Volume median aerodynamic diameter

Table III. Median Aerodynamic Diameters (MAD) and Geometric Standard Deviations (Og) of Pb-210 and Be-7 Based on Radioactivity and SO Measured with High-Volume Cascade Impactors...

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).

P3). The average mass median aerodynamic diameter was 3.55 0.07 pm with the geometric standard deviation of 2.55. The test was also conducted to compare the device performance with other small-volume nebulizers,and found to be more efficient in the delivery rate (mass out per minute) than that of other SVNs. 

Thus, Eqs. (2) and (3) enable us to convert the volume-equivalent particle size distribution into aerodynamic-equivalent distribution for any flow regime. The characteristic mean diameters are dy (volume geometric) and (volume aerodynamic). Although dp is numerically close to the mass-median aerodynamic diameter (MMAD) often used for the aerosols, it is better defined for asymmetrical distribution, so often observed for respiratory powders. 

Figure 11. Shaded areas indicate variability of deposition for a given mass median (aerodynamic) diameter in each compartment when the distribution parameter,

The median diameter corresponds with the 50 % value of a cumulative number, volume or mass percent distribution as function of the diameter. Fifty percent of the volume (number or mass) of the aerosol is in larger, and 50 % is in smaller particles than the median diameter. For a volume distribution it is the volume median diameter, for a mass distribution the mass median diameter. When the mass percent is expressed as a function of the aerod3mamic diameter, reference can be made to the mass median aerodynamic diameter (MMAD). 

All currently marketed inhaler devices produce polydisperse aerosols of which the individual particles have different sizes. Therefore, they cannot be characterised by a single diameter. In fact, for most solid aerosols from dry powder inhalers the particles may have different shapes too, which is the reason to characterise them with aerodynamic diameters. To be able to express polydisperse aerosols with a single parameter, the median aerodynamic diameter (MAD) was introduced. When the aerodynamic size range which covers the population of particles in the aerosol is divided into different classes and the volume or mass fraction within each size class is expressed as function of the class mean diameter, a volume or mass distribution as function of the aerodynamic diameter is obtained. This volume or mass frequency distribution can be transferred into a cumulative percent distribution of which the 50 % value corresponds with the volume or mass median aerodynamic diameter (VMAD or MMAD). This is the diameter indicating that 50 % of the total aerosol volume or mass is in larger, and 50 % is in smaller particles. When particles of all sizes in the aerosol have the same density, which is mostly the case, then VMAD equals MMAD. 

Thus, for spherical particles having a density equal to Ig/cm, the aerodynamic and eqnivalent volume diameters are mathematically equal, hi practice, all medical aerosols comprise particle size distributions thus, the value mass median aerodynamic diameter (MMAD) is freqnently used to characterize an aerosol. 

The mass median diameter (MMD) is the most common descriptor of primary particle size and may be determined by sieving or centrifugal sedimentation. The volume median diameter, as determined by laser diffraction, may be used as an approximation of MMD, provided that the particle density is known and does not vary with size, and that the particle shape is near spherical. The MMD of a powder can be used as a predictor of aerodynamic diameter by Eq. (1),... 

Aerodynamic diameter of a real median size particle Diffusion coefficient Sauter mean diameter Volume mean diameter Entrainment, mass liquid/mass gas Plate or stage efficiency, fractional Power dissipation per mass Murphree plate efficiency, with entrainment, gas concentrations, fractional... 

There are ver> limited data on deposition of particles in children. Knight and Gilberg (23) performed calcnlations, using deposition models and lung models (24), and age adjustment for respiratory frequency and tidal volume, according to Hoffman (25). These estimates indicate that, in the proximal lung generations 1-12, the deposition is double in a child of 6 months of age, compared with a 25-year-old adult, for small, 1.3-pm-aerodynamic mass median diameter, particles. 

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