of air per minute and so would substantially alter the aerosol size distribution in a volume as small as a bathroom. Since it is not feasible to make direct measurements of activity-weighted size distributions, they have to be calculated. For initial work with the field data, a series of simplifying assumptions were made. From the measured number-weighted size distributions, the activity-weighted size distributions could be calculated as follows. Using the equations given by Porstendörfer and others (1979), the attachment coefficients for 222Rn decay products to any size of particle can be calculated. With these coefficients and the experimental particle data, the attachment rates can be calculated; and with the steady-state equations given by Knutson (1988), the activity-weighted size distribution can be calculated (see figure 5.6). The steady-state approximation gives an upper bound to the calculated values, whereas a dynamic-model calculation (Datye and others 1997) gives results that are likely to be more representative of typical showering conditions.
Figure 5.6 clearly shows how the activity-weighted size distribution shifts during showering toward larger particles that are less efficient at delivering a dose to the bronchial tissues. Thus, although the activity suspended in the air increases because of enhanced attachment of the activity to the larger particles, the dose does not increase as sharply because the larger particles are less effectively deposited in the lung. The period during which the peak is shifted is short—around 5 to 10 min—and the particles return to their original size within about 15 min. The asymmetry in the peaks can in some measure be attributed to the variable nature of the particle size spectra over the sampling period and to the