Aerosol airflow

In order to control the movements of contaminants it is useful to be able to see how both the contaminant and the induced airflows move. A number of flow visualization methods have been developed some are more suitable for laboratory research applications whereas others are quite widely used in industrial situations. We are primarily interested in this latter category. The methods involve releasing a tracer (for example gas, aerosol, or heat) and making visible its path. While in most cases the methods are subjective, their use is invaluable. Ideally the tracer should be nontoxic, nonirritating, inexpensive, and highly visible at low concentrations. The system should be easily portable, self-contained, easy to use, and be controllable. 

Air contaminants in solid or liquid state (aerosols), e.g., wood dust, welding smoke, or oil mist, are all in principle directly visible. The dispersion of those contaminants and the airflow patterns around the source may therefore be studied without any special tools. It is, however, not always possible to see the contaminant if, for example, the concentration in the air is low, the size of the particles is small, or the lighting is poor. The fact that the contaminant can t be seen may stem from the acceptable low level of the concentration but that can of course not be used to conclude that the control is acceptable. That conclusion depends not only on the contaminant s toxicological qualities but on how visible it is iit air. The ability to see the particles directly is also, as said above, a function of their size. Small particles, able to be transported deep into the thinner airways of the lungs, are many times also difficult to see directly. 

A. Srecnath, 0. Ramachandian,. H Vincent. Experimenial invesugaiions into the nature ot airflows near blutf bodies with aspiration, with implications to aerosol sampling. Atmospherie F.nt tronment. 1 11.5), 1997... 

For extrathoracic deposition of particles, the model uses measured airway diameters and experimental data, where deposition is related to particle size and airflow parameters, and scales deposition for women and children from adult male data. Similar to the extrathoracic region, experimental data served as the basis for lung (bronchi, bronchioles, and alveoli) aerosol transport and deposition. A theoretical model of gas transport and particle deposition was used to interpret data and to predict deposition for compartments and subpopulations other than adult males. Table 3-4 provides reference respiratory values for the general Caucasian population during various intensities of physical exertion. 

A variety of species of laboratory animal are employed to study aerosol deposition for both efficacy and toxicity. It is important to recognize that the breathing parameters [17], not to mention the anatomy [18], of these animals differ substantially from humans. Table 3 shows a range of breathing parameters for several species of laboratory animal. Clearly there is a matter of scale involved in that small animals cannot generate the same airflow volumes as humans and to some extent compensate by increasing their respiratory rate. 

Diffusion is the dominant mechanism of lung deposition for radon daughter aerosols. It is generally assumed that airflow is laminar in the smaller airways and that deposition in each airway generation can be calculated adequately (Chamberlain and Dyson, 1936 Ingham, 1975). However, there is no such consensus on the treatment of deposition in the upper bronchi. Some authors (Jacobi and Eisfeld, 1980 NCRP, 1984) have considered deposition to be enhanced by secondary flow, on the basis of experimental results (Martin and Jacobi, 1972). It has been shown that this assumption reduces the calculated dose from unattached radon daughters by a factor of two (James, 1985). 

Sussman, R. G., B. S. Conen and M. Lippmann, The Distribution of Airflow in Casts of Human Lungs, Presented at the 1985 Annual Meeting of the American Association for Aerosol Research, Albuquerque, NM (November, 1985). 

Oral or aerosolized bronchodilators (e.g., albuterol aerosol) may be of benefit to some patients during acute pulmonary exacerbations. For patients who consistently demonstrate limitations in airflow, a therapeutic change of bronchodilators should be considered. 

Figure 19.2 Schematic depiction of the described approaches for deposition and subsequent cell-aerosol interaction measurement. Figure 19.2a based on Ref. [84], Figure 19.2b on Ref. [86], and Figure 19.2c on Refs. [54, 89, 90], respectively. Doted lines represent airflow.

There was more. A strain gauge on each mask would indicate the airflow, and a small tube would divert a sample of the aerosol to the spectrophotometers, designed to calculate cumulative dosage in real time. Airtight Plexiglas windows would provide visual access to the volunteers. Auditory input would feed into each soldier s earpiece from a microphone in the booth. 

The AERx pulmonary delivery system [40,41] can be regarded as a combination of a MDI and a nebulizer. This system forms an aerosol by extrusion of an aqueous drug-containing solution through a disposable nozzle containing an array of precisely micromachined holes. The droplets are entrained by the airflow passing over the blister. Control over the size distribution of the holes enables the formation of droplets having a narrow size distribution. 

This test should be performed by trained or certified personnel who introduce DOP aerosol upstream of the filter through a test port and search for leaks downstream with an aerosol photometer. Remove air diffusers and airflow laminators before performing the test (whatever is applicable) to get access to the filters. 

Even when the appropriate inhaler is chosen, the influence of the disease state cannot be ignored. Disease states can influence the dimension and properties of the airways and hence the disposition of any inhaled drug. Thus, great care must be taken when extrapolating the findings based on intratracheal administration to different animal species in order to predict deposition profiles after inhalation of aerosol formulations by patients suffering from airway disease. DPIs are not appropriate in many diseases when the ability to have sufficient airflow is hindered. Since many diseases that we would like to treat via pulmonary administration of biomolecules cause a decrease in airflow, we must be careful in the decision of which type of inhalation mechanism to choose. 

The ability of currently used aircraft probes to accurately sample aerosols has been questioned. Huebert et al. (8) conducted a comparative study of several different types of aerosol probes, all mounted on the same aircraft. The results suggested that substantial losses of particles occurred in all of the inlet systems. Because of the limited nature of the study, however, the causes of the aerosol losses could not be identified. The results of the Huebert study prompted a workshop to reexamine the entire issue of aerosol sampling from aircraft (9). An important conclusion of the workshop was that currently there is insufficient knowledge to adequately describe important characteristics of airflow and particle trajectories at flight speeds of aerosol sampling probes used on aircraft. 

The difference %i Xo in concentration of a vapour or aerosol in the free stream and at a surface is the driving force for deposition. Since the ratio Q/(x 1 — Xo) has the dimensions of a velocity, it is called the velocity of deposition, denoted vg. Alternatively, on the electrical analogy, vg is the conductance and its reciprocal, r is the resistance to mass transfer. If the boundary layer of an airflow over a surface has two or more parts, for example above and below the top of the roughness elements, the resistances of these layers are additive, since... 

Notice that for typical aerosol particle sizes, the exponential terms rapidly disappear. Note also that the particle is rapidly acquiring the velocity of the horizontal airflow. 

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