Quantitative evaluation of the performance of an industrial benchtop enclosing hood

Plain benchtop enclosing hoods are assumed to be highly effective in protecting workers from airborne contaminants, but there is little research published to support or rebut that assumption. The purpose of this research was to investigate the performance of a 36 in. wide, 30 in. high, and 40 in. deep benchtop enclosing hood. The study consisted of two parts: (1) investigating the effects of hood face velocity (five levels: 111, 140, 170, 200, and 229 ft/min) and wind tunnel cross-draft velocity (five levels: 14, 26, 36, 46, and 57 ft/min) on a plain benchtop enclosing hood, and (2) studying the effects of specific interventions (no-intervention, collar flange, bottom flange, cowling, and sash) added onto the same enclosing hood. A tracer gas method was used to study the hood's performance inside a 9 ft high, 12 ft wide, and 40 ft long wind tunnel. Freon-134a concentrations were measured at the mouth and nose of an anthropometrically scaled, heated, breathing manikin holding a source between its hands while standing at the enclosing hood's face. Roughly 3 L/min of pure Freon-134a mixed with 9 L/min of helium was released from the source during all tests. Results showed that hood face velocity, wind tunnel cross-draft velocity, and interventions had statistically significant effects (p < 0.05) on the concentrations measured at the manikin's breathing zone. Lower exposures were associated with higher face velocities and higher cross-draft velocities. The highest exposures occurred when the face velocity was at the lowest test value (111 ft/min), and the cross-draft velocity was at its lowest test value (14 ft/min). For the effects of interventions to the hood face, the results showed that flanges and the cowling failed to consistently reduce exposures and often exacerbated them. However, the customized sash reduced exposures to less than the detection limit of 0.1 ppm, so a similar sash should be considered when feasible. The hood face velocity should be at least 150 ft/min if a sash is not used.