Experimental methods to determine inhalability and personal sampler performance for aerosols in ultra-low windspeed environments

Abstract

Most previous experiments of aerosol inhalability as it relates to particle aerodynamic diameter were conducted in wind tunnels for windspeeds greater than 0.5 m s⁻¹. While that body of work was used to establish an inhalable aerosol convention, results from studies in calm air chambers (for essentially zero windspeed) are being discussed as the basis of a modified criterion. Meanwhile, however, information is lacking for windspeeds in the intermediate range, which—it so happens—pertain to most actual workplaces. With this in mind, we have developed a new experimental system to assess inhalability and personal sampler performance for aerosols with particle aerodynamic diameter within the range from 6 to 90 µm for ultra-low windspeed environments from about 0.1 to 0.5 m s⁻¹. In this range of conditions for particle size and windspeed, controlled aerosol experiments are very difficult to perform, most notably with respect to the problem of achieving uniform spatial distributions of both test aerosols and air velocity. In the work reported in this paper, we have addressed these difficulties in a new, custom-designed experimental facility. It is a novel wind tunnel design that provides stable and controllable low-turbulence air movement, and allows for the delivery of test aerosol to the working section both from upstream (as in conventional wind tunnel experiments) and from above (as in calm air studies). In this system, losses by elutriation of particles that are being convected in the horizontal aerosol flow are compensated by particles entering from above by gravitational settling. An important feature of the new facility is the life-size, breathing mannequin that contains physical means to achieve any combination of mouth and nasal inspiration and expiration, and allows any desired relevant breathing flowrate and pattern by means of an external computer-controlled breathing simulator. Special steps were taken in the detailed design to ensure that particles may be collected during the inspiration phase of the breathing cycle and that the air during the expiration phase re-enters the breathing zone through a separate pathway (in order to avoid re-entrainment of collected particles). The mannequin itself was heated (to body temperature) to allow for the possibility that, at such low windspeeds, the overall air movement may be influenced by updrafts associated with the enhanced buoyancy of warm air near the body of the mannequin. The new experimental system has been commissioned and calibrated. Experiments have been carried out to determine the role of expired air and body heat on the time-dependent flow near the mannequin which might be expected to influence the transport, and hence inhalation, of particles. These show that such effects may be expected for some parts of the ranges of conditions studied. Preliminary experiments have been carried out to assess the aspiration efficiency of the human head. The successful development of this novel experimental facility paves the way for an important new series of experiments to evaluate inhalability under realistic workplace conditions, along with assessments of the performance of personal inhalable aerosol samplers.