Particle deposition, alveolar regional depositions

Fig. 7.4. Deposition of particles in alveolar region (open symbols) and whole respiratory tract (closed symbols) (14 or 15 breaths/min by mouth, tidal volume 1.0 to 1.51). Experimental results of Chan Lippmann, 1980 (C>), Stahlhofen etal., 1980 ( ), Foord etal., 1978 (O), Pritchard etal., 1980 (A). Error bars are Is.e. Lines are theoretical calculations of Yu Diu, 1982.

Figure 14 Modeled local deposition in the tracheobronchial and alveolar region Deposition estimates were earried out for the symmetrical morphometric model of Weibel (5) at a tidal volume of700 cm an inspiratory flow rate of500 em s , and an end-inspiratory pause of 0.5 s. The percentage deposition per airway generation is given for partieles with aerodynamie diameters ranging between 1.6 and 15.8 pm. The percentage quoted is the fraction of all aerosol particles that pass through the glottis. (From Ref. 129.)...

Fig. 7-2. Particle deposition as a function of particle diameter in various regions of the lung. The nasopharyngeal region consists of the nose and throat the tracheobronchial region consists of the windpipe and large airways and the pulmonary region consists of the small bronchi and the alveolar sacs. Source Task Group on Lung Dynamics, Health Phys. 12, 173 (1966).

The ICRP deposition model estimates the fraction of inhaled material initially retained in each compartment (see Figure 3-2). The model was developed with five compartments (1) the anterior nasal passages (ET,) (2) all other extrathoracic airways (ET2) (posterior nasal passages, the naso- and oropharynx, and the larynx) (3) the bronchi (BB) (4) the bronchioles (bb) and (5) the alveolar interstitium (AI). Particles deposited in each of the regions may be removed and redistributed either upward into the respiratory tree or to the lymphatic system and blood by different particle removal mechanisms. 

If particles reach and become deposited in the alveoli, they tend to stay imbedded in the fluid on the alveolar surface or move into the lymph nodes. The one mechanism by which particles are physically resuspended and removed from the AI region is coughing. For modeling purposes, the AI region is divided into three subcompartments to represent different clearance rates, all of which are slow. 

The size of the fibrous particles that appear to induce disease in the animal models is compatible with the measured respiratory range in humans (Lipp-man, 1977). Most particulate deposition takes place not in the upper or conducting portion of the airways but in the alveolar region of the pulmonary tree (the respiratory unit). Some surface deposition may occur at bifurcations in the bronchial tree, but the actual amount at each location is influenced by anatomy, specific to the species—probably to an individual—as well as the variety of fiber. A large proportion of airborne particulates are rejected as part of the normal clearance mechanisms in animals, but in humans clearance mechanisms may be compromised by smoking, for example. We are unaware of any experiments on fiber toxicity using smoking rats ... 

Particles with an aerodynamic diameter of 1-5 p are deposited in the airways (tracheobronchial regions) hy sedimentation under gravitational forces. As the alveolar regions are approached, the velocity of the airflow decreases significantly, allowing more time for sedimentation. The very small particles, generally less than 1 p, that penetrate to the alveoli are deposited there mainly hy diffusion. 

On the other hand, particles from fossil fuel combustion and gas-to-particle conversion are generally much smaller (< 2.5-/Am diameter) and fall in the respirable size range. These particles can reach the alveolar region where gas exchange occurs. This region is not coated with a protective mucus layer, and here the clearance time for deposited particles is much greater than in the upper respiratory tract hence the potential for health effects is much greater (Phalen, 1984). 

Particles of aerodynamic diameter greater than 5 pm deposit primarily in the upper airways or mouth and throat region, while a significant percentage of those less than 1 pm do not deposit but are exhaled (Darquenne et al. 1997). Therefore, for optimum deposition in the alveolar region and systemic delivery, particles have to be between 1 and 5 pm. 

Fig. 1 Typical example of particle size distributions at roadside (Marylebone Road, London) and urban background site (North Kensington, London), both taken in 2007. Also are shown size dependent deposition in alveolar region [102]...

If the rate of ventilation of the lungs is 0.75 m3 IT1, the rate of deposition of potential alpha energy in the alveolar region is then 0.15 Ep Jh-1. The mass of the lung in the standard man is 1 kg, and by definition 1J kg-1 is 1 Gy, so the dose rate is 0.15 Gy per Jh m-3. Curve A of Fig. 1.15, following James (1987b), shows the variation of the alveolar dose with dp. As dp decreases, the dose increases to a peak at 18 nm, and then declines because smaller particles are trapped in the upper respiratory tract and fewer reach the alveolar region. 

Gravitational sedimentation is the predominant deposition mechanism in the last five to six generations of airways (e.g., the bronchioles and alveolar region), where the air velocity is very low [23]. It allows deposition of particles into the... 

This is particle deposition resulting from settling under gravity. It becomes increasingly important for particles that reach airways where the airstream velocity is relatively low, e g the bronchioles and alveolar region. The fraction of particles depositing by this mechanism will be dependent upon the time the particles spend in these regions. 

A number of models have been developed to attempt to predict respiratory deposition, especially in the lower airways—the alveolar region. Experimental studies have tended to confirm the validity of the models, recognizing that there is much individual variation and thus a great spread in the results. The results have been clear enough, however, to indicate that till else being equal, deposition of particles in the lungs is greatly influenced by particle size and particle density. 

With mass respirable sampling, an attempt is made to separate the aerosol into two fractions representing the mass that would be deposited in the alveolar region and the mass that would not be deposited in this region. To do this, it is necessary to define the size distribution of particles deposited in the alveolar region. This material is defined as respirable dust. 

High efficiency of deposition in the alveolar region due to particle size... 

Pulmonary administration of pharmaceutioal oompounds using aerosols is a oommon olinioal practice due to its relatively easy use. It is generally accepted that aerosol particles of 1-5 pm in size are required for deposition in the alveolar region of the lung, which exhibits the highest systemic absorption however, particles less than 1 pm in diameter are more easily incorporated into the "respirable percentage" of aerosolized droplets. 

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