Particulate size-selective sampling is an important consideration in determining ambient air quality standards and threshold limit values for workplace exposures. Hazard evaluations, as well as risk analyses, can benefit from an improved understanding of factors affecting regional respiratory tract deposition of particles in man. Here, thoracic deposition and its component parts are examined, as a function of particulate size, for ventilation rates ranging from normal respiration to heavy exercise in individuals who are habitual mouth breathers and in those who normally employ oronasal breathing when minute ventilations exceed approximately 35 1/Lmin. The logistic models yielded significantly improved fits of the experimental data compared to previously used linear regression models. Out analyses demonstrate that the activity level of the exposed population should be taken into account to assess the potential health consequences from ambient or workplace exposures.