This document is an introduction to the following documents on industrial ventilation:
The above documents do not cover indoor air quality and general ventilation as used in offices, homes, and other non-industrial settings. Please see the OSH Answers document Indoor Air Quality - General for more information.
Ventilation is the mechanical system in a building that brings in "fresh" outdoor air and removes the "contaminated" indoor air.
In a workplace, ventilation is used to control exposure to airborne contaminants. It is commonly used to remove contaminants such as fumes, dusts, and vapours, in order to provide a healthy and safe working environment. Ventilation can be accomplished by natural means (e.g., opening a window) or mechanical means (e.g., fans or blowers).
Industrial systems are designed to move a specific amount of air at a specific speed (velocity), which results in the removal (or "exhaust") of undesirable contaminants. While all ventilation systems follow the same basic principles, each system is designed specifically to match to the type of work and the rate of contaminant release at that workplace.
There are four purposes of ventilation:
Ventilation is considered an "engineering control" to remove or control contaminants released in indoor work environments. It is one of the preferred ways to control employee exposure to air contaminants.
Other ways to control contaminants include:
Systems are composed of many parts including:
Each of these parts are discussed in this series of documents.
There are two types of mechanical ventilation systems used in industrial settings:
Dilution (or general) ventilation reduces the concentration of the contaminant by mixing the contaminated air with clean, uncontaminated air.
Local exhaust ventilation captures contaminates at or very near the source and exhausts them outside.
Dilution, or "general", ventilation supplies and exhausts large amounts of air to and from an area or building. It usually involves large exhaust fans placed in the walls or roof of a room or building.
Dilution ventilation controls pollutants generated at a worksite by ventilating the entire workplace. The use of general ventilation distributes pollutants, to some degree, throughout the entire worksite and could therefore affect persons who are far from the source of contamination.
Dilution ventilation can be made 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 good and poor dilution ventilation design.
When used to control chemical pollutants, dilution must be limited to only situations where:
It is therefore unusual to recommend the use of general ventilation for the control of chemical substances except in the case of solvents which have admissible concentrations of more than 100 parts per million.
As a method for protecting workers, it is important to know that dilution ventilation:
Regular "floor" or "desk" fans are also sometimes used as a method of ventilation, but these fans typically blow the contaminant around the work area without effectively controlling it. Opening doors or windows can be used as dilution ventilation, but again, this method is not reliable since air movement is not controlled.
As a general note, the air or "volumetric" flow rate of dilution ventilation depends largely on the how fast the contaminant enters the air as well as the efficiency that fresh air mixes with workroom air.
Local exhaust system is used to control air contaminants by trapping them at or near the source, in contrast to dilution ventilation which lets the contaminant spread throughout the workplace. Local exhaust is generally a far more effective way of controlling highly toxic contaminants before they reach the workers' breathing zones. This type of system is usually the preferred control method if:
In a general way, a local exhaust system operates similar to a household vacuum cleaner with the hose as close as possible to the place where dirt would be created.
A local exhaust system has six basic elements (see Figure 6):
Basic components of a local exhaust system
All industrial ventilation systems, when designed properly, should be able to provide long-term worker protection. The two types of ventilation, dilution and local exhaust, are compared in the following table.
|Comparison of Ventilation Systems|
|Dilution Ventilation||Local Exhaust Ventilation|
|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 vapours.||Can handle many types of contaminants including dusts and metal fumes.|
|Effective control for flammable or combustible gases or vapours.||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 mobile or dispersed contaminant sources.||Ineffective for handling surges of gases or vapours or irregular emissions.||Less energy costs since there is less makeup air to heat or cool.|
Some limitations include:
The following is an example of changes that can affect how a system works:
A hood and branch are added to an existing duct. The local exhaust ventilation pulls air into the system from the new location, which will reduce the airflow from other locations that are further away from the exhaust fan. Again, airflow will all be affected. The result is that the system will plug more rapidly and the airflows at the other hoods may not be adequate enough to remove contaminants.
An important and sometimes overlooked aspect of local ventilation is the need to provide enough air to replace the air that is exhausted from the workplace. If enough make-up air is not provided when large volumes of air are exhausted, the workplace becomes "starved" for air and negative pressure is created.
Negative pressure in the workplace increases resistance on the ventilation system causing it to move less air. Air will also enter a building through cracks around doors or windows or other small openings to try to "equal" the rate of air being removed. The result is that workers may be exposed to cold air in the winter, and additional heating costs may occur. One simple way to judge if a building is under an excessive negative pressure is if you have difficulty opening a door that pushed into the room or building (the air wants to force the door closed).
A separate intake fan, located away from the exhaust fans, should be used to bring in fresh, uncontaminated air from outside. This air must be clean and heated in winter or cooled in summer, as needed.
Document last updated on January 10, 2008