Chemical Fume Hoods

Fume Hood Purpose and Importance

The laboratory chemical fume hood is the most common local exhaust ventilation system used in laboratories and is the primary method used to control inhalation exposures to hazardous substances. When used properly, fume hoods offer a significant degree of protection for the user. Understanding the limitations, the appropriate maintenance techniques, and overall design of the fume hood will ensure your safety while using hazardous materials. The purpose of a chemical fume hood is to prevent the release of hazardous substances into the general laboratory space by controlling and then exhausting hazardous and/or odorous chemicals. In the event of an accidental spill, the fume hood will contain the spilled chemicals and exhaust the fumes away from the user and laboratory zone.

Labconco Corporation Fume hoods
Labconco fume hoods
Photograph Courtesy of Labconco Corporation

What types of hazardous substances require a chemical fume hood?

To determine if a chemical is required to be used inside of a chemical fume hood, first check the Safety Data Sheet (SDS). Statements found in Section 2 on a SDS such as “Do not breathe dust, fumes, or vapors” or “Toxic by inhalation” indicate the need for a fume hood. As a best practice, always use a chemical fume hood for all work involving the handling of open chemicals (e.g., preparing solutions) whenever possible. For more information about hazardous chemicals visit the Chemical Hygiene Plan (CHP) webpage.

Fume Hood Design

A chemical fume hood is a ventilated enclosure used to trap and exhaust vapors, gases, and nanoparticles. The exhaust fan is typically stationed at the top of the building and pulls air through the duct work connected to the hood and exhausts it into the atmosphere.

Typical Fume Hood Design
 Diagram  Definitions
Fume hood air flow diagram
  • Sash: The glass “window” that opens and closes either vertically, horizontally, or a combination of both. The sash protects the user.

  • Baffles: Located at the rear of the hood, they direct air in the appropriate direction. Baffles may be adjustable and the position is dependent on the vapor density of the chemicals used.

  • Exhaust Duct: The duct connects the fume hood to the exhaust system and acts as the pathway for chemicals to travel toward the outside air.

  • Face: The opening to which air is drawn into the fume hood. The face of the hood is parallel to the vertical sash and is the plane where face velocity measuEHSents are taken.

  • Air Foil: The foil is positioned at the front edge of the hood and is designed aerodynamically to direct air into the hood to allow for less turbulence and more efficient containment of chemicals.

Sash Openings

Vertical: The sash rises up and down and is optimal for shielding the user from contaminants with a large glass window pane. The vertical sash is framed, moves along tracks mounted inside of the hood’s wall, and is pulled up and down by a cable and pulley or chain and sprocket system. When using a hood with a vertical opening, raise the sash to the designated working height, generally 18” above the work surface, indicated by the black arrow on EHS’s inspection sticker. When not in use, close the vertical fume hood sash for the greatest level of safety and containment.  This practice will improve air quality in laboratories and reduce energy use.

Horizontal: The sash moves side to side and is comprised of multiple window panes. This gives the user more freedom to work in a certain area of the fume hood while reducing costs because the sash is never completely open. EHS will post an approval sticker specific to the horizontal sash openings, affirming the horizontal sash opening in inches. When not in use, close the horizontal fume hood panels for the greatest level of safety and containment. This practice will improve air quality in laboratories and reduce energy use.

Combination: This is a sash designed with a horizontal pane system built into the vertical sash. EHS will measure the air velocities of each sash position and post the working height along with the horizontal sash sticker to the fume hood. Only use sash in one direction at a time. When not in use, close both the sash and horizontal panels for the greatest level of safety and containment.  This practice will improve air quality in laboratories and reduce energy use.  

Fume Hoods & Sash Enclosures
Vertical Sliding Sash Horizontal Sliding sash Combination Sliding Sash
Hood with vertical rising sash Hood with horizontal sliding sash Hood with combination sash
   Images courtesy of Labconco Corporation  


Constant Air Volume (CAV): This type of fume hood exhausts the same amount of air at all times, regardless of the horizontal or vertical sash position. As the sash is opened and closed, the air velocity at the face of the hood will change.

Variable Air Volume (VAV): This type of fume hood contains a face velocity control, which controls the fan speed to maintain a constant air velocity at the face of the hood. This type of exhaust allows for optimal hood performance regardless of the sash position, and provides significant energy savings by reducing the air flow rate when the sash is closed. When you are not working in the VAV fume hood, CLOSE THE SASH, turn off the lights, and conserve energy.

Think SAFE. Think GREEN.

You may notice this sticker placed on your fume hood; indicating that it operates as a VAV hood.

Sticker that May Appear on Variable Air Volume Fume Hood

Shut the sashes annual savings sticker.

Any fume hood not currently in use may be evaluated to determine whether it may be temporarily turned off (hibernated) by contacting Physical Facilities Engineering at PFengineering@purdue.eduCalling PF engineering to have your fume hood shut off can reduce the electricity used to power the fan, thus conserving energy, reducing greenhouse gas emissions, and saving your University thousands of dollars per year.

Types of Fume Hoods

General Lab Use: Conventional hoods found on Purdue’s campus and are approved for general chemistry, radioisotopes, and carcinogen or toxic chemical work.

High Performance: These fume hoods have containment-enhancing features allowing them to operate at lower face velocities while protecting the operator. Since less room air is exhausted, energy is conserved. EHS will tag high performance hoods with a special standard operating procedure sticker, informing users of the appropriate air velocity range determined by the University.

Perchloric Acid: Special hoods equipped with a stainless steel or PVC duct and properly timed water wash down system. The wash down system must be used following each use of the acid hood. Using perchloric acid in a general lab fume hood may cause the acid vapors to settle onto the ductwork and create explosive perchlorate crystals. Serious injury or fatality may result to hood users or maintenance staff if the acid crystals are exposed to vibration and detonate.

 Perchloric Acid Fume Hood Warning Label

Perchloric acid hood Danger sign.

Polypropylene (Acid Resistant): Dilute acids may be used at room temperature in most fume hoods, but if you are performing acid digestion, heating, or working with concentrated acids such as: HF, Aqua Regia, Nitric Acid, Piranha Solutions, etc., acid resistant hood and ductwork is required. Strong acids are corrosive to the duct work found in general lab fume hoods. Fume hoods constructed from polypropylene material are long-lasting and designed to resist harsh chemicals for years.

Walk-In: Equipped with a floor-mounted design, walk-in hoods specialize in exhausting chemicals that are used alongside large laboratory equipment. When using a walk-in hood, close the bottom sash to the floor and only raise the top sash to EHS’s designated working height. Do not obstruct the area at the face of the hood.

Ductless Filtered: Designed to EHSove potential hazardous fumes and vapors from the work area as the exhausted air passes through absorbent material, such as activated charcoal. Occasionally, the EHS department is asked to approve purchases of ductless, filtered fume hoods for use in research labs. We do not recommend ductless fume hoods. We do not believe ductless fume hoods provide reliable protection against chemical exposure, and think they may, in fact, give workers a false sense of security.

The ductless hood's appeal is largely economic because it does not require the expensive ductwork that traditional hoods need to exhaust fumes to the outside.  However, in practice these hoods require constant attention and, if not carefully selected, don’t provide adequate protection.  In many cases, the filter is designed for specific chemicals and will not protect against the variety of current and future chemicals used in a typical research university lab.  The problems associated with breakthrough and with desorption of vapors from the absorbent material plague ductless fume hoods.  Departments would also face expenses to change charcoal filters and to dispose of the old/used filters, which would be classified as hazardous waste.  Therefore, depending upon the amount of use, annual maintenance costs to the owner could exceed several hundred dollars. Internal blowers for ductless hoods have also been known to be loud and prevent effective communication within the lab.  Fume hoods, ducted or filtered, should only be installed in fully exhausted labs with minimum dilution ventilation rate of 6 to 8 air changes per hour. Ventilation codes do not allow general return or exhaust air from a laboratory space with a fume hood to be recirculated to classrooms or offices.

If a department is purchasing or owns a ductless fume hood, they must develop written laboratory standard operating procedures (SOPs) that must include the following:

  • Performing a hazard assessment related to its use in their location
  • Using it according to manufacturer instructions and recommendations
  • Using it only for nuisance vapors and dusts that do not present a fire or toxicity hazard
  • Prominently posted signage informing users, lab personnel, and maintenance personnel of the following:
    • The chemicals allowed to be used in the hood
    • The type of filter used, its limitations and change schedule
    • Filter collection efficiency and breakthrough properties may change where multiple chemicals are used, resulting in earlier filter breakthrough
    • Notice and warnings that the hood recirculates air inside the room and many low molecular weight chemicals can be stripped from the filter and reenter the room

Fume Hood Performance Indicators

Performance indicators are important safety devices that must be monitored regularly and are necessary for every chemical fume hood on Purdue’s campus. Each hood should be equipped with a monitoring device used to continuously measure air flow, and provide a visible reading to the hood user. The most common types of visual performance indicators found in Purdue’s laboratories are differential pressure manometers or gauges, and digital monitoring devices. It is important to the health and safety of the laboratory occupants to pay close attention to the digital or posted reading. A broken or missing performance indicator may result in lab occupants being unaware of air flow changes, and increasing the risk of chemical exposure. If you believe that your hood is missing a performance indicator, your current device has been damaged, or is out of range; please contact EHS immediately to have one installed.

Inclined Manometer: Detection system mounted in a slightly inclined position; capable of measuring pressure above and below atmospheric. EHS supplies the red gauge oil used to quantify the velocity pressure measured by a change in differential pressure from total atmospheric. Manometers found on Purdue’s campus will be one of the two types shown below. EHS will calibrate the device when air velocity measuEHSents are inspected annually and record the reading on a sticker along with the date.  A difference in ± 0.05 inches of water column of the recorded reading may indicate air velocity changes to your fume hood. Continuing to use a fume hood with high or low flow poses a risk to the user and lab occupants if the chemicals are not adequately contained and exhausted.

Examples of Inclined Manometers
Dwyer King
Dwyer Mark II manometer King Instruments Company manometer


Magnehelic Differential Pressure Gauge: Monitoring device that measures the difference in differential pressure across an orifice in the duct or between the laboratory and the fume hood exhaust duct. They are mounted on the outside of the hood and detect pressure differences from atmospheric, operating with an aneroid pressure gauge. EHS will calibrate the Magnehelic at the annual inspection of the fume hood and record the location of the pointer for the official reading along with the date. A difference in ± 0.05 inches of water column of the recorded reading may indicate air velocity changes to your fume hood. Continuing to use a fume hood with high or low flow poses a risk to the user and lab occupants if the chemicals are not adequately contained and exhausted.

Magnehelic Differential Pressure Gauge

Magnehelic static pressure gauge


Digital Monitoring Device: This device measures air velocity with a sensor and either has a display in feet per minute (fpm) and/or an alarm system, alerting users if the air flow is out of range. Electronic fume hood monitors/controllers measure either the air velocity or the sash position. Sash position is correlated to the air velocity at that sash opening area to determine face velocity. Digital monitors will display velocity on a screen, while some only contain an alarm system and is color coded accordingly:

Digital Monitoring Device Color Codes
Red Yellow Green
(Too High or Low Flow)
Use with CAUTION SAFE for Use

If your fume hood control device is going into alarm mode please contact EHS so we can assess the air velocity of the fume hood. A EHS representative will determine if the air flow is within the recommended range and either submit a work order for repair or suggest that the digital monitoring device be calibrated. If a monitoring device is not reading accurately and needs serviced, it may alarm even if the air velocity is within the safe zone. Calibration of digital monitors is serviced by the designated Zone Maintenance to your area by submitting a Request for Services, 18A work order.

Examples of Digital Monitoring Devices
Fisher Scientific Phoenix Controls Labconco Kewaunee
Fisher Scientific Safeaire fume hood monitor Phoenix Controls fume hood monitor Labconco 500 fume hood monitor Kewaunee Air Alert 500 fume hood monitor


Fume Hood Evaluation and Maintenance

Annual Inspection and Posted Stickers

Every chemical fume hood on Purdue’s campus is routinely inspected once per year by EHS’s Industrial Hygiene Technician as a protocol to the University’s health and safety program. Upon completion of the hood inspection, EHS will record:

  1. Average face velocity measuEHSent in feet per minute (fpm)
  2. Approved use status (storage, general chemistry, radioisotopes, carcinogen/toxic chemicals)
  3. Horizontal window sash opening measuEHSent (in inches) or an arrow indicating safe vertical sash open height
  4. Hood Identification (Ex. A, B, or C)
  5. Date of inspection
  6. Inspector's initials

This information will be labeled on a yellow sticker and placed at the indicated approved working height related to that face velocity measuEHSent. Users should raise the sash to the working height indicated by the black arrow on the label when operating the hood. Hood operators may raise the sash above this point for equipment set-up only. Do not manipulate chemicals or run experiments with the sash open above the designated working height. This sash height is specifically determined to contain and exhaust chemicals, and is necessary for your overall safety. Additionally, a horizontal sash sticker will be placed on the front of the hood if horizontal panels are utilized. All chemical fume hoods should also be labeled with a yellow lab hood operating procedures sticker specific to general and high performance fume hoods.

Face Velocity Range and Appropriate Use Fume Hood Stickers 

Current Standards: Face Velocity (FV) Range and Appropriate Use Stickers

Standard Fume Hood Operating Procedures
General Use: 80 fpm ≤ FV ≤125 fpm

Storage Only: FV < 80 fpm or FV > 125 fpm

High Performance Fume Hood Procedures
General Use: 70 fpm ≤ FV ≤100 fpm
Storage Only: FV < 70 fpm or FV > 100 fpm

Standard Fume Hood Operating Procedure Sticker High Performance Fume Hood Operating Procedure Sticker


Sash Opening Stickers 

Horizontal & Vertical Sash Opening Stickers
Vertical Horizontal
Vertical Sash Sticker Horizontal Sash Sticker


ASHRAE 110 Testing Procedure

Defined by the American Society of Heating, Refrigerating and Air-Conditioning Engineers; this published standard specifies a quantitative test procedure for evaluation of a laboratory fume hood.

EHS or a third party independent fume hood professional will conduct ASHRAE 110 testing to determine adequate containment As Installed (AI) or As Used (AU) methods in accordance with the published ASHRAE 110 Standard 110-1995. This practice is generally conducted at EHS’s discretion; typically for fume hoods at maximum capacity, complex/special cases, or newly installed or relocated fume hoods. This procedure consists of extensive smoke tests, face velocity and cross draft measuEHSents, tracer gas containment using sulfur hexafluoride (SF6), computer graphing of measurable SF6 inhaled by a positioned mannequin in parts per million (ppm), and a general performance report. If you have safety concerns regarding the containment of chemicals or air flow patterns of your fume hood, please contact the Industrial Hygiene section of EHS to discuss if your hood would be a good candidate for further testing.


Proper Fume Hood Techniques and Practices

Before using a fume hood: Verify it has been inspected within the last 12 months, airflow is sufficient, the lights are working, side panels are intact, and the hood performance indicator is in good standing.

Personal Protective Equipment (PPE): Must be worn in accordance to your lab’s hazard assessments including: lab coat, safety glasses/goggles, gloves, aprons, and the minimum lab PPE required in the CHP (Chemical Hygiene Plan). 

 Optimizing Fume Hood Performance

How to Optimize Fume Hood Performance
Do keep all work at least 6” inside the hood Do not block slots with bulky equipment
Do lower sash while conducting experiments Do not position fans so as to direct airflow across the face
Do inspect baffles to minimize obstructions Do not block the airfoil with absorbent paper
Do provide catch basins for containers Do not EHSove side panels
Do close the sash for more safety and less energy Do not leave sash/panels open


Material Placement

Examples: Material Placement Inside Fume Hoods
Poor Material Placement Good Material Placement Best Material Placement
Poor, good, and best placement of material inside a fume hood.   
Images courtesy of Kewaunee Scientific Corporation


Poorly placed materials on the work surface of the fume hood increases the risk for chemical exposure because hazards are not efficiently exhausted out of the hood. The first photo indicates the user does not have material at least 6” inside the hood and the hazardous chemicals are near the breathing zone. The second photo is an improvement of the first, but half-way inside is still not the optimal position for exhausting fumes. The best placement of material displayed in the third photo is approximately 3/4 inside of the hood, where chemicals can immediately be exhausted out the back and through the top duct work. Keep in mind that placing materials more than ¾ inside the hood can block air flow to the back baffles.

Large Equipment Placement

Examples: Large Equipment Placement Inside Fume Hoods
Poor Large Equipment Placement Good Large Equipment Placement
Large equipment placement inside fume hoods  
Images courtesy of Kewaunee Scientific Corporation


Fume Hood Misconceptions and Limitations

Never use a fume hood while in alarm mode. By muting the hood alarm you may be ignoring a possible malfunction with either the air flow or unsafe sash height. When a hood is alarming, lower the sash, and allow the fume hood to properly contain and balance the air velocity. Report continuing malfunctions or power failures to your designated Zone Maintenance crew or the Industrial Hygiene section of EHS.

Do NOT use “Storage Only” hoods for chemical use. Fume hoods posted with a Storage Only sign indicate the air flow is insufficient for toxic chemical work and may only be used for storing materials or closed waste containers.

Storage Only Sign

Storage Only sign

Post-pone all experiments and close open containers while hood repairs are being made. Once a work order is placed to Zone Maintenance by either your building deputy or EHS’s Industrial Hygiene section, a sign will be posted on the fume hood stating the hood is not safe to use and repairs are in process. Failure to relocate or end experiments while repairs are in process poses a health and safety risk to the lab as well as maintenance workers. During the repair process, fume hoods do not provide adequate ventilation for open chemicals.

Do Not Use Until Repaired Sign

 Do Not Use This Fume Hood sign 

Do NOT lean into the fume hood. It is unsafe for lab users to insert their body or head inside the hood, beyond the front sash. Leaning inside the hood positions contaminants in the breathing zone of the user ad disrupts air flow. This also increases the risk for chemicals spills and accidents. Only extend properly protected hands and arms into the hood and keep sash heights as low as possible, or even completely closed, always making sure the sash is between you and your work.

Do NOT use a fume hood for:

  • Highly Hazardous Substances – Consider a glove box.
  • Spray Painting – Use a paint booth equipped with filtering capabilities
  • Microorganisms/Biological Agents – Use a biosafety cabinet.
  • Waste Disposal via Evaporation – This is a violation of environmental regulations. A hood is not pollution control.

 If you have any questions or comments about your chemical fume hood, contact someone from Radiological and Environmental Management's Industrial Hygiene section.

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