Aerosol area median diameter

Example 2.9 Given a lognormally distributed aerosol with a geometric mean diameter of 1.5 pm and a og of 2.3, what are the surface-area median diameter and the mass median diameter of this aerosol ... 

A second comparison can be made if one compares to a still larger aerosol, with a mass median diameter of 20,000nm and a surface area median diameter of 200 to 400 nm (described in Reference 26). In this case, <500 nm would also contain >65% of the surface area available to the lung interstitial space. Since the aerosol in this exposure is only 1440 nm, an even higher percentage of the surface area should be less than 500 nm. Thus, most of the surface area would likely locate in the pulmonary interstitial space. Since TiOa is insoluble, the particles would be expected to persist in that location for some time, and serve as a possible source of irritation, inflammation, and injury. 

In treating some closed interiors with aerosols, it may be necessary to limit the time of application. In this case the particle size must be large enough to settle out in the time available. A 10- to 15-minute exposure time is the minimum for satisfactory results. An aerosol spray having a mass median diameter of 15 to 30 microns is sufficient for the short-exposure applications, but will not penetrate so completely as the smaller particle size. Furthermore, aerosols of this size must be released from more than one point if the radius of the area is more than 15 feet. When heat from thermal generators causes excess breakdown of the insecticide, equipment that produces larger particle sizes must be used. 

Plutonium is so toxic that processing and fabrication are always done in sealed cells or glove boxes, but accidental dispersions of aerosol occur from time to time. Following combustion of Pu metal chips in a production area at Rocky Flats, Colorado, in 1964, airborne contamination was widespread. Alpha tracks from individual particles caught on membrane filters were detected on nuclear film, and the Pu content, and hence the particle size, was deduced (Fig. 5.2, curve E). The activity median diameter was 0.3 /urn (Mann Kirchner, 1967). The same method, used during normal operations in a production area at Los Alamos, gave activity median diameters in the range 0.15 to 0.65 /urn (Moss et al., 1961). However, when a spill occurred, followed by clean-up operations, the Pu particles were found to be associated with inert dust particles of mass median diameter 7 /urn. 

Plotting the surface and volume distributions of a log-normal aerosol distribution on a log-probability graph would also result in straight lines parallel to each other (same standard deviation). For the distribution shown in Figure 8.8 with Dpg = 1.0 pm and ag = 2.0, the resulting surface area and volume median diameters are approximately 2.6 pm and 4.2 pm, respectively. 

Fig. 1. Deposition of inhaled particles of different sizes (mass median aerodynamic diameters) in the three regions of the respiratory tract. Each shaded area indicates the variability of deposition when the aerosol distribution parameter, o, (geometric standard deviation) was varied from 1.2 to 4.5. The assumed tidal volume was 1450 cm3. (Reproduced from Health Physics, vol. 12, pp. 173-207,1966 by permission of the Health Physics Society).

Pulmonary deposition of an aerosol preparation is determined primarily by its size. Aerosols with a mass median aerodynamic diameter of 1-5 xm produce the best therapeutic results and are the target particle size for inhalation therapy. These small particles penetrate deep within the respiratory tract to ensure drug deposition in peripheral airways. The cross-sectional area (cm ) of the lung increases dramatically at the level of the respiratory zone therefore, the velocity of gas flow during inspiration rapidly decreases at this level. Moderate-sized particles (5-10 (xm) frequently settle out by sedimentation in larger more central airways because the velocity of gas falls rapidly in the region of the terminal bronchioles. 

Figure 3. An example of the log-normal distribution function in normalized linear form for CMD = 1.0 and Og = 2.0, showing the mode, median and mean of the size distribution, the surface area distribution median and mean diameters, the mass distribution median and mean diameters, and the diameter of average mass. Reproduced with permission from Raabe OG (1970). Generation and characterization of aerosols. In Inhalation Carcinogenesis (MG Hanna, P Nettersheim and JR Gilbert, eds), pp. 123-172. Proceedings of a Biology Division, Oak Ridge National Laboratory Conference. Oak Ridge, TN, USA US Atomic Energy Commission...

The median is defined as the diameter for which one-half the total munber of particles are smaller and one-half are larger. The median is also the diameter that divides the frequency distribution curve into equal areas, and the diameter corresponding to a cumulative fraction of 0.5. The mode is the most frequent size, or the diameter associated with the highest point on the frequency function curve. The mode can be determined by. setting the derivative of the frequency function equal to zero and solving for d. For symmetrical distributions such as the normal distribution, the mean, median, and mode will have the same value, which is the diameter of the axis of synunetry. For an asymmetrical or skewed distribution, these quantities will have different values. The median is conunonly used with skewed distributions, because extreme values in the tail have less effect on the median than on the mean. Most aerosol size distributions are skewed, with a long tail to the right, as shown in Fig. 4.4. For such a distribution,... 

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