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Industrial Ventilation


Industrial ventilation is a method of controlling worker exposure to airborne toxic chemicals or flammable vapors by exhausting contaminated air away from the work area and replacing it with clean air. It is one alternative to control employee exposure to air contaminants in the workplace.

Other alternatives include process changes, work practice changes, substitution with less toxic chemicals, or elimination of the use of toxic chemicals.

Industrial ventilation is typically used to remove welding fumes, solvent vapors, oil mists or dusts from a work location and exhaust these contaminants outdoors.

The design and troubleshooting of industrial ventilation systems should be handled by a qualified ventilation engineer or firms specializing in this field. However, a basic knowledge of how exhaust ventilation systems work and some basic troubleshooting tips is included in this guideline.

Regulatory information

When workers are exposed to amounts of chemicals in the air that are hazardous (such as in excess of permissible exposure limits), WISHA requires that employers take steps to reduce their exposure. WISHA regulations specifically address ventilation in such operations as spray painting, abrasive blasting, welding, use of dip tanks, and work in confined spaces.

The following WISHA standards have industrial ventilation components:

Chapter 296-62 - General Occupational Health Standards

Chapter 296-24 - General Safety & Health Standards

Chapter 296-155 - Construction

Chapter 296-841, WAC Airborne Contaminants

WAC 296-809-600 - Confined Spaces (519 KB PDF)

Chapter 296-818, WAC - Abrasive Blasting

Chapter 296-835, WAC - Dip Tanks

Control of worker exposure by ventilation or other means is required by WISHA occupational health regulations under the following three conditions:

  1. When levels of airborne contaminants (chemicals, dusts, vapors, fumes) are hazardous, such as above their permissible exposure limits,
  2. When the lower explosive limit of flammable vapors is exceeded,
  3. When the oxygen level drops below 19.5 % in the air.

WISHA regulations also require that exposure controls, such as ventilation must be implemented before resorting to the use of respirators. Respirators may be used to control worker exposure only when other exposure controls such as ventilation are not feasible or when they do not lower the air contaminant levels below their permissible exposure limits or in emergency situations.  Forced air (mechanical) ventilation is always required when workers enter confined spaces where there is a potential for exposure to toxic and, flammable vapors or dust or oxygen deficiency."

Types and components of ventilation systems 

Dilution Ventilation

Local Exhaust Ventilation


Ducts and Ducting

Air Cleaners


Exhaust Stacks


There are 3 types of workplace ventilation:

  1. "Indoor air quality ventilation" used primarily to provide fresh, heated or cooled air to buildings as part of the heating, ventilating and air-conditioning system,
  2. "Dilution ventilation" which dilutes contaminated air in a whole building or room by blowing in clean air and exhausting some dirty air,
  3. "Local exhaust ventilation" which captures contaminate emissions at or very near the source and exhausts them outside.

Indoor air quality ventilation, used primarily in offices and other non-industrial buildings, will not be covered in this guideline.

There are advantages and disadvantages to the use of either dilution ventilation or local exhaust ventilation in terms of costs and effectiveness. Table 1 compares the two types.

Table 1
Advantages Disadvantages Advantages Disadvantages

Usually lower equipment and installation costs.

Does not completely remove contaminants.

Captures contaminant at source and removes it from the workplace.

Higher cost for design, installation and equipment.

Requires less maintenance.

Cannot be used for highly toxic chemicals.

Only choice for highly toxic airborne chemicals.

Requires regular cleaning, inspection and maintenance.

Effective control for small amounts of low toxicity chemicals.

Ineffective for dusts or metal fumes or large amounts of gases or vapors.

Can handle all sorts of contaminants including dusts and metal fumes.


Effective control for flammable or combustible gases or vapors.

Requires large amounts of heated or cooled makeup air.

Requires smaller amount of makeup air since smaller amounts of air are being exhausted.


Best ventilation for small dispersed contaminant sources or mobile sources.

Ineffective for handling surges of gases or vapors or irregular emissions.

Less energy costs since less makeup air to heat or cool.


Dilution ventilation

Dilution ventilation is usually accomplished with the use of large exhaust fans in the walls or roof of a building or room. Opening doors or windows can be used as dilution ventilation, but this is not always a reliable method since air movement is not controlled. Cooling fans (floor fans) are also sometimes used as a method of ventilation, but these fans usually just blow the contaminant around the work area without effectively controlling it.

Dilution ventilation can be more effective if the exhaust fan is located close to exposed workers and the makeup air is located behind the worker so that contaminated air is drawn away from the worker's breathing zone. See Figure 1 for examples of best locations for exhaust fans and air inlets.

figure 1 (illustration) click on picture to enlarge Figure 1.

In cases where the source of contamination is widely scattered or is from a mobile source, like carbon monoxide from a forklift, large wall or roof exhaust fans can be effective. Makeup air to replace the air exhausted is necessary for the best control. Simple openings in walls or doors can be sources of makeup air, or a second fan can draw makeup air into the building or room. However, makeup air may require heating in the winter resulting in increased heating bills.

Local exhaust ventilation

Local exhaust ventilation is needed when employees are exposed to high toxicity chemicals, when large amounts of dusts or welding fumes are generated, or when increased heating costs from ventilation in cold weather are a concern.

Local exhaust ventilation operates on the principle that air moves from an area of high pressure to an area of low pressure. The difference in low pressure is created by a fan that draws or sucks air through the ventilation system. Local exhaust systems are located as close as possible to the source of contamination to capture the contaminate before it is released into the work area. A local exhaust system operates in the same manner as a household vacuum cleaner.

A local exhaust system has 5 basic elements:

    1. A "hood" or opening that captures the contaminant at the source,
    2. Ducts that transport the airborne chemicals through the system,
    3. An air cleaning device (not always required) that removes the contaminant from the moving air in the system,
    4. A fan that moves the air through the system and discharges (blows) it outdoors,
    5. An exhaust stack through which the contaminated air is discharged.

As with dilution ventilation, makeup air must be provided to replace the air exhausted in order for the system to operate properly.

Figure 2 illustrates the basic parts of a local exhaust system.

figure 2 (illustration) click on picture to enlarge Figure 2.


A hood is designed to confine or capture the contaminant at its source. The air velocity at the hood opening and inside the hood must be sufficient to capture and carry the air contaminants. The hood should enclose the source of contaminant as much as possible or be placed as close to the source as possible. Examples of different types of hoods are illustrated in Figure 3.

figure 3 (illustration) click on picture to enlarge Figure 3.

Air velocity in local exhaust systems is measured in feet per minute (fpm). Volume of air though a local exhaust system is measured in cubic feet per minute (cfm) which is simply the air velocity times the area of the hood opening. Sometimes air velocity is measured indirectly by measuring air pressure in the ductwork of the system. The pressure inside a local exhaust system is slightly negative compared to the pressure outside the system and is measured in units called "inches of water". This negative pressure varies through the system and is usually measured to determine how well the system is functioning.

Although enclosing hoods provide the best control, they are often not feasible because they would interfere with the work being done by the employee. In those cases, a capture exhaust hood can only be located near the source of the contaminant. These type of hoods "reach out" to capture the contaminant much like a vacuum cleaner sucking dirt off a floor. However, the distance between the face of the hood and source must be short to effectively capture the contaminant. A hood moved from two inches away from a source to four inches away from a source will require four times the amount of air volume through the system to provide the same degree of capture. Adding a flange to the edges of the capturing hood provides more efficient capture of contaminants. See Figure 4. Wide and flat hoods or hoods with slots do not have a greater "reach", rather they just spread out the airflow over a wide distance. Their most common use is along the edge of tanks containing volatile chemicals.

figure 4 (illustration) click on picture to enlarge Figure 4.

Canopy hoods are not recommended for use in local exhaust ventilation because even slight cross-drafts can push contaminants out into the work area and because they often draw air through the breathing zone of an employee working at them. Figure 5 illustrates some additional problems with canopy hoods.

figure 5 (illustration) click on picture to enlarge Figure 5.

Ducts and ducting

Ducts carry the airborne contaminant through the local exhaust system. To do this effectively, there should be as little resistance in the form of turbulence or friction as possible. Air moving too slow through the system will cause settling out of dusts and eventual clogging of the duct. Air moving too fast is wasteful of power, can create noise problems and may cause excessive abrasion if dusts are being exhausted. Smooth, round ducts are recommended for local exhaust systems. Dust can get trapped in the corners of square ducts, and air turbulence is higher inside them, reducing air velocity. Even though flexible ducting is sometimes necessary in some situations, it has a rougher surface than smooth ducting resulting in more friction and reduced air velocity.

Sharp bends or tees should be avoided in ducts as well as abrupt changes in diameter. Also, smaller diameter duct will have greater friction than larger diameter ductwork. Ducting should be straight at least two duct diameters before entering the fan to maintain smooth airflow into the fan. Duct connections must also be as tight as possible to prevent a reduction in air velocity at the hood because of leaks at joints. Figure 6 illustrates some basic duct design principles.

figure 6 (illustration) click on picture to enlargeFigure 6.

Air cleaners

Air cleaning devices on ventilation systems are sometimes necessary to capture large amounts of dust. In some instances, they may be required by air pollution regulations. The type of air cleaner depends on the type of contaminant being removed, its concentration in the air, the amount of contaminant that must be removed, and other factors. Dust filters are the most common type of air cleaners found in industry. Other types of air cleaners remove gases and vapors. Local air pollution regulations dictate the type of air cleaner required. The cost and extra resistance that these air cleaners add to an exhaust ventilation system must be considered in the design. For more information on air pollution regulations and requirements, contact the local air pollution control authority ( for your locality.


Fans are the workhorses of exhaust ventilation systems. They must be the appropriate size and type to make the ventilation system work effectively. They must provide enough air pressure difference ("suction") to capture contaminants at the source, draw them through the hood, carry them through the ducting and exhaust them outdoors.

There are two main types of exhaust fans - axial fans and centrifugal fans. Axial fans, usually resembling propellers, draw air straight through the fan. Centrifugal fans, resembling squirrel cages, draw air into the center of the fan and exhaust it at a 90-degree angle. Figure 7 illustrates the different types of fans.

figure 7 (illustration) click on picture to enlarge Figure 7.

Axial or propeller fans are most commonly used for dilution ventilation or for cooling. These fans are often mounted in a wall or ceiling. They can move large amounts of air if there is little resistance, but are not suited for local exhaust ventilation because they do not provide enough suction to draw air through the system.

Centrifugal fans can operate at against a high resistance and are typically used in local exhaust ventilation systems. There are several types of centrifugal fans. The rugged radial blade centrifugal fans are the best type for exhausting heavy amounts of dust because they are less likely to become clogged or abraded by the dust.

Selection of the proper fan can be a complicated task and should be handled by ventilation or fan experts. For additional information on exhaust ventilation fans, see the latest edition of Industrial Ventilation: A Manual of Recommended Practices published by the American Conference of Governmental Industrial Hygienists (ACGIH) (

Exhaust stacks

Exhaust stacks also need to be designed and located properly for the most efficient operation of a local exhaust system. A common mistake is to locate them too close to building fresh air intakes. Generally they should be located no closer than 50 feet to prevent re-circulation of contaminants. Stacks work best when they are tall, usually at least 10 feet above the roof line. Air velocity out of the stack should be at least 3000 feet per minute to overcome the effects of downdrafts from wind blowing over the building. Rain caps on stacks should be avoided because they tend to force contaminants back down to the building where they can be pulled into the fresh air intakes. They are also not very effective in keeping rain out of the stack. See Figure 8 for an example of a well-designed stack.

figure 8 (illustration) click on picture to enlargeFigure 8.


If an existing ventilation system appears to not be functioning properly, the following simple checks can be made without extensive measurements or expert help:

  • Is the fan belt broken or slipping?
  • Is the fan wired backward (reversed polarity)?
  • Is ductwork clogged with dust?
  • Is there holes, cracks or openings in the ducting?
  • Is the air cleaner clogged?
  • Are any dampers in the ductwork closed?
  • Is there insufficient makeup air?
  • Has ductwork been changed to include more length, more or sharper bends, or abrupt diameter changes?
  • Have additional hoods and ductwork been added? Without proper airflow balancing, some hoods in a multiple system may have inadequate flow. Or the fan may be too small to handle the additional resistance.
  • Has the contaminant source been moved further away from the hood opening?
  • Is more contaminant being generated at the source?
  • Are cooling fans causing crossdrafts?
  • Have employees modified the hood because it interferes with their job tasks?

Many of these problems can be avoided by periodic maintenance and measurements of air velocities or air pressures of ventilation systems. Airflow at the hood can be visually checked with inexpensive smoke generators (smoke tubes) or measured with air velometers. WISHA industrial hygiene consultants can do limited ventilation investigations. Ventilation specialists may be needed to remedy or redesign more complicated ventilation problems.


Companies that provide industrial ventilation/HVAC services

Note: L&I does not endorse any of the following companies — the list is provided for convenience only. More information is available in L&I's Intended Use/External Content policy.


Area of experience or specialty

Air Systems Engineering
3602 South Pine Street
Tacoma WA 98409
Tacoma: 253-572-9484
Seattle: 206-628-9484


Anvil Corporation
1675 W Bakerview Rd
Bellingham WA 98226

Industrial, municipal, commercial and institutional ventilation and HVAC systems

Argo Blower
5400 E Marginal Way S
Seattle WA 98134


Industrial Ventilation

Argus Pacific
1900 W Nickerson St Suite 315
Seattle WA 98119 


Berona Engineers Inc.
4630 200th St SW
Lynnwood WA 98036 


Industrial ventilation and HVAC systems

Bouillon Inc.
34734 Pilot Point Rd NE
Kingston WA 98346

Design of Industrial ventilation and HVAC systems

CB Engineers
600 108 th Ave NE
Bellevue WA 98004


Industrial ventilation and HVAC systems

CNL Design LLC
PO Box 1577
Coeur d'Alene ID 83816


EISI Inc. 
1900 W Emerson Suite 200
Seattle WA 98119-1649 


Industrial ventilation and HVAC systems

Environmetrics Inc.
4128 Burke Ave N
Seattle WA 98103 


Specialize in ventilation system location assessments

Finishing Consultants
720 132nd St SW
Everett WA 98204

Spray Finishing / Booths

Finishing Technologies
976 Industry Drive
Tukwila WA 98188
2211 NW Nicolai St
Portland OR 97210

Spray finishing equipment and technology

FSI Consulting Engineers 
605 First Ave Suite 400 
Seattle WA 98104-2224 


Industrial ventilation and HVAC systems

2407 N 31 st St
Tacoma WA 98407


Industrial Ventilation and HVAC design

Industrial Air Systems
17739 15th Ave NE
Seattle, WA 98155


HVAC and Spray Booths

L & S Engineering Associates
216 W Pacific Suite 211
Spokane WA 99201


Maiani Construction Services Inc.
7536 East Parks Road
Athol ID 83801


Notkin Engineering Inc. 
2601 Fourth Ave Suite 420 
Seattle WA 98121 

Industrial ventilation, laboratory ventilation and HVAC systems

Serbaco Inc.
PO Box 301007
Portland OR 97294

Industrial ventilation and air pollution control (design and service)

Skye Industrial
26235 292nd Ave SE
Ravensdale WA 98051

Industrial Ventilation and Spray Booth installation contractor

401 Second Ave South 
Seattle WA 98104 

Industrial ventilation and HVAC systems

2211 East Sprague Ave
Spokane WA 99202

HVAC systems

Additional links

Expand or collapse. Please read an important message about links on this page.


"Links" to other information sources are provided as a courtesy, but we cannot vouch for or take responsibility for information contained beyond files administered by the Washington State Department of Labor and Industries. Links from this page do not represent or imply the endorsement of commercial products by the State of Washington, Labor and Industries, or by departmental staff. For more information, read L&I's Intended Usage policy.

Fire and explosion hazards posed by dust collectors located indoors ( (WorkSafeBC Hazard Alert).

For more technical information on industrial ventilation, see the latest edition of Industrial Ventilation: A Manual of Recommended Practices published by the American Conference of Governmental Industrial Hygienists (ACGIH). To purchase a copy of this book or other texts on ventilation, contact ACGIH at or by phone at 513-742-2020.

Self-study courses on industrial ventilation are offered by Jeff Burton of IVE Inc., a nationally known expert on ventilation. He also provides in-person training courses periodically through ACGIH. Contact him at or by phone at 1-801-298-8996.

The Field Research & Consultation Group in the School of Public Health & Community Medicine at the University of Washington can provide some assistance for small or under-served businesses who may need assistance in designing and installing exhaust ventilation systems. For more information contact the Field Group at 206-543-9711, or visit their website at

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