirements (80–120 fpm for conventional hoods), requires annual certification, mandates airflow alarms, sets exhaust discharge height requirements above roof level, and requires that makeup air equal 100% of exhaust with the room maintained at slight negative pressure relative to corridors. It is the primary reference standard cited by OSHA for laboratory compliance.

OSHA requires retention of records related to employee exposure for 30 years (medical records) and at least 3 years for other exposure-related records. For practical purposes, most EH&S programs retain fume hood certification records for a minimum of 3 years. Training records should be kept for the duration of employment plus 3 years. Chemical Hygiene Plans must include the current version plus the previous year's version. Filter replacement logs for ductless hoods should be kept for the life of the hood plus 3 years.

A single conventional 6-foot fume hood operated at 100 fpm with an 18-inch sash opening exhausts approximately 1,600 CFM of conditioned air. Replacing this air (heating or cooling) consumes roughly 12,000–18,000 kWh per year depending on climate — costing $1,200–$2,000/year at $0.10/kWh. A VAV hood with users maintaining a low sash position can reduce this by 50–70%. Across a large research building with 50+ fume hoods, fume hood exhaust is often the single largest energy expense, sometimes exceeding the energy used by all laboratory equipment combined.

The single highest-impact behavior is keeping the sash at the lowest position needed for current work and closing it fully when not actively working. On a VAV hood, reducing the sash from 18 inches to 6 inches can cut exhaust volume by ~65%. Other measures: install VAV controls on existing CAV hoods (payback typically 2–4 years), upgrade to certified high-performance low-flow hoods, add sash-open alarms and occupancy sensors, implement a formal 'sash down' lab culture program, and ensure HVAC controls are properly balanced (an unbalanced system wastes energy without benefit).

Third-party fume hood certification typically costs $100–$300 per hood for a basic face velocity and smoke test, depending on location and contractor. Adding an ASHRAE 110 tracer gas (SF6) test adds $150–$400 per hood. Large institutions with many hoods often negotiate volume pricing. In-house certification programs (where EH&S staff are trained) reduce per-hood cost significantly but require upfront equipment investment ($2,000–$5,000 for calibrated anemometers and training). Annual certification is a regulatory requirement, not optional.

Persistent or frequent airflow alarms can be caused by: HVAC system imbalance (common during peak demand or seasonal changeover), exhaust fan issues (worn bearings, belt slip), duct obstruction (buildup or inadvertent blockage), building pressure effects (elevator shafts, storm pressure), or a faulty sensor or alarm controller. Transient alarms during door-opening events are usually HVAC-related. Persistent alarms require facilities investigation — do not disable or ignore them. If the alarm is triggering because the face velocity is genuinely low, the hood must be taken out of service until repaired.

Several issues can cause odor escape from a hood that passed annual certification. The most common: equipment has been moved to a new configuration that blocks baffles or creates turbulence (test 'as used'); cross-drafts from HVAC diffusers, doors, or windows disrupt inflow (check room conditions); the sash is raised above the certified height during work; the chemical has an exceptionally low odor threshold (you can smell it at concentrations far below safety limits — odor doesn't mean the hood is failing safety criteria). Request an ASHRAE 110 'as used' test with your actual equipment in place for definitive assessment.

Uneven face velocity across the sash opening is usually caused by: blockage of rear baffle slots by equipment (creates high-resistance zones that reduce exhaust draw in some areas), damaged or warped baffles (disrupt the designed airflow pattern), the sash raised unevenly (for horizontal sliding sashes), equipment placed near the sash face that deflects airflow, or inadequate makeup air causing room depressurization that affects flow distribution. A face velocity measurement grid will identify low-velocity zones. Resolving baffle obstructions and proper equipment placement are the first corrective steps.

In a standard power outage, the fume hood exhaust fan stops — the hood provides no protection and should not be used for hazardous materials. Critical research facilities typically have emergency power for at least the exhaust fans (but not necessarily heating/cooling of makeup air). Check your facility's emergency power plan. If you are mid-experiment during a power outage: stop work immediately, cover all open containers, close the sash, and if hazardous vapors are present, evacuate and ventilate the room by opening windows or doors (away from the hood). Do not assume hood function without verifying fan operation.