size before, during, and after a shower. If an aerosol particle can grow, the increase in size will result in a larger decay-product attachment rate. If the particles can grow in the high humidities during showering, they can grow in the respiratory tract (George 1993). However, measurements show that this growth is minimal (Dua and Hopke 1996).

Changes in particle size will have two effects. First, the changes will affect the deposition of the particles onto the room surfaces, thereby affecting the amount of aerosol decay product available for inhalation. Second, the changes will alter where the aerosol particles deposit in the respiratory tract, and so affect the dose delivered to the lung (National Research Council 1991a). Furthermore, depending on how close the bathroom humidity is to 100%, the amount of possible growth when the particles enter the lungs is reduced, further altering deposition patterns in the lung. Figure 5.5 presents a series of number-weighted particle size distributions based on measurements with a scanning electrical mobility spectrometer. From the number-weighted size distributions, activity-weighted distributions can be calculated by using the attachment coefficients of Porstendörfer and others (1979). Figure 5.6 shows the estimated activity-weighted size distributions. Tu and Knutson (1991) have shown that this method provides a reasonable approximation of the directly measured activity-weighted distributions.

It is possible to make direct activity-weighted size distribution measurements (Ramamurthi and Hopke 1991; Tu and others 1991). However, the system available for such activity-weighted size distribution measurements draws 90 L

Figure 5.5

A series of number-weighted particle size distributions based on measurements with a scanning electrical mobility spectrometer.