INHALED AEROSOL DOSIMETRY: SOME CURRENT RESEARCH NEEDS.

After the presentation of 60 papers at the conference "Advancing Aerosol Dosimetry Research" (October 24-25, 2014 in Irvine, CA, USA), attendees submitted written descriptions of needed research. About 40 research needs were submitted. The suggestions fell into six broad categories: 1) Access to detailed anatomic data; 2) Access to subject-specific aerosol deposition datasets; 3) Improving current inhaled aerosol deposition models; 4) Some current experimental data needs and hot topics; 5) Linking exposure and deposition modeling to health endpoints; and 6) Developing guidelines for appropriate validation of dosimetry and risk assessment models. Summaries of suggestions are provided here as an update on research needs related to inhaled aerosol dosimetry modeling. Taken together, the recommendations support the overarching need for increased collaborations between dose modelers and those that use the models for risk assessments, aerosol medicine applications, design of toxicology experiments, and extrapolation across species. This paper is only a snapshot in time of perceived research needs from the conference attendees; it does not carry the approval of any agency or other group that plans research priorities or that funds research.

Inhaled aerosol particle dosimetry in mice: a review.

The availability of molecular and genetic tools has made the mouse the most common animal model for a variety of human diseases in toxicology studies. However, little is known about the factors that will influence the dose delivery to murine lungs during an inhalation study. Among these factors are the respiratory tract anatomy, lung physiology, and clearance characteristics. Therefore, the objective of this paper is to briefly review the current knowledge on the aforementioned factors in mice and their implications to the dose delivered to mouse models during inhalation studies. Representative scientific publications were chosen from searches using the NCBI PubMed and ISI Web of Knowledge databases. Relevant respiratory physiological differences have been widely reported for different mouse strains and sexes. The limited data on anatomical morphometry that is available for the murine respiratory tract indicates significant differences between mouse strains. These differences have implications to the dose delivered and the biological outcomes of inhalation studies.

New developments in aerosol dosimetry.

Dosimetry provides information linking environmental exposures to sites of deposition, removal from these sites, and translocation of deposited materials. Dosimetry also aids in extrapolating laboratory animal and in vitro data to humans. Recent progress has shed light on: properties of particles in relation to their fates in the body; influence of age, gender, body size, and lung diseases on inhaled particle doses; particle movement to the brain via the olfactory nerves; and particle deposition hot spots in the respiratory tract. Ultrafine size has emerged as an important dosimetric characteristic. Particle count, composition, and surface properties are recognized as potentially important toxicology-related considerations. Differences in body size influence airway sizes, inhaled particle deposition, specific ventilation, and specific doses (e.g. per unit body mass). Related to body size, age, gender, species, and strain are also dosimetric considerations. Diseases, such as chronic obstructive pulmonary disease (COPD) and bronchitis, produce uneven doses within the respiratory tract. Traditional concepts of the translocation and clearance of deposited particles have been challenged. Ultrafine particles can translocate to the brain via olfactory nerves, and from the lung to other organs. The clearance rates of particles from tracheobronchial airways are slowed by respiratory tract infections, but newer evidence implies that slow particle clearance from this region also exists in healthy lungs. Finally, hot spots of particle deposition are seen in hollow models, lung tissue, and dosimetric simulations. Local doses to groups of epithelial cells can be much greater than those to surrounding cells. The new insights challenge dosimetry scientists.

Inhaled aerosol particle dosimetry in mice: a review.

The availability of molecular and genetic tools has made the mouse the most common animal model for a variety of human diseases in toxicology studies. However, little is known about the factors that will influence the dose delivery to murine lungs during an inhalation study. Among these factors are the respiratory tract anatomy, lung physiology, and clearance characteristics. Therefore, the objective of this paper is to briefly review the current knowledge on the aforementioned factors in mice and their implications to the dose delivered to mouse models during inhalation studies. Representative scientific publications were chosen from searches using the NCBI PubMed and ISI Web of Knowledge databases. Relevant respiratory physiological differences have been widely reported for different mouse strains and sexes. The limited data on anatomical morphometry that is available for the murine respiratory tract indicates significant differences between mouse strains. These differences have implications to the dose delivered and the biological outcomes of inhalation studies.

Commentary on ''Toxicity testing in the 21st century: a vision and a strategy''.

Toxicity Testing in the 21st Century: A Vision and a Strategy (NRC, 2007) presents a bold plan for chemical toxicity testing that replaces whole-animal tests with cell-culture, genetic, other in-vitro techniques, computational methods, and human monitoring. Although the proposed vision is eloquently described, and recent advances in in-vitro and in-silico methods are impressive, it is difficult believe that replacing in-vitro testing is either practical or wise. It is not clear that the toxicity-related events that occur in whole animals can be adequately replicated using the proposed methods. Protecting public health is a serious endeavor that should not be limited by denying animal testing. Toxicologists and regulators are encouraged to read the report, carefully consider its implications, and share their thoughts. The vision is for too important to ignore.

Dosimetry considerations for animal aerosol inhalation studies.

The determination of the dose of inhaled aerosol particles in animal subjects is not a trivial exercise. In its simplest form, the dose is the amount (particle number, mass or other relevant metric) that deposits in the respiratory tract. The amount deposited will depend on the aerosol particle sizes (e.g. the aerodynamic diameter size distribution), the duration of exposure, the exposure system's delivery efficiency, the subject's ventilation rate, the species and strain, and other factors. Similarly, species differences in the clearance rates of deposited particles will influence the time integrated particle doses. In practice, particle doses are estimated using mathematical models, previous experimental dosimetry data, tracers of the inhaled particles and biomarkers of exposure. With care, desired aerosol doses can be achieved and documented.

The relevance of animal models for aerosol studies.

Animal models are essential for understanding the fates and effects of inhaled materials, because invasive methods are frequently necessary to provide the desired information. Because of the variability in humans of particle deposition, clearance, and effects, numerous animal models have been used in inhalation studies. Furthermore, humans are not typical mammals in some ways that affect inhalation phenomena. Humans have less fur, longer gestation and life times, simplified nasal structure, and symmetric bronchial branching in relation to other mammals. However, experience, plus the genetic similarity among mammals, underpins the use of animal models. Mammals are varied with respect to their inhaled particle deposition and clearance phenomena. Total inhaled aerosol deposition probability versus particle-size curves are qualitatively similar for various mammals of similar body mass, despite airway anatomy differences. However, more species variation is seen in regional particle deposition curves, complicating aerosol study design. The rates of clearance of deposited slowly dissolving particles are animal species dependent, apparently due to differences in gross, subgross, and cellular respiratory tract biology. Clearance rates for rapidly dissolving particles are not strongly species dependent. Inhalation toxicology studies require several animal species. Rodents are among the most frequently used, but for studies of lung development, diseases, exercise, etc., and for extrapolation to humans, larger mammals are also needed. Fortunately, the research database, and excellent monographs on inhalation phenomena provide ample guidance for study design.

Biological stress response terminology: Integrating the concepts of adaptive response and preconditioning stress within a hormetic dose-response framework.

Many biological subdisciplines that regularly assess dose-response relationships have identified an evolutionarily conserved process in which a low dose of a stressful stimulus activates an adaptive response that increases the resistance of the cell or organism to a moderate to severe level of stress. Due to a lack of frequent interaction among scientists in these many areas, there has emerged a broad range of terms that describe such dose-response relationships. This situation has become problematic because the different terms describe a family of similar biological responses (e.g., adaptive response, preconditioning, hormesis), adversely affecting interdisciplinary communication, and possibly even obscuring generalizable features and central biological concepts. With support from scientists in a broad range of disciplines, this article offers a set of recommendations we believe can achieve greater conceptual harmony in dose-response terminology, as well as better understanding and communication across the broad spectrum of biological disciplines.

Aerosol dosimetry considerations.

The concept of dose is fundamental to the discipline of toxicology. For inhaled particles, dose considerations include the sequential processes of inhalation, particle deposition, and particle clearance. Several important parameters modify each of these processes, including environmental, anatomic, and physiologic factors. When such factors are considered, it is possible to identify subpopulations and individuals who are likely to receive particle doses that greatly exceed those for the average population. Higher than average doses can be expected for people who are young, have certain acute or chronic lung diseases, are engaged in exercise, or are exposed in close proximity to sources of air pollutants. Although considerable research has improved the understanding of inhaled particle doses, much is still to be learned before high-risk groups and individuals can be protected properly.

Tracheobronchial particle dose considerations for in vitro toxicology studies.

The purpose of this paper is to present a method for estimating particle doses that may be used to reconcile particle deposition doses used in in vitro toxicology studies with in vivo exposure levels. The focus is on the tracheobronchial (TB) tree of heavily exposed individuals. A review of the factors that influence inhaled particle deposition doses in environmental exposures leads to the identification of cases in which greater than average TB tree doses are received. Exercising individuals and those with chronic obstructive pulmonary disease not only inhale increased volumes of air but they also may have uneven ventilation that leads to greater than average particle deposition doses per unit of TB tree surface area. In addition, deposition hot spots, as occur at airway bifurcations, will greatly increase the particle exposures of target cells in the TB tree. Three particle exposure scenarios are proposed, and the average and local doses to the TB epithelium are calculated. When various factors that enhance particle doses (enhancement factors, or EFs) in vivo are considered, substantial particle doses may be justified for in vitro tissue culture studies that use TB target cells, such as epithelial cell cultures. The use of such EFs is intended to improve in vitro dosing with particles. Although the exposure of cells in vitro cannot fully replicate the complexity of in vivo exposures, it is possible to calculate toxicologically relevant doses that may define adverse health effects in potentially sensitive human populations. Local groups of TB cells in high-dose individuals are predicted to receive particle doses that are 3000-25,000 times higher than the doses averaged over the entire TB region.

Toxicological interactions in the respiratory system after inhalation of ozone and sulfuric acid aerosol mixtures.

A factorial design study was performed to examine the acute effects of inhaled acid particles alone and in mixtures with ozone to test the hypothesis that acid particles and ozone would act synergistically. Sprague-Dawley rats were exposed nose-only for a single 4-h period to all 9 possible combinations of purified air and 2 concentrations each of O(3) (0.3 and 0.6 ppm) and submicrometer (0.3 µm mass median diameter [MMD]) sulfuric acid aerosols H(2)SO(4) (0.5 and 1.0 mg/m(3)). Respiratory-tract injury and impairment of alveolar macrophage functions were evaluated. Two-way analyses of variance were used to test for significance of main effects and statistical interactions, and Tukey multiple comparison tests were used to test the significance of differences between group mean values. Addition of H(2)SO(4) to O(3)-containing atmospheres resulted in significant H(2)SO(4) concentration-dependent reductions in O(3)-induced inflammatory responses, and H(2)SO(4), alone and in combination with O(3), depressed some functions of innate immunity. DNA synthesis in nasal, tracheal, and lung tissue following pollutant exposure, which is an index of injury or killing of epithelial cells, was significantly increased by O(3) but not by H(2)SO(4) when administered alone, compared to purified air. When administered with O(3), H(2)SO(4) did not reduce the effects of O(3) on DNA synthesis in the trachea or the lung, but did reduce the DNA synthesis response to O(3) in the nose. No significant changes in antibody-directed Fc receptor (FcR) binding of sheep red blood cells by alveolar macrophages were observed, but macrophage phagocytic activity was significantly reduced by the pollutant exposures. In summary, the results of this study indicate significant interactions between O(3) and H(2)SO(4) in concurrent exposures; however, the findings do not support the hypothesis that O(3) and H(2)SO(4) act synergistically in rats after single 4-h exposures.

The particulate air pollution controversy.

Scientists, regulators, legislators, and segments of industry and the lay public are attempting to understand and respond to epidemiology findings of associations between measures of modern particulate air pollutants (PM) and adverse health outcomes in urban dwellers. The associations have been interpreted to imply that tens of thousands of Americans are killed annually by small daily increments in PM. These epidemiology studies and their interpretations have been challenged, although it is accepted that high concentrations of air pollutants have claimed many lives in the past. Although reproducible and statistically significant, the relative risks associated with modern PM are very small and confounded by many factors. Neither toxicology studies nor human clinical investigations have identified the components and/or characteristics of PM that might be causing the health-effect associations. Currently, a massive worldwide research effort is under way in an attempt to identify whom might be harmed and by what substances and mechanisms. Finding the answers is important, because control measures have the potential not only to be costly but also to limit the availability of goods and services that are important to public health.

Performance of a portable whole-body mouse exposure system.

A mobile whole-body exposure system was developed for exposing mice to concentrated ambient particulate matter smaller than 2.5 microm in mass median aerodynamic diameter (MMAD). Each 20-L exposure cage was designed to hold 9 mice within individual compartments. This allowed for transport and subsequent exposure. Airflow mixing and the potential for stagnant areas within the compartments were modeled using computational fluid dynamic modeling (CFD). CFD analysis showed no stagnant areas and good mixing throughout the exposure cage. The actual performance of the exposure system was determined for 0.5 to 2.0 microm diameter aerosols by measuring (1) uniformity of aerosol distribution and (2) particle deposition in the tracheobronchial and pulmonary regions of mice exposed in the system. A 0.6-microm MMAD (GSD=2.0) cigarette smoke aerosol was used to experimentally measure the uniformity of aerosol distribution to the nine individual compartments. The average data from three runs showed no statistically significant difference among individual compartments. Particle deposition efficiency in adult male BALB/c mice was measured after exposure (30 min) in the system using monodisperse fluorescent polystyrene latex particles (0.5, 1, and 2 microm aerodynamic diameter). The measured deposition efficiency in this mobile exposure system for the combined tracheobronchial and pulmonary regions of the adult male BALBc mice was 21% for 0.5 microm, 11% for 1.0 microm, and 6.5% for 2.0 microm particles. These deposition efficiencies are similar to those reported for mice exposed in a nose-only exposure system, which indicates that particle losses to animal fur and exposure system surfaces were acceptable.

Particulate air pollutants and asthma. A paradigm for the role of oxidative stress in PM-induced adverse health effects.

Asthma is a chronic inflammatory disease, which involves a variety of different mediators, including reactive oxygen species. There is growing awareness that particulate pollutants act as adjuvants during allergic sensitization and can also induce acute asthma exacerbations. In this communication we review the role of oxidative stress in asthma, with an emphasis on the pro-oxidative effects of diesel exhaust particles and their chemicals in the respiratory tract. We review the biology of oxidative stress, including protective and injurious effects that explain the impact of particulate matter-induced oxidative stress in asthma.

Dosimetry implications of upper tracheobronchial airway anatomy in two mouse varieties.

Strain- and variety-related differences in responses of mice have been reported for a variety of inhaled particulate and gaseous materials. It is important to understand the potential contributions to such responses of differences in delivered doses to the respiratory tract as well as differences in biochemical processes. Deposition doses of inhaled particles are influenced by several factors, including airway anatomy, ventilation, and particle characteristics. Tracheobronchial airway morphometry for airway generations 1-6 of the BALB/c mouse was generated using replica lung casts prepared in situ. Measurements were performed on two groups: control and ovalbumin-sensitized male BALB/c mice. These measurements were compared with previously published airway morphometry of male B6C3F(1) mice. Sensitization did not significantly change measured airway dimensions in the BALB/c mouse. However, the two mouse varieties had significant differences in airway anatomy. The differences found in airway anatomy between mouse varieties correlated with differences in body length and chest circumference. Particle deposition predictions for both varieties of mice were performed for unit density spherical particles from 0.1 to 10 microm in diameter at two ventilation rates using a published aerosol dosimetry computer code. Particle deposition in the proximal tracheobronchial tree ranged up to 3 times greater for the BALB/c mouse for a 2 microm particle diameter and high ventilation rate. These differences in predicted particle deposition suggest that observed strain and variety differences in response to inhaled particulate matter may be in part due to differences in delivered doses to the respiratory tract.