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 In most cases, they should be more feasible than elimination or substitution and, given the potential toxicity of many nanomaterials, should prove to be more protective than administrative controls and PPE.

Engineering controls are divided into two broad categories for discussion below: ventilation and nonventilation controls.

2.3.1 Ventilation

The general concept behind ventilation for controlling occupational exposures to air contaminants, including nanomaterials, is to remove contaminated air from the work environment. The efficiency of the ventilation system can be affected by its configuration and flow volumes of both the air supplied to and the air exhausted from the work space. Effective ventilation applies to a wide range of applications including office heating, ventilating, and air conditioning (HVAC); infection control in healthcare; and control of emissions in industrial processes. Ventilation for occupant comfort, HVAC, is a specialized application of dilution ventilation and is not within the scope of this document. Filtration is a topic directly affecting ventilation; exhaust air laden with nanomaterials may need to be cleaned before being released into the environment.

General ventilation can be used to achieve several goals for workplace contaminant control. A properly designed supply air ventilation system can provide plant ventilation, building pressurization, and exhaust air replacement. As new local exhaust hoods are installed in the production area, it is important to consider the need for replacement air, the location of the hood installation, and the need to rebalance the ventilation system. In general, it is necessary to balance the amount of exhausted air with a nearly equal amount of supply air. Without this replacement air, uncontrolled drafts will occur at doors, windows, and other openings; doors will become difficult to open due to the high pressure difference, and exhaust fan performance may degrade. In addition, turbulence created through high pressure differentials can defeat the design intent of the ventilation. Placement of the air supply registers in relation to other exhaust ventilation systems is important so that they do not negatively impact the desired performance. The use of general ventilation for dilution of contaminants being generated in the space should be restricted in its use depending on several factors discussed below.

General ventilation used for dilution of contaminants by its nature is inefficient. One of two methods, recirculated air or single-pass air, may be used for this purpose. As the terms imply, recirculated air involves the treatment of exhaust air prior to its being returned to the area from which it was exhausted. Single-pass air is exhausted to the outside and may or may not require treatment prior to discharge. Both of these methods are expensive—the treatment of the recirculated air involves both first-cost and operating-cost penalties, while makeup-air treatment for single-pass air is inherently costly.

According to the American Conference of Governmental Industrial Hygienists (ACGIH) Industrial Ventilation: A Manual of Recommended Practice for Design (hereafter referred to as the Industrial Ventilation Manual), dilution ventilation (i.e., air changes) to control exposure should be used only under specific conditions. Dilution ventilation for controlling health hazards is restricted by four limiting factors: (1) the quantity of contaminant generated must Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes 11