Fibers aerodynamic diameter

Both of these approximations differ from Eq. 5.15 in the value of the coefficient and in the value of the exponent of the aspect ratio (Ve versus Vi). Spurny et al. (1978) reported experimental measurements of asbestos fiber aerodynamic diameters which indicate a range of exponential values of 0.116 to 0.171 with a coefficient of about 1.34. However, even if the details are still not clear, it is clear that for fibers the... 

St. Louis Sample Collection. Ambient aerosols were collected in St. Louis in 6-h Intervals with a TWOMASS automated sequential tape sampler. This sampler fractionated the aerosol into two size classes, fine particles having aerodynamic diameters less than 3pm, and coarse particles with diameters greater than 3pm. It was equipped with a beta-attenuation mass monitor to determine fine-particle mass (11). Only the fine particle filter was examined in this study. Pallflex E70 glass-fiber filter tape with a detachable cellulose backing (Pallflex Inc. Putnam, CT) was used with this sampler. An aerosol sampler operating from the same inlet manifold as the... 

Particles which are non-spherical will have at least one physical dimension which is greater than the aerodynamic diameter. Environmental fibers 50 pm in length can reach the A region because they align with the inspired airflow. Such materials then impact in the airways by a process of interception with the airway walls. 

With fibers, measurements are usually in terms of fiber length L and diameter d. Writing the aerodynamic diameter as... 

In many practical cases Leith s approach to the definition of the aerosol shape factor has greatly simplified the understanding of this correction to Stokes law. For example, consider again the aerodynamic diameter of a fiber having a cross-sectional diameter df, length L, and density p. This can be approximated by using Eqs. 5.17 and 5.18 for the case of long axis motion parallel to the flow as... 

Example 5.10 Estimate the aerodynamic diameter of the asbestos fiber in Example 5.9, using Eq. 5.21. 

As mentioned above, Eq. 5.15 implies that the aerodynamic diameter of a rod or fiber will be influenced very little by its length, being much more dependent on its cross-sectional diameter. Hence fibers of different lengths but similar cross-sections will have similar aerodynamic properties, despite large differences in mass. 

What length of 1-pm-diameter fiber will have the same aerodynamic diameter as a 10-pm unit-density sphere Assume the fiber density is 2.65 g/cm3. 

Example 6.5 An asbestos fiber is reported to have an aerodynamic diameter of... 

Asbestos fibers are nonvolatile and insoluble, so their natural tendency is to settle out of air and water, and deposit in soil or sediment (EPA 1977, 1979c). However, some fibers are sufficiently small that they can remain in suspension in both air and water and be transported long distances. For example, fibers with aerodynamic diameters of 0.1-1 pm can be carried thousands of kilometers in air (Jaenicke 1980), and transport of fibers over 75 miles has been reported in the water of Lake Superior (EPA 1979c). Adsorptive interactions between the fibers and natural organic contaminants may favor coagulation and precipitation of the fibers (EPA 1979c). 

Sampling of airborne particles was carried out at the rooftop level of a 6-story building of the National Institute of Public Health surrounded with arterial roads in central Tokyo (30 m above the ground) between lO and 22 October, 2000. Airborne particles smaller than 10 pm in aerodynamic diameter were collected on quartz fiber filters with a high-volume air sampler with 10-pm cut-off stage for 24 h from 1224 m of the air. Three quarters of the filter samples spiked with 2-NFL internal standard were cut into small pieces and put into dichloromethane to extract SOF from the airborne particle. The extracted SOF solution was isolated by filtration and washed sequentially with 5% sodium hydroxide, 20% sulfuric acid solution, and purified water. After removal of water, the sample solution was concentrated by drying under nitrogen and was dissolved into 1 mL of acetonitrile for subsequent analysis. 

Glass fiber filter (VDI 3498, Part 1,1998) External diameter 120 mm (LIB apparatus) or 50 mm (small filter apparatus) with a collection efficiency of 99.8 % for particles with an aerodynamic diameter >0.3 //m. The filter must be baked-out at 250-400 °C before use. 

To be able to selectively adjust the aerodynamic diameters, the nanofibers fabricated by electrospinning have to be shortened to a defined axis ratio. This task can be achieved by laser or mechanical cutting. To control the aerodynamic radius via the density, highly porous fibers may be used. Inhalation therapy will have to be based on polymers which are biocompatible with particular emphasis on the specific reactions within the lung. 

Stober, W., Flaschsbart, H., and Hochrainer, D. (1970). The aerodynamic diameter of latex aggregates and asbestos fibers. Staub-Reinhalt Li 30 1-12. 

Equation (1) points to a number of important particle properties. Clearly the particle diameter, by any definition, plays a role in the behavior of the particle. Two other particle properties, density and shape, are of significance. The shape becomes important if particles deviate significantly from sphericity. The majority of pharmaceutical aerosol particles exhibit a high level of rotational symmetry and consequently do not deviate substantially from spherical behavior. The notable exception is that of elongated particles, fibers, or needles, which exhibit shape factors, kp, substantially greater than 1. Density will frequently deviate from unity and must be considered in comparing aerodynamic and equivalent volume diameters. 

The aerodynamic behavior of a fibrous particle is neither simple nor well understood (Timbrell, 1965). However, once any fibrous particle is suspended in ambient air, there is always the opportunity for inhalation. Fibrous particulate aerodynamics may be related, as a first approximation, to fiber diameter (Timbrell, 1979). 

Using radioactive labeled UICC samples, the deposition and distribution of asbestiform fibers in the pulmonary cavity have been studied. For example, after thirty minutes of inhalation, the deposition of fibers in the respiratory track was shown to be proportional to the median aerodynamic particle diameter for the two UICC chrysotiles, amosite, anthophyllite, and crocidolite. The percentage of total deposited fiber in the lower respiratory tract varied inversely as the square root of the particle diameter (Morgan et al., 1975). 

However, rather long fibers are known to be able to penetrate deeply into the lung. The reason is that the aerodynamic radius controls this process with the aerod5mamic radius of rods being controlled mainly by the diameter and only weakly by the length, as detailed later in more detail. So, inhalation therapy based on nanorods as accessible via nanofibers offers great benefits. 

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