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During the past two decades, there has been considerable progress in developing particle size-selective criteria for aerosol sampling and exposure assessment that relate more realistically to actual human exposures than previously. An important aspect has been the aspiration efficiency-the 'inhalability'-with which particles enter through the nose and mouth of aerosol-exposed individuals during breathing. Most of the reported experiments to determine inhalability have been conducted in wind tunnels with life-sized, breathing mannequins, for windspeeds from 0.5 m s(-1) and above. A few experiments have been reported for calm air. However, nothing has been reported for the intermediate range from 0.5 m s(-1) downward, and it so happens-as we now know-that this corresponds to most industrial workplaces. The research described in this paper represents a first step toward filling this knowledge gap. It focuses on identifying the features of the airflow near the mannequin at such low windspeeds that might have important influences on the nature of particle transport, and hence on inhalability, and eventually the performances of personal aerosol samplers mounted in the breathing zone. We have carried out flow visualization experiments for the realistic range of windspeeds indicated, investigating specifically the effect of the air jet released into the freestream during expiration and the effect of the upward-moving boundary layer near the body associated with the buoyancy of air in that region as a result of heat received from the warm body. We set out to identify the combinations of conditions-external windspeed, breathing mode (nose versus mouth breathing), breathing rate and body temperature-where such factors need to be taken into account. We developed an experimental system that allowed the visualization of smoke traces, providing very good observation of how the flow was modified as conditions changed. From inspection of a large number of moving pictures, we developed a matrix of regimes-categorized by windspeed and breathing rate-where the effect of the expired air is sufficient to permanently and seriously destabilize the airflow approaching the mannequin. It was found that the effect of body temperature was minimal. Such results will be important in the interpretation of current and future inhalability experiments carried out at realistic low windspeeds.

Visualization of the Airflow around a Life-Sized, Heated, Breathing Mannequin at Ultralow Windspeeds