J. Ferin

Correlation between Particle Size, In Vivo Particle Persistence, and Lung Injury

Dosimetry parameters such as deposition, clearance, retention, and translocation and dissolution of inhaled particles in and to different lung compartments may be important for the persistence of particles in the lung and may correlate with adverse pulmonary effects. We investigated such correlations using a model involving TiO2 particles of two particle sizes (20 nm diameter, ultrafine; 250 nm diameter, fine) of the same crystalline structure (anatase). A 12-week inhalation experiment in rats resulted in a similar mass deposition of the two particle types in the lower respiratory tract. The ultrafine particles elicited a persistently high inflammatory reaction in the lungs of the animals compared to the larger-sized particles. In the postexposure period (up to 1 year) retention in the alveolar space per se was not different between fine and ultrafine TiO2. However, the following differences between the particle types were noted: a significantly different total pulmonary retention, both quantitatively (significantly prolonged retention of the ultrafine TiO2) and qualitatively (increased translocation to the pulmonary interstitium and persistence there of the ultrafine TiO2); greater epithelial effects (Type II cell proliferation; occlusion of pores of Kohn) and the beginning of interstitial fibrotic foci with ultrafine TiO2; significant sustained impairment of alveolar macrophage function after ultrafine TiO2 exposure as measured by the clearance of test particles. A correlation between particle surface area and effects was observed. A comparison of the adverse reactions with dosimetric parameters of TiO2 in different lung compartments in the postexposure period showed a correlation of the persistence of effects in both the alveolar and interstitial space with the persistence of particles in the respective compartment.

Association of particulate air pollution and acute mortality: involvement of ultrafine particles?

Recent epidemiological studies show an association between particulate air pollution and acute mortality and morbidity down to ambient particle concentrations below 100 micrograms/m3. Whether this association also implies a causality between acute health effects and particle exposure at these low levels is unclear at this time; no mechanism is known that would explain such dramatic effects of low ambient particle concentrations. Based on results of our past and most recent inhalation studies with ultrafine particles in rats, we propose that such particles, that is, particles below approximately 50 nm in diameter, may contribute to the observed increased mortality and morbidity In the past we demonstrated that inhalation of highly insoluble particles of low intrinsic toxicity, such as TiO2, results in significantly increased pulmonary inflammatory responses when their size is in the ultrafine particle range, approximately 20 nm in diameter. However, these effects were not of an acute nature and occurred only after prolonged inhalation exposure of the aggregated ultrafine particles at concentrations in the milligrams per cubic meter range. In contrast, in the course of our most recent studies with thermodegradation products of polytetrafluoroethylene (PTFE) we found that freshly generated PTFE fumes containing singlet ultrafine particles (median diameter 26 nm) were highly toxic to rats at inhaled concentrations of 0.7-1.0 x 10(6) particles/cm3, resulting in acute hemorrhagic pulmonary inflammation and death after 10-30 min of exposure. We also found that work performance of the rats in a running wheel was severely affected by PTFE fume exposure. These results confirm reports from other laboratories of the highly toxic nature of PTFE fumes, which cannot be attributed to gas-phase components of these fumes such as HF, carbonylfluoride, or perfluoroisobutylene, or to reactive radicals. The calculated mass concentration of the inhaled ultrafine PTFE particles in our studies was less than 60 micrograms/m3, a very low value to cause mortality in healthy rats. Aging of the fumes with concomitant aggregation of the ultrafine particles significantly decreases their toxicity. Since ultrafine particles are always present in the urban atmosphere, we suggest that they play a role in causing acute lung injury in sensitive parts of the population.

Increased pulmonary toxicity of ultrafine particles? I. Particle clearance, translocation, morphology

The purpose of our studies is to elucidate the basic mechanism of lung tissue injury which may be common for particles of high or low toxicity. In experiments on rats we compared particle translocation of two types of TiO 2 and of two types of Al 2 O 3 . The types of TiO 2 or Al 2 O 3 differ in origin, manufacturing technology and most importantly in the size of the primary particles, but not in chemical or crystallographic characteristics

Increased pulmonary toxicity of ultrafine particles? II. Lung lavage studies

We determined the acute and late inflammatory reaction in the lung after instillation of equal amounts of two different dusts, commonly labelled as «nuisance» dusts, but each with two distinctly different particle sizes in the 15-50 nm and 0.2-0.5 μm range, TiO 2 and Al 2 O 3

Regression of Pulmonary Lesions Produced by Inhaled Titanium Dioxide in Rats

Inhaled ultrafine particles of TiO2 (TiO2-D, 20 nm particle size) lead to a greater pulmonary inflammatory response than larger pigment-grade particles (TiO2-F, 250 nm). Male Fisher 344 rats were exposed for 6 hours a day, 5 days a week, for 3 months to 1) filtered air (control); 2) TiO2-F, 22.3 mg/m3; 3) TiO2-D, 23.5 mg/m3; or 4) crystalline SiO2, a positive control particle (∼ 800 nm particle size, 1.3 mg/m3). Groups of 3-4 animals were sacrificed at 6 and 12 months following the completion of exposure. Pulmonary effects of exposure were evaluated using standard hematoxylin and eosin–stain sections, histochemical stains for collagen, and immunohistochemical assays for cell turnover. Six months after animals were exposed to SiO2, they had moderate focal interstitial fibrosis and moderately severe focal alveolitis. Animals exposed to TiO2-D had slightly less fibrosis. The least fibrosis was seen in the TiO2-F group. At 1 year after exposure, fibrosis was still present but decreased in the SiO2 group. The amount of interstitial fibrosis in the TiO2-D– and TiO2-F–treated animals had largely returned to untreated control levels, although an increased number of alveolar macrophages persisted, usually with retained particles. There was discordance between bromodeoxyuridine and proliferating cell nuclear antigen indices, most probably due to cytokine elaboration in the areas of inflammation, which may have altered the expression of proliferating cell nuclear antigens. There was no detectable fibroblast labeling at the 6-month observation and only very low levels at 12 months. Thus, although initially irritant, TiO2-induced lesions regressed during a 1-year period following cessation of exposure.

Volumetric loading of alveolar macrophages (AM): a possible basis for diminished AM-mediated particle clearance.

Using intratracheal instillation of radioactively labeled plastic microspheres of 3.3 and 10.3 microns diameter at two dose levels, this 7-month study in male Fischer 344 rats was designed to test a volumetric particulate burden hypothesis that has been proposed as a mechanistic basis for the condition of dust overloading of the lungs with highly insoluble particles of very low toxicity and to explain the prolongation of pulmonary particle retention. The study utilized airway and deep lung lavage techniques, scanning electron and optical microscopy of lavaged cells and lungs of sacrificed animals, particle distribution in alveolar macrophages (AM), fecal recovery of radioactive particles, and lung retention measurements by external counting. Microscopic assessments revealed that essentially all of the 3.3- and 10.3-microns-diameter particles were phagocytized by AM within 24 h postinstillation. One phagocytized 10.3-microns particle is capable of producing the hypothesized 600-microns 3/AM overload criterion for virtual AM immobilization. Neither the number nor the volume of 3.3-microns-diameter particles instilled was large enough to produce volumetric overloading assuming uniform distribution of the particles in the lung. In contrast to the 3.3-microns particles, the 10.3-microns particles were apparently sequestered to a greater extent and capable of greatly prolonging AM-mediated clearance of particles from the pulmonary region. The measured pulmonary retention half-times for the small and large particles were 86-109 days and 870-1020 days, respectively. Fecal recovery data closely complemented pulmonary clearance data for both particle sizes. The two-particle approach was found supportive of the volumetric overload hypothesis.

Pulmonary retention and clearance of particles

Research in pulmonary retention and clearance of particles intensified in the fifties in connection with interests in pneumoconiosis and in inhalation of radioactive particles, and more recently with the increased interest in the effects of environmental particles. The studies enhanced our understanding of clearance mechanisms, the various clearance pathways, the different clearance capacities of various species and the importance of other factors affecting lung clearance. Based on research in recent years, the historical concept of inert and fibrogenic particles was abandoned. It seems that particles even at surprisingly low concentrations may have negative health effects and that ultrafine particles have higher than expected toxicity when compared to similar particles of a larger size.

Modification of rat operant behavior by ozone exposure

Abstract Ozone exposure may induce extrapulmonary consequences, such as reductions in the spontaneous activity of rodents, or complaints in humans of lethargy, headache, chest discomfort, and other subjective symptoms. Further evidence of behavioral disruption was provided by this experiment. Twelve rats were trained to perform a bar-pressing response maintained by food reward. Food pellets were delivered according to a fixed interval 5-min reinforcement schedule. Exposures encompassed a range of concentrations from 0.1 to 2.0 ppm. A single exposure lasted 6 hr, and successive exposures were separated by at least 6 days. Compared to performance under control conditions (ambient air), response rates fell linearly from 0.1 to 1.4 ppm. During individual 6-hr exposures, the rates declined from the beginning to the end of the session. These results, given the sedentary nature of the task, reflect reduced inclination rather than impaired physiological capacity to respond.

The Effect of SO2 on Lung Clearance of TiO2 Particles in Rats

By use of the nontoxic-particle-challenge system, the integrated alveolar clearance was measured in rats by serially sacrificing them after exposure to TiO2 aerosol and determining the amount of TiO2 retained in the lung. Prior to exposure to TiO2, groups of animals were exposed to 0.1, 1 or 20 ppm of SO2 for 7 hours/day, 5 days/week for total exposure of 70 to 170 hours. The results indicate that SO2 affects the clearance of “inert” particles. The lowest exposures showed slight stimulation or no effect on clearance whereas a depressing of clearance was observed after 170 hours of exposure at 1 ppm. Short-term exposure at higher concentrations appear to be tolerated better than longer exposures at low concentrations.

OBSERVATIONS CONCERNING ALVEOLAR DUST CLEARANCE

Before beginning my observations on alveolar dust clearance, some of the problems inherent in studies of this subject should be mentioned. In their excellent book, Pulmonary Deposition and Retention of Inhaled Aerosols, Hatch and Gross state, “Owing to the wide difference in experimental results reported by the various investigators, it is not possible to draw firm conclusions with respect to pulmonary clearance characteristics.” In 1970 Kilburn commented further in his paper “Alveolar Microenvironment”: 2 “Because no one has even seen or placed a tracer into a living alveolus this paper must be deductive, hypothetical, and speculative.” After this conciliatory introduction, let me mention a further weak point. In reviewing basic attributes of alveolar clearance, one has to rely on data which very often lead only indirectly to the description of the processes under discussion. One example is the technique used in the quantitative determination of lung clearance. The amount of particles cleared in a certain period of time after exposure is estimated indirectly, by subtracting the amount retained in the lung after a given period from the amount deposited during exposure. Although a number of questions remain to be answered, the mechanism of particle clearance from the respiratory system has been broken down into at least two basic components. The first, called rapid or early, is connected mostly with clearance of particles deposited from the inhaled air at the respiratory airways. The second, called slow or long-term, is associated with all the remaining possibilities, e.g., alveolar and lymphatic clearance. The terms used to describe the components are related to clearance velocity and time, the anatomy of the lung, or the mechanisms involved. The Task Group on Lung Dynamics 3 proposed a dust clearance model that enumerated ten different absorption and translocation processes associated with the clearance of three lung compartments. The model defines two clearance phases from the pulmonary region-the compartment consisting of the respiratory bronchioles and all the structures connected with the alveoli-phases that depend on macrophages coupled to the ciliary-mucous transport process. It is to this part of the lung clearance process that the present paper is addressed.