The relevance of physical activity for the kinetics of inhaled gaseous substances

Inhalation is the most important route of absorption for many volatile subs tances. The inhaled chemical is distributed via the bloodstream into the or gans and tissues. It is eliminated mainly unchanged by exhalation and also via metabolism. The blood concentration can be considered as a surrogate fo r the body burden of the chemical. It depends on the rate of uptake and on the rate of elimination. The rate of uptake by inhalation is determined by the blood:air partition coefficient of the gaseous compound, the actual con centration of the chemical already in the blood entering the lungs, the blo od flow through the lungs, and the alveolar ventilation. The latter is grea tly influenced by physical activity, which thus has a crucial impact on the rate of uptake. Consequently, the blood concentration of an inhaled chemic al and the resulting alveolar retention, representing the rate of metabolis m at steady-state, are dependent on the intensity of physical work. Both pa rameters can be calculated for steady-state conditions using simple algebra ic equations, if one assumes that the rate of metabolic elimination is limi ted by the blood flow through the metabolizing organs. This assumption is v alid for many rapidly metabolized inhaled gases and vapours at low concentr ations present under workplace conditions. The derived equations give the t heoretical background for the observations presented from a series of exper imental studies which demonstrate that physical activity can be a major det erminant of the toxicokinetics of inhaled compounds. Practical examples ill ustrate the procedure. We conclude that workplace-related physical activity should be taken into account for compounds with blood:air partition coeffi cients above 6 in the determination of occupational limit concentrations in air.