Respiratory tract, particulate deposition

The likelihood that materials will produce local effects in the respiratory tract depends on their physical and chemical properties, solubiHty, reactivity with fluid-lining layers of the respiratory tract, reactivity with local tissue components, and (in the case of particulates) the site of deposition. Depending on the nature of the material, and the conditions of the exposure, the types of local response produced include acute inflammation and damage, chronic... 

The ICRP (1994b, 1995) developed a Human Respiratory Tract Model for Radiological Protection, which contains respiratory tract deposition and clearance compartmental models for inhalation exposure that may be applied to particulate aerosols of americium compounds. The ICRP (1986, 1989) has a biokinetic model for human oral exposure that applies to americium. The National Council on Radiation Protection and Measurement (NCRP) has also developed a respiratory tract model for inhaled radionuclides (NCRP 1997). At this time, the NCRP recommends the use of the ICRP model for calculating exposures for radiation workers and the general public. Readers interested in this topic are referred to NCRP Report No. 125 Deposition, Retention and Dosimetry of Inhaled Radioactive Substances (NCRP 1997). In the appendix to the report, NCRP provides the animal testing clearance data and equations fitting the data that supported the development of the human mode for americium. 

During occupational exposure, respiratory absorption of soluble and insoluble nickel compounds is the major route of entry, with gastrointestinal absorption secondary (WHO 1991). Inhalation exposure studies of nickel in humans and test animals show that nickel localizes in the lungs, with much lower levels in liver and kidneys (USPHS 1993). About half the inhaled nickel is deposited on bronchial mucosa and swept upward in mucous to be swallowed about 25% of the inhaled nickel is deposited in the pulmonary parenchyma (NAS 1975). The relative amount of inhaled nickel absorbed from the pulmonary tract is dependent on the chemical and physical properties of the nickel compound (USEPA 1986). Pulmonary absorption into the blood is greatest for nickel carbonyl vapor about half the inhaled amount is absorbed (USEPA 1980). Nickel in particulate matter is absorbed from the pulmonary tract to a lesser degree than nickel carbonyl however, smaller particles are absorbed more readily than larger ones (USEPA 1980). Large nickel particles (>2 pm in diameter) are deposited in the upper respiratory tract smaller particles tend to enter the lower respiratory tract. In humans, 35% of the inhaled nickel is absorbed into the blood from the respiratory tract the remainder is either swallowed or expectorated. Soluble nickel compounds... 

The importance of tobacco includes both those constituents in smoke that may interact with nicotine directly, as well as those that indirectly influence a smoker s perception and behaviors. For example, some tobacco smoke constituents may alter the site of absorption of nicotine, such as bronchodilators (e.g., cocoa, licorice), which allow deeper inhalation and subsequent deposition of constituents in more highly permeable areas of the respiratory tract. Likewise, product changes to alter or control particle size, or to provide particulate carriers for vapor-phase smoke constituents, also could facilitate changes at the site of absorption (Ingebrethsen 1993). This would also include the use of acids or bases to alter the form of nicotine and basicity of smoke. Again, a wide range of relevant findings is indicated by internal documents (Ferris Wayne et al. 2006 Keithly et al. 2005 Pankow 2001). 

Particle deposition in the respiratory tract can initiate inflammatory responses. With repeated deposition, inflammation becomes chronic, and the site or sites of deposition beeome laden, not only with the particulates, but with several types of cells—fibroblasts, macrophages, leukocytes, and lymphocytes. These cells are normal constituents of the lung, an organ composed predominantly of connective tissue. Lung connective tissue forms the thin membrane that defines the functional alveolar-capillary unit. Inside this air sac and on the membrane are specialized eells required for gas exchange, maintenance, and repair (Fig. 3.6). 

The chief factor that determines the site of deposition of particulate matter in the respiratory tract is its size. Particles having an aerodynamic diameter of 5-30 pm are primarily... 

The need to determine accurately the phase-specific concentrations of these pollutants reflects several concerns Compared to gaseous materials, particle-phase materials may penetrate more deeply into the human respiratory tract particle-phase pollutants scatter light much more effectively than gaseous materials, and they thus have a greater contribution to visibility reduction gaseous nitric acid has a much higher deposition velocity than particulate nitrates and can be a substantial contributor to the acidification of lakes, streams, forests, and vegetation. 

Pritchard, J.N. Black, A. (1984) An estimate of the tar particulate matter depositing in the respiratory tracts of healthy male middle- and low-tar cigarette smokers. In Aerosols, ed. B.Y.H. Liu D.Y.H. Piu. Amsterdam Elsevier. 

Figure 27.15. The deposition site of particles in the lung depends on particle size. Shown are predicted deposition patterns of (a) 5-pm particles, (b) 1-pm particles, and (c) 0.1-pm particles. (Adapted from Bennett, W. D., and Brown, J. S. Particulate dosimetry in the respiratory tract. In Foster, W. M., and Costa, D. L. (Eds.). Air Pollutants and the Respiratory Tract, 2nd edition. Taylor Francis, New York, 2005, pp. 21-73. This figure was completely redrawn by the author from materials cited.)...

Volcanic ash Atmospheric particulates generated hy eruptions. Natural and anthropogenic disturbance of volcanic ash deposits, such as earthquakes, landshdes, construction activities. Inhalation. Irritation of respiratory tract asthma potential effects of crystalline silica and iron-rich particles within ash. 

The solubility of an inhaled contaminant influences the disposition of gases, vapors, and particulates. In general, those substances that are highly water soluble, such as ammonia, formaldehyde, and hydrogen chloride, will be removed by the upper respiratory tract. Formaldehyde is concentrated in the nasal mucosa and is a nasal carcinogen in the rat. Chemicals with intermediate solubility, such as halogens and ozone, deposit in both the upper respiratory tract and the lung, while chemicals with low... 

Although inhalation is probably one of the oldest modes of drug delivery, nature has designed the respiratory tract to prevent deposition of particulate matter in the lungs. The respiratory tract is an efficient anatomical barrier for most particulate mass with aerodynamic diameter above approximately 5 jam. This limit is defined by the mechanism of inertial impaction and sed-... 

Because TCDO has a very low volatility, TCDD uptake via inhalation is directly related to the concentration of airborne dust due to wind-blown soil. It has been estimated that of all airborne, respirable particulates, only about 30-50% comes from soil, while the rest is apparently due to products of combustion, tire wear and other sources (32). Of the total suspended particulates, usually no more than 50% are respirable (i.e., particles less than 10 urn). Of these, about 50% of the respirable particles are deposited in the upper airways and ultimately swallowed while the rest reach the alveoli or are expired. An analysis of CDC s data indicates that CDC assumed that 100% of the TCDD present on all the inhaled particles would be retained and absorbed in the respiratory tract. In contrast, the EPA assessment (2) assumed that only 25% of the inhaled particles would be absorbed in the lower airways since at least 50% of the particles would be non-respirable (especially by weight) and these will be swallowed due to impaction in the throat and only about 50% of the respirable particles would be absorbed. In any assessment, it is important to recognize that of those particles swallowed, no more than 10-30% should be absorbed since they will pass through the G.I. tract (assuming 10-30% oral bioavailability). 

Uncertainty of Iodine Particulate Deposition in the Respiratory Tract... 

Inhalation is a prominent pathway for radiation dose contributions from environmental or occupational intake of radioactive iodine. The ability to model and predict the kinetics of iodine in the body can be used for internal dosimetric assessments that predict the radiation dose delivered to various tissues. These assessments may be used to assess risk to the individual from inhalation intakes of radioactive iodine, which may occur through two distinct processes depending on its physical form. Iodine bound to particulates may be deposited in the respiratory tract, and iodine gas may be taken up by various tissues in the respiratory system during the breathing cycle. Inhalation models are necessary for estimates of iodine uptake and deposition that may result in radiation dose to workers or members of the public. 

---