The human health effects that result from breathing air pollutants depend on the amount of pollutant inhaled from the air (exposure dose) and the amount of inhaled material that stays in the respiratory tract (retained dose). Because the retained dose of a pollutant may damage the respiratory tract and cause disease, it is a key factor for determining appropriate government regulations for air pollutants. Drs. Kleinman and colleagues at the University of California, Irvine examined the factors that influence the retained dose of a pollutant and its potential adverse health effects on the respiratory system. The investigators exposed two groups of dogs to nitrogen dioxide, formaldehyde, ammonium nitrate particles, or a mixture of formaldehyde and the particles. They exposed each dog in the first group via a mask while resting and during exercise. They calculated the exposure dose and then determined the percentage of the exposure dose retained by the respiratory tract by measuring the amount of pollutant that exited through the mask. For the second group, they designed a special exposure system that permitted separate delivery of pollutants either to the upper respiratory tract only, via the nose or mouth, or directly into the lungs. By measuring the amount of expired pollutant, they determined the percentage of the exposure dose retained by either the upper respiratory tract or the lungs at rest and under conditions that simulated breathing patterns during exercise.