This work measured the nasal inhalability of ten human subjects in different inspiratory flow rates under calm air condition and compared the results with the manikin studies in the literature. The results obtained indicate significant inhalability differences exist when human subjects breathe at rest versus moderate exercise. The difference however is within 10% of uncertainty limit and is not practically important in designing air sampler. The cutoff size for nasal inhalation was found to be approximately 135 pro, particles larger than this size will not be the concern for human health. The comparison of the in vivo and manikin measurements suggests the natural convective air current, resulting from the human body heat, might interact with the settling of airborne particles in calm air and cause a slightly higher inhalability for particles smaller than 50 mu m. This study concludes that the revision of the current ACGIH IPM is recommended and the human inhalability under typical windspeeds at workplaces should be taken into account. (C) 2005 Elsevier Ltd. All rights reserved.