Aerosol mass median diameter

Ambient air Pb in submicron vapor aerosols Mass median diameters for fi and coarse fractions Lin et al. ne (1993)... 

Punte et al. I tested volunteers in a wind tunnel at an airspeed of 5 mph to establish CN tolerance time—the length of time a subject could remain in the CN-containing airstream. The tolerance time varied with the subject. CN aerosols were generated from acetone solutions and had a mass median diameter of about 0.6 ym. Such particles can reach and remain in the deep region of the respiratory tract. The immediate effects of such exposures were tingling of the nose ind rhinorrhea, burning of throat and eyes, lacrimation, and blurred vision. Some subjects suffered dyspnea. Mild conjunctivitis was observed, but this and the other symptoms passed rapidly when the subject left the wind tunnel. 

Inhalation toxicity Aerosol exposure (mass median diameter, 1.8 um) produced similar toxic signs In rats, mice, and guinea pigs. 

For control of flying insects with aerosols, research showed that there is an optimum particle size needed to obtain maximum kill. The size was determined through extensive research in wind tunnels (2) and Peet-Grady chambers in which the most nearly homogeneous sprays possible were used. For adult mosquitoes the optimum particle size was found to be between 10 and 15 microns mass median diameter and for adult houseflies between 15 and 20 microns. 

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. 

The relationship between particle size and foliage injury was shown in work with liquefied gas aerosol formulations of hexaethyl tetraphosphate for greenhouse applications (I). Particles larger than 20 microns diameter injured some varieties of foliage. Increasing pressure and using smaller nozzles lowered the mass median diameter but did not eliminate the maximum sized particles. The problem was finally solved with a low-concentration formulation, in which the particles larger than 20 microns in diameter could be eliminated. 

Atomization—Liquid aerosols of readily melted solids may be generated by direct atomization using, for example, a pneumatic nebulizer such as the Collision. When a pneumatic atomizer is used, the aerosol concentration may be varied by changing the pressure of the atomizer air supply. The mass median diameter of the aerosols produced by the small pneumatic nebulizer is typically in the micrometer range. The direct atomization method was applied to dimethyl-phthalate, a liquid, and dibutyl phosphate, a low-melting solid. 

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. 

Table 7.2 Comparison of the mass median diameter of aerosols over the North Sea and at Enewetak North Central Pacific (pm)...

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 ... 

Using the Hatch-Choate equation, compute the mass median diameter from the information developed in Prob. 5. If the aerosol contains 1 million particles per cubic foot and the particle density is 1 g/cm3, find the aerosol concentration in micrograms per cubic meter. 

Choice of which average diameter to use in a given situation depends on how the diameter was measured or how it is to be used. For the case where aerosol mass is measured and the fractions collected are associated with specific particle diameters, the resulting average value is the mass median diameter. In studying chemical reaction rates, the volume-surface mean diameter may be more important than just the arithmetic mean or geometric number mean. 

From the plot (Fig. 7.8) an MMAD of 1.2 p.m is found and a ag of 2.0 determined. This is a mass median diameter because the weight or mass of aerosol collected on each stage was used in the analysis. 

Nebulizers and atomizers used in aerosol research produce a polydisperse aerosol consisting of particles under 10 pm in diameter. Most nebulizers use compressed air for atomization, whereas some use ultrasonics. Many models of compressed-air nebulizers have been developed, but they basically use the principle of air blast atomization of liquids issuing through a small orifice. Impaction plates or baffles are used to remove the larger droplets. Mass median diameters normally range from 2 to 5 pm, with a compressed-air pressure of 20-30psig. A detailed discussion of nebulizers can be found in Raabe [5]. Most nebulizers or atomizers tend to have a small liquid reservoir and cannot be used for long duration unless the reservoir is refilled continuously. 

High levels of SOi removal (>98%) with up to 80% NO j removal have been reported for inlet concentrations of 3000 and 500 ppm of SOj anti NO.t, respectively. The mass median diameter of the aerosol product falls in the size range 0.6 to 1.0 /mi. The particles tend to be sticky because of their hygroscopicity and the high relative humidity, and particle collection is difficult. Recent efforts at commercialization have focused on the use of a pulse power... 

From Figure 8 it will be seen that only 10% of the particles in the aerosol were greater than 1 pm in diameter but that these accounted for about 80% of the total mass of the particles in the aerosol. More dramatically, 50% of the mass was made up by particles of diameter greater than 1.39 pm (mass median diameter = 1.39 pm) but these comprised only about 4% of the particles in the aerosol. 

A salt aerosol is generated in an enclosure, usually up to a concentration of 13 mg m 3, with a particle mass median diameter of 0.3 to 1.3 pm, depending on the aerosol generator used. The subject dons the respirator, enters the enclosure and performs a set of simple exercises while the salt concentration inside and outside the respirator are measured by flame photometry. The Dstl, Porton Down uses a specially built high-sensitivity flame photometer to measure very low salt concentrations (and hence to confirm very high PFs). 

The leak test ( DOP test situ test to verify that filters do not leak on inatidlation. The leak test is not a second efficiency test. It is intended to dis> close leaks around the frames and damage to the filter medium. An aerosol of oil particles with mass median diameter of 0.3 pm is used to challenge the filter deteaion Is by aerosol photometry on the downstream side. Standards of integrity are specified as maximum permissable percentages of the upstream concentration of particles that can be recovered downstream of the filler. 

Table 15.4 presents typical aerosol size distributions and concentrations based on measurements in many locations, including mass median diameters Dpg, geometric standard deviations volume concentrations V(pm3cm 3) of the nuclei,... 

Table 22.4 presents typical aerosol size distributions and concentrations based on measurements in many locations, including mass median diameters, Dpg, geometric standard deviations, ag, and volume concentrations V((Mm cm ) of the nuclei, accumulation, and coarse particle modes of eight classifications of aerosol. We note only a small variability in the accumulation and coarse mode size distributions for the variety of aerosol classes (excluding the marine aerosol). The average specifications for the accumulation and coarse... 

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. 

Figure 13 Regional mass depositions estimated with the ICRP semiempirical deposition model (3) for oral breathing of polydisperse aerosols with unit-density spheres at the reference resting pattern for an adult Caucasian male sitting awake (9) (5-s breathing-cycle period and flow rate of 300 cm s ). The particle size distributions are log-normal mass distributions characterized by various mass median diameters and a geometric standard deviation of 2.

The latter approach—completely drying the aerosol— while valid, is diffi-cnlt to perform experimentally (29). The low solids content of pharmaceutical nebulizer solutions results in dry aerosols that are too fine for reliable enough measurements to be made to allow back calculation with any accuracy. For example a typical nebulizer droplet distribution with a mass median diameter of 3 pm generated from a 0.1% drug solution would result in a dry particle distribution with a mass median diameter of 0.1 pm. Obtaining size distribution data with any reasonable resolution at this size is very problematic. A further issue is that the solids content of the original droplets must be known in order to perform the calculation, and this can vary considerably during the course of nebulization. This technique has therefore not been used extensively. 

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