page_title: Engineering controls
section_title: Local exhaust ventilation
hierarchical_section_title: Engineering controls / Chemical and biological hazards / Ventilation / Local exhaust ventilation
caption_attribution_description: English: Figure 10. Nano containment hood adapted from a pharmaceutical balance enclosure. The use of bench-mounted weighing enclosures is common for the manipulation of small amounts of material. These fume hood-like local exhaust ventilation (LEV) devices typically operate at airflow rates lower than those in traditional fume hoods and use airfoils at enclosure sills to reduce turbulence and potential for leakage. They also have face velocity alarms to alert the user to potentially unsafe operating conditions. Based on the hazards assessment, these fume hood-like LEV devices can be outfitted with HEPA filtration or connected to the ventilation exhaust system.
context_page_description: Engineering controls are strategies designed to protect workers from hazardous conditions by placing a barrier between the worker and the hazard or by removing a hazardous substance through air ventilation. Engineering controls involve a physical change to the workplace itself, rather than relying on workers' behavior or requiring workers to wear protective clothing.
  Engineering controls is the third of five members of the hierarchy of hazard controls, which orders control strategies by their feasibility and effectiveness. Engineering controls are preferred over administrative controls and personal protective equipment because they are designed to remove the hazard at the source, before it comes in contact with the worker. Well-designed engineering controls can be highly effective in protecting workers and will typically be independent of worker interactions to provide this high level of protection. The initial cost of engineering controls can be higher than the cost of administrative controls or PPE, but over the longer term, operating costs are frequently lower, and in some instances, can provide a cost savings in other areas of the process.
context_section_description: Local exhaust ventilation (LEV) is the application of an exhaust system at or near the source of contamination. If properly designed, it will be much more efficient at removing contaminants than dilution ventilation, requiring lower exhaust volumes, less make-up air, and, in many cases, lower costs. By applying exhaust at the source, contaminants are removed before they get into the general work environment.  Examples of local exhaust systems include fume hoods, vented balance enclosures, and biosafety cabinets.  Exhaust hoods lacking an enclosure are less preferable, and laminar flow hoods are not recommended because they direct air outwards towards the worker.
  Fume hoods are recommended to have an average inward velocity of 80–100 feet per minute (fpm) at the face of the hood. For higher toxicity materials, a higher face velocity of 100–120 fpm is recommended in order to provide better protection. However, face velocities exceeding 150 fpm are not believed to improve performance, and could increase hood leakage.  It is recommended that air exiting a fume hood should be passed through a HEPA filter and exhausted outside the work environment, with used filters being handled as hazardous waste.  Turbulence can cause materials to exit the front of the hood, and can be avoided by keeping the sash in the proper position, keeping the interior of the hood uncluttered with equipment, and not making fast movements while working.
  Low-turbulence balance enclosures were initially developed for the weighing of pharmaceutical powders and are also used for nanomaterials; these provide adequate containment at lower face velocities, typically operating at 65–85 fpm.  They are useful for weighing operations, which disturb the material and increase its aerosolization.
  Biosafety cabinets are designed to contain bioaerosols.  However, common biosafety cabinets are more prone to turbulence.  As with fume hoods, they are recommended to be exhausted outside the facility.
  Dedicated large-scale ventilated enclosures for large pieces of equipment can also be used.
caption: Vented balance enclosures used in the pharmaceutical industry can be used for nanomaterials, with the advantages of smaller size and lower turbulence.
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