Research on common air filter materials has shown that fractional efficiency for collection of particles of different sizes is consistent with the single fiber theory [Heim et al. 2005; Kim et al. 2007; Shin et al. 2008]. Kim et al. [2006] found that humidity has little effect on particle collection efficiency. Huang et al. [2007b] determined that the use of electrostatic filters (commonly used for respirators) improves particle collection in the 0.1–1-µm particle size range. Testing of respirator filters showed that the most penetrating particle size (MPPS) shifted from 30–60 nm to 200–300 nm following treatment of respirators by liquid isopropanol, which removes electrostatic charges on the filter materials [Rengasamy et al. 2009]. This result suggests that capture by electrostatic forces is important for particles in the 250–300-µm range. Overall, filters appear to behave in a manner consistent with theoretical predictions that common filter materials allow for efficient collection through diffusion of nanoparticles less than about 10 nm [Heim et al. 2005; Huang et al. 2007b; Kim et al. 2007; Shin et al. 2008].
Some researchers have found evidence of thermal rebound, which increases particle penetration through filters for nanoparticles in the size range of 1–10 nm [Bałazy et al. 2004; Kim et al. 2006]; however, several other filter testing studies did not reveal this effect, even at higher temperatures [Heim et al. 2005; Huang et al. 2007b; Kim et al. 2007; Shin et al. 2008]. The thermal rebound effect is a result of the thermal velocity of the particle exceeding the critical sticking velocity for a particle on a filter, allowing the particle to move past the filter fiber and penetrate the filter. The critical sticking velocity of an incident particle is defined as the maximum impact speed at which the particle will stick to a surface; above this velocity, the particle will bounce and not stick to the filter. The primary adhesive forces for nanomater-sized particles are the London-van der Waals forces. These forces are caused by random movement of electrons creating complementary dipoles between particle and filter material [Hinds 1999]. As the particle gets smaller, it is more difficult to remove the particle from surfaces.
High efficiency particulate air (HEPA) filtration is commonly used for applications requiring reliably high filtration. By definition, HEPA filters are 99.97% efficient at the most penetrating particle size of 0.3 microns (Figure 6). These filters are disposable and are usually replaced when the pressure drop exceeds a predetermined number, typically 100 mm water gauge (wg). When properly sized and installed, HEPA filtration is appropriate for nanomaterial applications both for ventilation systems and respiratory protection.
2.3.2 Nonventilation Engineering Controls
Nonventilation engineering controls cover a range of control measures (e.g., guards and barricades, material treatment, or additives). Nonventilation controls can be used in conjunction with ventilation measures to provide an enhanced level of protection for nanomaterial workers.
A variety of dust control methods have been used and evaluated in many industries and may be applicable to the processes used in the manufacturing and processing of nanomaterials [Smandych et al. 1998]. These methods include the enclosure of material-conveying equipment, such as belt and screw conveyers, as well as the use of pneumatic conveyance systems. Other work practices have been used to reduce the aerosolization of dust during bag 16
Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes
