Comparing Mechanical Ventilation Systems-June 2014

Monthly Round Up of Important Ideas and Standards in
Industrial Hygiene and Safety 

June 2014

IN THIS ISSUE: Comparing Mechanical Ventilation Systems

Mechanical ventilation systems are the most common engineering control for hazardous chemicals and pollutants. When properly designed, installed, and maintained they can be very effective in maintaining worker health. Two classes of mechanical ventilation exist: dilution ventilation and local exhaust ventilation. 

Dilution ventilation (DV) uses large exhaust fans placed in the roof or walls of a room or building to reduce the amount of a contaminant by removing polluted air and replacing it with fresh air. Since DV removes air from the entire workspace, it can affect employees far from the source of a pollutant. DV is typically measured in air changes per hour. The greater the concentration of a contaminant, the more air change is needed. DV has the following limitations:

  • Reduces but does not eliminate pollutants
  • Should not be used in with highly toxic or chemicals in high concentrations
  • Does not work with heavy materials such as dusts or metal fumes
  • Although initial investment may be low, DV needs large amounts of makeup air, which may be expensive to heat and cool.

Local exhaust ventilation (LEV) is the preferred method to reduce chemicals in the air in most cases. The primary difference is that LEV captures the contaminant at the source and removes it from the workplace. LEV uses a hood that can be placed near the contaminant. The hood is connected to a duct, fan, and stack. Air cleaners may be placed at various stages along the system.

LEV has the following strengths and limitations:

  • Capable of completely removing a contaminant from the air
  • Works well with highly toxic chemicals, dusts, and materials with greater concentration
  • Requires less makeup air and may have lower energy cost
  • May have greater initial cost for design and materials 
  • Requires regular cleaning, inspection, and maintenance

Examples of LEV exist in many forms:

  • Portable fume extractors are typically used for welding. The hood, fan, air cleaner, and duct are one unit that can be moved anywhere in the workspace, and the articulating arm can move the hood as close to the work as needed.
  • Downdraft tables are designed to allow work to be done at or near waist height. Work is done over a grate with LEV that removes contaminant at the source. Units are stand alone with built fans, cleaner, and exhaust.  They can be effective for dust collection from sanding, grinding, polishing and welding.
  • Ventilated tools are powered hand tools such as sanders and grinders that are connected to a specialized vacuum cleaner. Each tool is equipped with a shroud that focuses airflow and allows contaminant to be captured at the source and removed from the operator’s breathing zone.
  • Paint booths are fully or partially enclosed structures that allow paint spray to be confined and removed. Ventilation systems can remove or lessen vapors. They are equipped with filter banks to remove contaminants before they are exhausted through a roof or wall.
  • Fixed systems come in many sizes and configurations.  Unlike other LEV units, they must be designed specific to the machinery, building or workspace, and contaminant. Fixed systems have the greatest cost but may be more effective in controlling hazardous chemicals and contaminants.

For more information on this topic and to discuss your company’s safety and industrial hygiene needs call OccuSafe at (214) 662-6005 or visit us at

OccuSafe Industrial Hygiene & Safety, Inc. provides skills and expertise to recognize, evaluate and control hazards and injuries in the areas of industrial hygiene, occupational safety and health. OccuSafe services companies of all sizes in a range of industries.

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