Laboratory Ventilation Guide

Laboratory ventilation doesn't affect me, right?

Do you rely on your hood or biological safety cabinet? What if the hood or cabinet you are using does not function properly? Can you assume that you are being protected if you don’t smell anything or aren’t overwhelmed by a chemical? Do you know if your hood or cabinet is doing its job?

Laboratory Hoods

What is adequate flow?

Recommended Flow Rates

Most laboratory hoods at UNR require an average face velocity between 80-120 feet per minute (fpm) measured at a sash height of 16”. Hoods used for high hazard or highly toxic substances may require higher average face velocities of up to 150 fpm. Face velocities higher than 150 fpm can cause turbulence around the periphery of the sash opening and actually reduce the capture efficiency of the hood.

Appropriate Sash Height:

Hood performance is generally evaluated at a level of 16”. This recommended sash height is marked with the following label:

A label featuring a red arrow which indicates the recommended sash height.  The label also contains the FPM and the surveyor information.

The sash level of 16” is based on recommended practice and should enable comfortable operation for most researchers and operations while providing adequate barrier protection. It is understood that the sash position may need to be adjusted for different users or unique operations, however, higher sash positions will result in decreasing contaminant capture. If you must routinely operate above the recommended sash height, the Environmental Health and Safety Department can help evaluate your specific application.

The following two pictures represent a “typical” hood which would meet or pass accepted testing criteria. Notice the significant drop in capture efficiency and lack of barrier protection with increased sash height.

An example of typical use of a hood with a sash at 16 inches.  Visible fumes are being contained within the hood.

Typical hood with sash at 16"

An example of typical use of a hood with an open sash.  Visible fumes are being released from the hood.

Typical hood with open sash

How can I tell if my hood has adequate flow?

The Environmental Health & Safety Department performs annual surveys of campus laboratory hoods. If your hood does not have a current survey label, contact the Environmental Health and Safety Department. If a hood is found to be inadequate or non-operational the lab director will be contacted so that other options or alternatives can be discussed until improvement or repairs can be implemented.

New hoods should be installed with flow indicators to warn you of low flow, however, most hoods at UNR are not equipped with this feature. You can get a visual indication that your hood is operating by hanging a kimwipe from the sash and looking for inward movement, however, appropriate instrumentation is required to determine if flow is adequate. If your hood is obviously not functioning contact Buildings and Grounds and/or submit a work order with the building, room, hood location, and a description of the problem. Buildings and Grounds is the only group that can make repairs to your hood.

Laboratory Hood Designs

General Design Guidelines

Ideally, laboratory hoods should not be located adjacent to high traffic areas, doors, windows, or supply air diffusers. Hood exhaust should be designed to prevent the exhaust from entering the buildings air intakes; ideally the exhaust should be at least 10 feet above any adjacent roofline or intake within 50 feet.

Hood Types

Standard Hoods (non-bypass)

Standard or non-bypass hoods exhaust a constant volume of air, regardless of sash height. As the sash is lowered the opening area decreases, resulting in an increase in face velocity.

A diagram of the airflow inside a standard hood.
Bypass Hoods

Bypass hoods incorporate an additional source of room air when the sash is closed. As the sash is lowered, additional room air is introduced through the bypass (usually an opening above the sash), effectively minimizing face velocity fluctuations at lower sash heights.

A diagram of the airflow inside a bypass hood.
Auxiliary Air Hoods

Auxiliary hoods provide a dedicated supply of air to the face of a bypass hood which results in cost savings by reducing the amount of heated or air conditioned room air exhausted by the hood. However, even when adjusted properly, auxiliary air hoods can result in significantly higher worker exposure compared to non-auxiliary hoods. As a result, auxiliary air hoods should not be used at UNR unless special energy conditions require their use.

A diagram of the airflow inside an auxilary hood.
Variable Air Volume Hoods

Variable air volume hoods maintain a constant face velocity as sash height changes. This provides an acceptable face velocity over a relatively large sash opening.

A diagram showing the location of the sash position sensor, fiberglass damper and velocity sensor on the variable air volume hood.

Specific Configurations

Perchloric Acid Hoods

Perchloric acid hoods are designed especially for use with perchloric acid and other materials that can deposit shock sensitive crystalline materials in the hood and exhaust system. Perchloric acid should only be used in a dedicated and approved hood labeled “For Perchloric Acid Operations Only”. The hood, ductwork, and exhaust fan should be constructed of acid resistant, non-reactive materials with sealed joints and designed to meet the requirements of NFPA 45. Exhaust should not be manifolded with other types of fume hoods. Perchloric acid hoods, ductwork, and fan systems should also be equipped with water-wash systems to minimize deposits of shock-sensitive crystalline materials. Do not use organic or oxidizable materials in perchloric acid hoods.

Glove Box Hoods

Glove box hoods should maintain a static pressure difference of 0.1 inches water gauge and at least 100 fpm inward face velocity when the largest port is open. Glove boxes should be filtered or scrubbed prior to being discharged to the atmosphere.

Laboratory Hood User Checklist

  • check for a current certification sticker
  • verify that adequate face velocity exists
  • always keep the hood sash between the operator’s face and the work
  • do not place upper body/head inside the hood during contaminant generating activities
  • keep the slots of hood baffles free of obstruction by apparatus or containers
  • work and place equipment at least 6 inches behind the sash
  • keep storage to a minimum
  • elevate bulky equipment and materials on racks or stoppers (approx. 1.5”)
  • reduce cross drafts at hood face from fans, HVAC, doors, or pedestrian traffic
  • limit arm movement and rapid movements in and out of the hood
  • clean hoods before maintenance personnel work on them
  • know the nearest location of safety showers, eyewashes, and fire extinguishers
  • do not turn the hood off

Laboratory Hood HELP!!!

If you have questions about potential chemical or physical hazards in your work area, have a hood that has questionable airflow, or need a hood survey please contact the Environmental Health & Safety Department.

  • Environmental Health & Safety Department - 327-5040
  • Rich Stone, Industrial Hygienist - 327-5055
  • Ben Owens, Chemical Hygiene Officer - 327-5196

If you have a hood, which does not operate, or is in need of repairs, you may contact Buildings and Grounds.

  • Building & Grounds - 784-8020
  • Work Order

Biological Safety Cabinets

Biological Safety Cabinet Designs

Class I:

Class I cabinets allow “dirty” room air to constantly enter cabinet and flow across work surface, hence provide no product protection. Unlike conventional fume hoods, the HEPA filter protects the environment from by filtering air before it is exhausted outside.

A diagram of the class I biological safety cabinet.  A set of arrows and symbols show the path of air flow through the bench.  See the figure caption for the key.
The following symbols appear in the image above:
  • a rectangle filled with vertical linesRoom air
  • a rectangle filled with spotsContaminated air
  • a blank rectangleHEPA-filtered air
Class IIA

Class IIA cabinets exhausts 30% air through a HEPA filter at top of the cabinet (room or outside) while 70% air is re-circulated back down to the work area through another HEPA. Air flows back to the work area in a vertical, unidirectional flow where it splits as it flows through perforated grilles in front and back. Since much of the air is re-circulated, this cabinet is not for volatile toxic or radionuclide materials.

A diagram of the class IIA biological safety cabinet.  A set of arrows and symbols show the path of air flow through the bench.  See the figure caption for the key.
The following symbols appear in the image above:
  • a rectangle filled with vertical linesRoom air
  • a rectangle filled with spotsContaminated air
  • a blank rectangleHEPA-filtered air
Class IIB1

Class IIB1 cabinets are similar to IIA cabinets except that approx. 70% air is exhausted (outside) at the back of the cabinet and 30% is recirculated. Minute quantities of volatile toxics or radionuclides may be used in the back portion of the cabinet.

A diagram of the class IIB1 biological safety cabinet.  A set of arrows and symbols show the path of air flow through the bench.  See the figure caption for the key.
The following symbols appear in the image above:
  • a rectangle filled with vertical linesRoom air
  • a rectangle filled with spotsContaminated air
  • a blank rectangleHEPA-filtered air
Class IIB2

Class IIB2 cabinets have no re-circulation. Air is brought into the cabinet through a HEPA filter where it splits over the work area and is exhausted through another HEPA filter. Volatile toxics and radionuclides may be used.

A diagram of the class IIB2 biological safety cabinet.  A set of arrows and symbols show the path of air flow through the bench.  See the figure caption for the key.
The following symbols appear in the image above:
  • a rectangle filled with vertical linesRoom air
  • a rectangle filled with spotsContaminated air
  • a blank rectangleHEPA-filtered air
Class IIB3

ClassIIB3 cabinets are similar to Class IIA cabinets except the air must be exhausted outside. Volatile toxics and radionuclides may be used.

A diagram of the class IIB3 biological safety cabinet.  A set of arrows and symbols show the path of air flow through the bench.  See the figure caption for the key.
The following symbols appear in the image above:
  • a rectangle filled with vertical linesRoom air
  • a rectangle filled with spotsContaminated air
  • a blank rectangleHEPA-filtered air
Class III

Class III cabinets are totally enclosed, gas-tight and designed for use with high risk (BSL 3&4) agents. Supply air is HEPA filtered while exhaust air passes through two HEPA filters and is sterilized before being exhausted outside.

A diagram of the class III biological safety cabinet.  A set of arrows and symbols show the path of air flow through the bench.  See the figure caption for the key.
The following symbols appear in the image above:
  • a rectangle filled with vertical linesRoom air
  • a rectangle filled with spotsContaminated air
  • a blank rectangleHEPA-filtered air
Biological Safety Cabinet Usage Summary
TypeFace Velocity (1fpm)Airflow PatternRadionuclides/Toxic ChemicalsBiosafety LevelsProduct Protection
Class I* open front 75 In at front; rear and top through HEPA filter No 2, 3 No
Class II Type A 75 70% recirculated through HEPA; exhaust through HEPA No 2, 3 Yes
Type B1 100 30% recirculated through HEPA; exhaust via HEPA and hard ducted Yes (Low levels/volatility) 2, 3 Yes
Type B2 100 No recirculation; total exhaust via HEPA and hard ducted Yes 2, 3 Yes
Type B3 100 Same as IIA, but plena under negative pressure to room and air exhaust is ducted Yes 2, 3 Yes
Class III N/A Supply air inlets and exhaust through 2 HEPA filter Yes 3, 4 Yes

Biological Safety Cabinet Help

If you have questions about potential biological hazards in your work area or are unsure of who can perform maintenance on your biosafety cabinet please contact the Environmental Health & Safety Department.

  • Environmental Health & Safety Department - 327-5040
  • Rich Stone, Industrial Hygienist - 327-5055
  • Ben Owens, Chemical Hygiene Officer - 327-5196

If you have a biological safety cabinet that does not operate, is in need of repairs, or requires certification you may contact the following biological safety cabinet certifiers.

MARISCHAL, INC.
750 South State
Richmond, UT 84333
Phone: (800) 491-1776 or (801) 258-5463
Fax: (801) 258-2792

Laboratory Ventilation Resources

Laboratory Hoods:
  • UNR Exhaust Ventilation System Policy
  • UNR Chemical Hygiene Plan
  • University Of Wisconsin-Milwaukee Hood Procedures & Practices
  • University of Louisville Fume Hood Users Guide
  • MIT Hood Survey Program
Biological Safety Cabinets
  • UNR Biosafety Manual
  • UNR Institutional Biosafety Committee
  • CDC Selection, Installation and Use of Biosafety Cabinets
  • NSF Biological Safety Cabinet Information
  • CDC Biosafety in Microbiological and Biomedical Laboratories