Dust removal from the lung parenchyma: an investigation of clearance stimulants

Bioaccessibilities of metal(loid)s and organic contaminants in particulates measured in simulated human lung fluids: A critical review

Particle-bound pollutants can pose a health risk to humans. Inhalation exposure evaluated by total contaminant concentrations significantly overestimates the potential risk. To assess the risk more accurately, bioavailability, which is the fraction that enters into the systemic circulation, should be considered. Researchers have replaced bioavailability by bioaccessibility due to the rapid and cost-efficient measurement for the latter, especially for assessment by oral ingestion. However, contaminants in particulates have different behavior when inhaled than when orally ingested. Some of the contaminants are exhaled along with exhalation, and others are deposited in the lung with the particulates. In addition, a fraction of the contaminants is released into the lung fluid and absorbed by the lung, and another fraction enters systemic circulation under the action of cell phagocytosis on particulates. Even if the release fraction, i.e., release bioaccessibility, is considered, the measurement faces many challenges. The present study highlights the factors influencing release bioaccessibility and the incorporation of inhalation bioaccessibility into the risk assessment of inhaled contaminants. Currently, there are three types of extraction techniques for simulated human lung fluids, including simple chemical solutions, sequential extraction techniques, and physiologically based techniques. The last technique generally uses three kinds of solution: Gamble's solution, Hatch's solution, and artificial lysosomal fluid, which are the most widely used physiologically based simulated human lung fluids. External factors such as simulated lung fluid composition, pH, extraction time, and sorption sinks can affect release bioaccessibility, whereas particle size and contaminant properties are important internal factors. Overall, release bioaccessibility is less used than bioaccessibility considering the deposition fraction when assessing the risk of contaminants in inhaled particulates. The release bioaccessibility measurement poses two main challenges: developing a unified, accurate, stable, simple, and systematic biologically based method, and validating the method through in-vivo assays.

ICRP Publication 141: Occupational Intakes of Radionuclides: Part 4

The 2007 Recommendations (ICRP, 2007) introduced changes that affect the calculation of effective dose, and implied a revision of the dose coefficients for internal exposure, published previously in the Publication 30 series (ICRP, 1979a,b, 1980a, 1981, 1988) and Publication 68 (ICRP, 1994b). In addition, new data are now available that support an update of the radionuclide-specific information given in Publications 54 and 78 (ICRP, 1989a, 1997) for the design of monitoring programmes and retrospective assessment of occupational internal doses. Provision of new biokinetic models, dose coefficients, monitoring methods, and bioassay data was performed by Committee 2 and its task groups. A new series, the Occupational Intakes of Radionuclides (OIR) series, will replace the Publication 30 series and Publications 54, 68, and 78. OIR Part 1 (ICRP, 2015) describes the assessment of internal occupational exposure to radionuclides, biokinetic and dosimetric models, methods of individual and workplace monitoring, and general aspects of retrospective dose assessment. OIR Part 2 (ICRP, 2016), OIR Part 3 (ICRP, 2017), this current publication, and the final publication in the OIR series (OIR Part 5) provide data on individual elements and their radioisotopes, including information on chemical forms encountered in the workplace; a list of principal radioisotopes and their physical half-lives and decay modes; the parameter values of the reference biokinetic models; and data on monitoring techniques for the radioisotopes most commonly encountered in workplaces. Reviews of data on inhalation, ingestion, and systemic biokinetics are also provided for most of the elements. Dosimetric data provided in the printed publications of the OIR series include tables of committed effective dose per intake (Sv per Bq intake) for inhalation and ingestion, tables of committed effective dose per content (Sv per Bq measurement) for inhalation, and graphs of retention and excretion data per Bq intake for inhalation. These data are provided for all absorption types and for the most common isotope(s) of each element. The online electronic files that accompany the OIR series of publications contains a comprehensive set of committed effective and equivalent dose coefficients, committed effective dose per content functions, and reference bioassay functions. Data are provided for inhalation, ingestion, and direct input to blood. This fourth publication in the OIR series provides the above data for the following elements: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), actinium (Ac), protactinium (Pa), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (Cf), einsteinium (Es), and fermium (Fm).

A critical review of approaches and limitations of inhalation bioavailability and bioaccessibility of metal(loid)s from ambient particulate matter or dust

Inhalation of metal(loid)s in ambient particulate matter (APM) represents a significant exposure pathway to humans. Although exposure assessment associated with this pathway is currently based on total metal(loid) content, a bioavailability (i.e. absorption in the systemic circulation) and/or bioaccessibility (i.e. solubility in simulated lung fluid) based approach may more accurately quantify exposure. Metal(loid) bioavailability-bioaccessibility assessment from APM is inherently complex and lacks consensus. This paper reviews the discrepancies that impede the adoption of a universal protocol for the assessment of inhalation bioaccessibility. Exposure assessment approaches for in-vivo bioavailability, in-vitro cell culture and in-vitro bioaccessibility (composition of simulated lungs fluid, physico-chemical and methodological considerations) are critiqued in the context of inhalation exposure refinement. An important limitation of bioavailability and bioaccessibility studies is the use of considerably higher than environmental metal(loid) concentration, which diminishing their relevance to human exposure scenarios. Similarly, individual metal(loid) studies have been criticised due to complexities of APM metal(loid) mixtures which may impart synergistic or antagonistic effects compared to single metal(loid) exposure. Although a number of different simulated lung fluid (SLF) compositions have been used in metal(loid) bioaccessibility studies, information regarding the comparative leaching efficiency among these different SLF and comparisons to in-vivo bioavailability data is lacking. In addition, the particle size utilised is often not representative of what is deposited in the lungs while assay parameters (extraction time, solid to liquid ratio, temperature and agitation) are often not biologically relevant. Research needs are identified in order to develop robust in-vitro bioaccessibility protocols for the assessment or prediction of metal(loid) bioavailability in APM for the refinement of inhalation exposure.

The Setting of Particulate Exposure Levels for Chronic Inhalation Toxicity Studies

This paper briefly reviews the development of regulatory agency guidelines for toxicity testing and the impact of the maximum tolerated dose (MTD) concept on chronic study designs within and outside of the regulatory arena. From the orientation of an inhalation toxicologist, the MTD is viewed as a difficult and costly concept to implement and one that is generally inappropriate as a general basis for setting dust exposure levels in sub-chronic and chronic inhalation toxicity studies. Two possible alternatives are discussed, especially in the context of accumulating evidence that most dusts, even the most innocuous, when presented in excessive amounts produce a spectrum of pulmonary responses that cannot be dissociated from injury.

The biokinetics of inorganic cobalt in the human body

This paper reviews information on the biological behavior of inorganic cobalt in humans and laboratory animals and proposes a model of the systemic biokinetics of inorganic cobalt in adult humans. The model was developed as part of an effort to update the models of the International Commission on Radiological Protection (ICRP) for addressing intakes of radionuclides by workers but is also applicable to environmental or medical exposures to inorganic forms of radiocobalt. The model can be used in conjunction with any respiratory, gastrointestinal, or wound model that provides predictions of the time-dependent feed of cobalt to blood. In contrast to the ICRP's current systemic model for cobalt, which is a simple open catenary system, the proposed model is constructed within a physiologically realistic framework that depicts recycling of cobalt between blood and tissues and transfer from blood to excretion pathways. Compared with the ICRP's current model, the proposed model yields similar predictions of whole-body retention but substantially different predictions of the systemic distribution of cobalt as a function of time after uptake to blood.

Dust overloading of the lungs: update and appraisal

This article reviews recent studies which involve, or impact on, the condition of dust overloading in the lungs of several species, especially the Fischer 344 rat. Its main purpose is to provide an update of the overload concept and new information of possible mechanistic relevance. At present, the most likely general explanation for the suppression of particle transport by the alveolar macrophage (AM) and the development of concurrent events, e.g., increased interstitial dust uptake and prolonged inflammatory response, is the persistent, possibly excessive, elaboration of chemotactic and chemokinetic factors by the AM. The induction of these interrelated events is hypothesized as related to the volume of dust phagocytized by the AM pool. The review concludes, inter alia, that information is badly needed on dust overload in nonrodent species and on the normal role of the AM in dust removal from the human lungs.

In vitro dissolution of uniform cobalt oxide particles by human and canine alveolar macrophages

Intracellular dissolution of inhaled particles is an important pathway of clearance of potentially toxic materials. To study this process, monolayers of human and canine alveolar macrophages (AM) were maintained alive and functional in vitro for more than 2 wk. Complete phagocytosis of moderately soluble, monodisperse 57Co3O4 test particles of four different sizes was obtained by optimizing the cell density of the monolayer and the particle-to-cell ratio. The fraction of the initial particle mass that was soluble increased over time when the particles were ingested by AM but remained constant when in culture medium alone. Smaller particle sizes had a faster characteristic intracellular dissolution rate constant than did larger particles. The dissolution rates differed between AM obtained from two human volunteers as compared to those obtained from six mongrel dogs. These in vitro dissolution rates were very similar to in vivo translocation rates previously obtained from human and canine lung clearance studies after inhalation of the same or similar monodisperse, homogeneous 57Co3O4 test particles. We believe an important clearance mechanism for inhaled aerosol particles deposited in the lungs can be simulated in vitro in a cell culture system.

Feasibility of dose adjustment based on differences in long-term clearance rates of inhaled particulate matter in humans and laboratory animals

Long-term pulmonary clearance rates were evaluated for several laboratory animal species, dogs, and humans to determine if differences among species exist, and if so, the adequacy of the data for dose adjustment. Within each species, large variations in clearance rates were seen, probably as a result of differences in solubility of the aerosol particles, differences in measurement techniques, possible lung damage, transport to lung-associated lymph nodes, and binding of dissolved chemicals to cellular macromolecules in the lung. While few direct comparisons among species using the same aerosol were available, mechanical clearance of particles from the alveolar regions of dogs and humans was generally slower than in most laboratory species, with t1/2 values several-fold longer. Particle clearance rate variations of this magnitude were shown to induce potentially large differences in bioavailability. This can result in large errors in assessing human risk from animal studies unless a dose adjustment is made. It is suggested that despite limitations on available data, a two- to threefold adjustment of dose when extrapolating from small laboratory animals to humans, for quantitative risk assessment, should be considered, unless solubility half-times are very short.

Characteristics of alveolar macrophages following the deposition of a low burden or iron oxide in the lung

Rats were exposed to aerosols of iron-59 oxide (mass median aerodynamic diameter, MMAD = 1.6 micron, sigma g = 3.0) at a nominal concentration of 20 mg/m3 for 2 h to determine how a low lung burden (approximately 30 micrograms) of innocuous particles affects the size of the alveolar macrophage (AM) pool, and the functional status of the AM as assessed in vitro by their ability to exclude Trypan blue, adhere to plastic substrate, and bind and phagocytize sheep erythrocytes opsonized with immunoglobulin G (SRBC-IgG). Iron oxide deposition did not bring about significant changes in cell types or numbers of AM lavaged, AM viabilities, or the plastic substrate adherence characteristics of the AM. As of 1 d post exposure, however, the ability of AM to phagocytize SRBC-IgG increased. Phagocytosis was maximally enhanced 3-7 d post exposure and returned to control levels by 20 d after exposure. The increase in phagocytic activity correlated with an increase in AM avidities for SRBC-IgG. The kinetics of subsidence of the phagocytic response did not parallel the alveolar clearance rate of the deposited particles [t1/2 (biol)/53 d]. These studies show the deposition of a low lung burden of a noncytotoxic dust can transiently enhance Fc gamma-receptor-mediated particle binding and phagocytosis by AM.

Association of 59iron oxide with alveolar macrophages during alveolar clearance

Long-Evans hooded rats were exposed for 2 h to aerosols of hydrated, radiolabeled iron (59Fe) oxide (MMAD = 1.6 micron; sigma g = 3.0) in order to produce a low mass burden of particles (approximately equal to 30 micrograms) in the lung. The kinetics of particle clearance and the association of the particles with alveolar macrophages (AM) were measured. Two to four hours after exposure, lavaged particles were linearly related to AM numbers harvested, and 60% of the 59Fe activity was physically associated with AM. By 24 h, greater than 90% of the lavaged particles were associated with AM. Such an association was found for at least 75% of the particulate burdens in the lungs. If all the 59Fe is assumed to be AM associated, the 59Fe per AM predicts the total AM population size to be 2.14 X 10(7) cells. This number, in conjunction with the alveolar clearance rate of the particles, suggested the number of AM leaving the lung daily was 2.8 X 10(5) cells.

Dissolution rates of uranium compounds in simulated lung fluid

Maximum dissolution rates of uranium into simulated lung fluid were measured at 37 degrees C to estimate clearance rates from the deep lung. The materials tested included: ore and yellowcake, an airborne sample from an industrial site, and purified samples of (NH4)2U2O7, U3O8, UO2 and UF4. A batch procedure was developed to test samples containing as little as 10 micrograms of natural uranium. Values of dissolution halftimes varied from 0.01 day to several thousand days depending on the physical and chemical form of the uranium. Dissolution occurred predominantly by formation of the #UO2(CO3)3 ]4-ion; and as a result, tetravalent uranium compounds dissolved slowly. Dissolution rates of size-separated yellowcake aerosols were found to be more closely correlated with specific surface area than with aerodynamic diameter.

Alveolar macrophage mediated pulmonary clearance suppressed by drug-induced phospholipidosis

Drug-induced phospholipidosis in rats treated with chlorphentermine (CP) for 4-7 days suppressed totally alveolar pulmonary clearance in the first days after a TiO2 aerosol exposure. Reversing phospholipidosis by treatment interruption led to a recovery of particle clearance. Morphological observations indicated that "foam cells" contained less TiO2 particles than alveolar macrophages (AM) of control rats. Clearance of ZnO particles which seems not to be mediated by AM was not affected by CP treatment. A grand average retention curve based on data from control groups of past experiments suggests that alveolar clearance of TiO2 particles has a phase 1 (T 1/2 = 7 days) lasting about 2 weeks and a phase 2 (T 1/2 = 69 days). The results with drug-induced phospholipidosis suggest that phase 1 is practically totally AM-mediated. Drug-induced phospholipidosis is a promising method for the study of AM involvement in defensive functions.

Deposition, retention, and clearance of inhaled particles

The relation between the concentrations and characteristics of air contaminants in the work place and the resultant toxic doses and potential hazards after their inhalation depends greatly on their patterns of deposition and the rates and pathways for their clearance from the deposition sites. The distribution of the deposition sites of inhaled particles is strongly dependent on their aerodynamic diameters. For normal man, inhaled non-hygroscopic particles greater than or equal to 2 micrometers that deposit in the conducting airways by impaction are concentrated on to a small fraction of the surface. Cigarette smoking and bronchitis produce a proximal shift in the deposition pattern. The major factor affecting the deposition of smaller particles is their transfer from tidal to reserve air. For particles soluble in respiratory tract fluid, systemic uptake may be relatively complete for all deposition patterns, and there may be local toxic or irritant effects or both. On the other hand, slowly soluble particles depositing in the conducting airways are carried on the surface to the glottis and are swallowed within one day. Mucociliary transport rates are highly variable, both along the ciliated airways of a given individual and between individuals. The changes in clearance rates produced by drugs, cigarette smoke, and other environmental pollutants can greatly increase or decrease these rates. Particles deposited in non-ciliated airways have large surface-to-volume ratios, and clearance by dissolution can occur for materials generally considered insoluble. They may also be cleared as free particles either by passive transport along surface liquids or, after phagocytosis, by transport within alveolar macrophages. If the particles penetrate the epithelium, either bare or within macrophages, they may be sequestered within cells or enter the lymphatic circulation and be carried to pleural, hilar, and more distant lymph nodes. Non-toxic insoluble particles are cleared from the alveolar region in a series of temporal phases. The earliest, lasting several weeks, appears to include the clearance of phagocytosed particles via the bronchial tree. The terminal phases appear to be related to solubility at interstitial sites. While the mechanisms and dynamics of particle deposition and clearance are reasonably well established in broad outline, reliable quantitative data are lacking in many specific areas. More information is needed on: (1) normal behaviour, (2) the extent of the reserve capacity of the system to cope with occupational exposures, and (3) the role of compensatory changes in airway sizes and in secretory and transport rates in providing protection against occupational exposures, and in relation to the development and progression of dysfunction and disease.

Pulmonary retention of neutron-activated coal dust

Coal dust aerosols with cesium-134 and scandium-46 labels were studied in dogs and rats following brief inhalation exposures by external measurement of gamma photons in the 0.6 to 0.8 and 0.9 to 1.1 MeV regions, respectively. Ancillary in vitro studies of the leaching characteristics of the two radionuclides from coal were made and control studies utilizing the "free" radionuclides were undertaken for each of the investigations with radioactive coal dust. The biological data strongly infer that coal dust retention in canine lungs is extremely protracted with a biological half-life no shorter than approximately 4.3 yr and probably much longer. The biological model which was formulated and analyzed to obtain this finding is discussed along with its limitations.

Studies of the lymphatic drainage of dog lungs

The lymphatic system of the lungs has proved difficult to study and characterize because it is technically complex to investigate, its properties are difficult to quantitate, and anatomically it is extraordinarily variable. This study was part of a more comprehensive effort to understand the nature of lymphatic permeation by extrinsic materials (e.g., dusts) arising from alveolar deposition, and had three main objectives; (1) to develop a reliable surgical approach for the collection of lymph from the right duct; (2) to investigate some of the inconsistencies in lymphatic structure and function, especially the relationship of the right lymph duct (RLD) and thoracic lymph duct (TLD) outflows to the pulmonic lymph; and (3) to begin a systematic investigation of lymphatic uptake of administered materials by varying their physicochemical parameters. Ultimately, we utilized a modification of the surgical approach of Meyer, which we believe is less prone to blood contamination than the venous-sac procedure of Leeds and Uhley and provides purer pulmonic lymph. By this means we obtained average RLD flow rates of 4.5 ml/h or 0.35 ml/h . kg body weight in 24 dogs, which are comparable to those in the recent literature. For demarcation of the pulmonic drainage in relation to the RLD and TLD, we found in 13 dogs that 75% or more of the lung lymph returned to the venous circulation through the RLD, wheras less than 3% of the thoracic lymph entered the RLD. Radioactive tracers were administered by intralymphatic, intrabronchial, inhalation, and intravenous routes to obtain these findings and the uptake data. Lymphatic uptake values for iron, cadmium, and lead were obtained principally after intrabronchial administration. The uptake data, while preliminary, indicate that both the chemical species and their physical states are important in affecting alveolar permeation into the pulmonic lymph. Evidence for varying lymphatic roles in the alveolar retention of these heavy metals is also presented.

The fate of ( 14 C)disodium cromoglycate in man

The fate of [14C]disodium cromoglycate (DSCG) has been examined in 12 asthmatic patients. Maximum plasma concentrations (mean 9.2 ng/ml) were obtained within 15 min of inhaling DSCG (20 mg) and the average plasma half-life was 81 min. Although absorption from the lung is rapid, most of the inhaled dose is swallowed. Only 2.0% of the dose was excreted in the urine, and 84% was recovered from the faeces. DSCG is poorly absorbed from the gastrointestinal tract, only 0.4 % of an oral dose (20 mg) appeared in the 24 h urine and 83 % was recovered from the faeces. Intravenous administration of DSCG resulted in approximately equal amounts (30–50% of dose) being excreted via the urine and faeces. No metabolites of DSCG were detected chromatographically.