| Inhalability is the efficiency with which people inhale airborne particles through the nose and/or mouth during breathing. Most of the previous studies used to set criteria for this were based on high-speed wind tunnels, using breathing mannequins to measure aspiration efficiency as a function of particle size. However, it has been shown that ultra-low windspeeds (between 0.05 and 0.5 m/s) are the most representative of modern workplaces. Bearing that in mind, studies performed in completely calm air have indicated that inhalability is greater in environments with essentially no air movements, casting doubt on the applicability of the current convention at ultra-low windspeeds as well. However, there is a lack of information for human inhalability at these windspeeds of interest. The hypothesis of this research was that inhalability at ultra-low windspeeds is more similar to calm air than fast moving air, on the basis that convective inertial forces will not completely overcome the effects of gravity, resulting in altered particle trajectories. In order to test this, entirely new facilities were necessary---including a new heated, breathing mannequin and a novel wind tunnel that combined the principles and modes of operation of both conventional wind tunnels and calm air chambers. Flow visualizations were performed that indicated expired air was a potentially influential factor for air patterns around a breathing mannequin; body heat was not shown to be important. Experiments to directly assess inhalability---as well as the sampling efficiency of personal samplers commonly used to quantify such exposures---were carried out for particle sizes between 7 and 90 mum, at three different windspeeds covering the ultra-low range. Several different breathing patterns were also examined to assess the influence of breathing flowrate and mode (i.e., nose versus mouth). Results showed that aspiration efficiency, for both the mannequin and the personal samplers, was dependent on windspeed, with the greatest values at the lowest windspeed. At 0.10 m/s, inhalability was more similar to a proposed calm air criterion while exposures above 0.25 m/s were still described well by the current convention, indicating the need for dual criteria with which to define inhalability based on windspeed. |
