Page:Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes.pdf/27

 not be too great or the airflow rate necessary for dilution will be impractical, (2) workers must be far enough away from the contaminant source or the evolution of contaminant must be in sufficiently low concentrations so that workers will not have an exposure in excess of the established threshold limit values (TLV®), (3) the toxicity of the contaminant must be low, and (4) the evolution of contaminants must be reasonably uniform [ACGIH 2013]. There are several issues with using dilution ventilation to control nanomaterial concentrations, including (1) there are no occupational exposure limits (TLVs mentioned above) or health effects data for many of the nanomaterials, (2) the toxicology data from some nanomaterials indicate that they may be associated with adverse health effects, and (3) it is difficult or impossible to calculate proper air change rates for contaminant control due to the variability in most operations. Therefore, local exhaust ventilation and good work practices should be used for controlling exposure, and air change rates should be based on the heat load requirements, general air movement, and comfort needs.

The use of supply air for maintaining proper pressurization between production and nonproduction areas is a reasonable approach to reducing the exposure to nanomaterials outside of the immediate work zone. The fugitive emissions from nanomaterial production and processing may result in high background concentrations in the production area. When adjacent plant areas are nonproduction areas (e.g., office, quality assurance/control labs) or production areas where nanomaterials are not used, infiltration of nanoparticles may occur and result in the exposure of workers in those areas. Therefore, a negative air pressure differential should be maintained in the nanomaterial production area with respect to adjacent rooms/areas. This will help reduce the potential migration of airborne nanomaterials and exposure to other workers in adjacent rooms or areas. To maintain a slight negative pressure, the room supply air volume should be slightly less than the exhaust air. A general guide is to set a 5% flow difference between supply and exhaust flow rates but no less than 50 cfm [ACGIH 2013]. As with any good engineering control, a real-time monitor of differential pressure between areas should be employed, preferably with the control capability to modify airflows to maintain the required pressure differential.

2.3.1.1 Local Exhaust Ventilation

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. When designing a local exhaust ventilation system, it is important to understand the transport mechanisms of the contaminants that are to be removed. This will allow the design to use optimal flow rates and capture locations, maximizing the contaminant capture while minimizing impact on the process and reducing operating costs. LEV typically involves five components [Washington State L & I, no date]:

Exhaust hood. Examples include an enclosing hood to contain the contaminant, a receiving hood to capture or receive a contaminant that is released at a high velocity (e.g., grinding swarf), or simply an open duct.  12

Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes