Abstract Human studies have linked air pollution (AP) to multiple neurodevelopmental disorders as well as to multiple neurodegenerative diseases and disorders, all conditions with increasing global prevalence that are subserved by broad underlying brain neurotransmitter dysregulation. Currently undetermined are the critical periods of AP exposures associated with such consequences and the potential for a subsequent trajectory of effects at different life stages and in response to different conditions and physiological demands. To further this understanding, pregnant mice were inhalationally exposed to concentrated ambient ultrafine particle air pollution for 4 hr/day from gestational day 0.5 to 16.5 with changes in frontal cortex and striatal neurotransmitter levels and frontostriatal correlations in offspring assessed at postnatal day (PND) 14, 41, 62 and 418, with the last time point derived from mice with and without behavioural experience. Predominant effects in females emerged after adolescence (PND62) with reductions in both frontal cortex and striatum in all three classes of neurotransmitters. In males, onset of adolescence (PND41) was accompanied by reductions in both glutamatergic and serotonergic neurotransmitter classes. In both sexes, the dynamic increases in neurotransmitters required by behavioural experience (PND418 Beh) were not evident in female frontal cortex serotonergic neurotransmitters and in male frontal cortex in all three classes of neurotransmitter systems. Collectively, this trajectory of effects underscores the persistence of developmental AP exposures and the importance of context, including sex, brain region and life stage in later emergent consequences, such that reliance on a single time point/condition may be misleading.

Abstract A hallmark of neurodegenerative disorders, including Alzheimer’s Disease (AD), is regional brain “iron (Fe) overload”. What has not been considered is the potential life-long exposure to Fe via air pollution (AP) as a contributor to elevated brain Fe and AD risk. AP has been linked in multiple studies to increased risk for AD with PM2.5 associated with increased risk for dementia, reduced memory and processing speed, and increased cognitive impairment. Our data and others examining Fe in olfactory bulb and frontal cortex of AD brain report the abundant presence of magnetite (Fe3+ iron oxide) nanoparticles, consistent with an exogenous rather than endogenous source. Filling this gap is a critical step forward for modelling and determining the mechanistic underpinnings of how PM2.5 contributes to memory loss and neurodegenerative risk. To address this, 10-wk-old C57Bl/6 mice were randomly exposed to HEPA-filtered air or 1.5 ng/L iron-oxide UFP particles for two hours daily for 20 d and then allowed to age. Acutely and at 12-mo, Fe-exposed females had memory deficits, and lower novel object recognition indices. At 18, but not 12 mo, Fe-exposed male mice were significantly hypoactive in locomotor assays, with more foot slips on balance beam. Significant shifts in brain amino acid metabolism and neurotransmitter balance were observed at each timepoint. Further, sex-specific tau accumulation, redox activation, and white matter damage was seen in olfactory bulb and frontal cortex. Taken together, these data indicate that iron-oxide exposures alter neuroanatomical, neurochemical, and behavioural patterns in a sex-dependent fashion with relevance for neurodegenerative risk.

Abstract Details on biodistribution of airborne “invader” exogenous nanoparticles (NP) are essential to understand hypothesized oxidative stress-induced effects involving neurodegeneration. Our analysis of NP in different brain regions selected two nose-to-brain neuronal pathways: (1) The rostral olfactory pathway from the olfactory bulb to deeper cerebral regions and (2) the caudal pathway to the trigeminal ganglion to the cerebellum. High-resolution imaging of human autopsied brain tissues and lab-particle-exposed mice brains shows exogenous-NP are typically located and bioprocessed next to NP-damaged neurons that have extensive demyelination. These pro-inflammatory NP are habitually surrounded by anti-inflammatory endogenous “ferritin”-NP, indicating a redox-protective mechanism by ferritin’s electron pumping capacity. Exogenous NP in different brain regions are diverse metals/metal-oxides, carbons (soot and nanotubes), and plastic/polymeric fragments. High-resolution analytical imaging (down to atomic scale) coupled with immunohistochemical (IHC) labelling shows NPs together with debris-collecting (eg myelin fragments) and inflammation and age-related corpora amylacea. Cerebral and cerebellar biodistribution occurs via axonal transport which—due to its particle size limitation of < 250 nm—cannot explain our findings of larger micro-sized NP in the cerebellum or other regions. Additional entry routes to the brain include blood-borne exogenous-NP that had entered the pulmonary interstitial compartment from deposits in the alveolar region. The brain’s capillaries through their tight junctions present an effective protection however, with age and increasing inflammatory conditions, these tight junctions become leaky, affording blood-borne NP-access. Discoveries of hypothesized further access portals of airborne NP reaching the brain, eg via the choroid plexus, optic nerve, or nervus terminalis, are future goals.

Abstract Iron-overload, a hallmark and potential contributor to neurodegeneration, comprises a significant portion of ultrafine particulate (UFP) matter of air pollution, with especially high concentrations in US subway systems. Previous research demonstrated that Fe-oxide nanoparticles can translocate along cranial nerves into brain, form deposits within the brain, and associate with degenerating myelin. We hypothesize that the essential role of Fe in myelination and the direct physical damage of particle transport along axons can contribute to the sex-specific white matter dysregulation observed in neurodegeneration. To assess the impact of Fe-oxide nanoparticle inhalation and age-related increases in brain Fe content on white matter, two cohorts of 10-week-old C57Bl/6 mice were randomly exposed to HEPA-filtered air or 1.5 ng/L iron-oxide UFPs for two hours daily for 20 d. An acute cohort was either acutely sacrificed followed by MRH or subjected to behaviour. The other cohort aged undisturbed until behaviour was examined at 12 and 18 mo. Acutely, Fe-exposure produced sex-specific diffusivity differences in olfactory bulbs. At 12 mo, only caudal regions on the olfactory circuit had significantly higher fractional anisotropy. Fe-exposed female mice had persistently lower novel recognition indices. Only at 18 mo, Fe-exposed male mice exhibited hypoactivity and motor deficits. Ultrastructural 18-mo corpus callosum myelin analyses demonstrated Fe-induced female-specific decreases in g-ratio and increases in axon area. Both sexes exhibited increases in fibre area, myelin area, and split myelin area. Fe-exposure produced a sex-specific effect on axon circularity indices in Fe-exposed animals potentially underlying sex differences in diffusivity.

Both excess brain Fe and air pollution (AP) exposures are associated with increased risk for multiple neurodegenerative disorders. Fe is a redox-active metal that is abundant in AP and even further elevated in U.S. subway systems. Exposures to AP and associated contaminants, such as Fe, are lifelong and could therefore contribute to elevated brain Fe observed in neurodegenerative diseases, particularly via nasal olfactory uptake of ultrafine particle AP. These studies tested the hypotheses that exogenously generated Fe oxide nanoparticles could reach the brain following inhalational exposures and produce neurotoxic effects consistent with neurodegenerative diseases and disorders in adult C57/Bl6J mice exposed by inhalation to Fe nanoparticles at a concentration similar to those found in underground subway systems ( 150 µg/m3) for 20 days. Olfactory bulb sections and exposure chamber TEM grids were analyzed for Fe speciation. Measures included brain volumetric and diffusivity changes; levels of striatal and cerebellar neurotransmitters and trans-sulfuration markers; quantification of frontal cortical and hippocampal Aβ42, total tau, and phosphorylated tau; and behavioral alterations in locomotor activity and memory. Particle speciation confirmed similarity of Fe oxides (mostly magnetite) found on chamber TEM grids and in olfactory bulb. Alzheimer’s disease (AD) like characteristics were seen in Fe-exposed females including increased olfactory bulb diffusivity, impaired memory, and increased accumulation of total and phosphorylated tau, with total hippocampal tau levels significantly correlated with increased errors in the radial arm maze. Fe-exposed males showed increased volume of the substantia nigra pars compacta, a region critical to the motor impairments seen in Parkinson’s disease (PD), in conjunction with reduced volume of the trigeminal nerve and optic tract and chiasm. Inhaled Fe oxide nanoparticles appeared to lead to olfactory bulb uptake. Further, these exposures reproduced characteristic features of neurodegenerative diseases in a sex-dependent manner, with females evidencing features similar to those seen in AD and effects in regions in males associated with PD. As such, prolonged inhaled Fe exposure via AP should be considered as a source of elevated brain Fe with aging, and as a risk factor for neurodegenerative diseases. The bases for dichotomous sex effects of inhaled Fe nanoparticles is as of yet unclear. Also as of yet unknown is how duration of such Fe exposures affect outcome, and/or whether exposures to inhaled Fe during early brain development enhances vulnerability to subsequent Fe exposures. Collectively, these findings suggest that regulation of air Fe levels, particularly in enclosed areas like subway stations, may have broad public health protective effects.

Two Ceria nanomaterials (NM-211 and NM-212) were tested for inhalation toxicity and organ burdens in order to design a chronic and carcinogenicity inhalation study (OECD TG No. 453). Rats inhaled aerosol concentrations of 0.5, 5, and 25 mg/m3 by whole-body exposure for 6 h/day on 5 consecutive days for 1 or 4 weeks with a post-exposure period of 24 or 129 days, respectively. Lungs were examined by bronchoalveolar lavage and histopathology. Inhaled Ceria is deposited in the lung and cleared with a half-time of 40 days; at aerosol concentrations higher than 0.5 mg/m3, this clearance was impaired resulting in a half-time above 200 days (25 mg/m3). After 5 days, Ceria (>0.5 mg/m3) induced an early inflammatory reaction by increases of neutrophils in the lung which decreased with time, with sustained exposure, and also after the exposure was terminated (during the post-exposure period). The neutrophil number observed in bronchoalveolar lavage fluid (BALF) was decreasing and supplemented by mononuclear cells, especially macrophages which were visible in histopathology but not in BALF. Further progression to granulomatous inflammation was observed 4 weeks post-exposure. The surface area of the particles provided a dose metrics with the best correlation of the two Ceria's inflammatory responses; hence, the inflammation appears to be directed by the particle surface rather than mass or volume in the lung. Observing the time course of lung burden and inflammation, it appears that the dose rate of particle deposition drove an initial inflammatory reaction by neutrophils. The later phase (after 4 weeks) was dominated by mononuclear cells, especially macrophages. The progression toward the subsequent granulomatous reaction was driven by the duration and amount of the particles in the lung. The further progression of the biological response will be determined in the ongoing long-term study.

IntroductionAir pollution (AP) has been associated with increased risk for multiple neurodevelopmental disorders. As one of the most abundant contaminants of AP, iron (Fe) is critical to brain function, with both deficiencies and excesses leading to potential neurotoxicity. Our prior studies examining the impact of developmental exposures of mice to inhaled Fe (1.0 μg/m3) alone or in conjunction with sulfur dioxide SO2 (1.31 mg/m3; FeS) from postnatal days (PND) 4–7 and 10–13 (human 3rd trimester brain equivalent period) revealed alterations in brain neurotransmitter levels at PND14 which had generally recovered by PND60, but which were, nevertheless, followed by behavioral impairments. The current study sought to determine whether subsequent behavioral experience, which requires neurochemical mediation, had unmasked residual deficits in neurotransmitter function in response to developmental FeS or Fe inhalation.MethodsConsequently, levels of brain neurotransmitters and trans-sulfuration markers were measured in mice that had either behavioral experience (BE) or no behavioral experience (NB) at PND 215 (Fe only) or 357 (FeS).ResultsBE itself markedly increased brain neurotransmitter and trans-sulfuration marker levels, particularly in males. These increases were prevented in males in both frontal cortex and striatum by prior developmental FeS exposures. In females, developmental Fe exposure was associated with residual increases particularly in striatal serotonergic function and levels of homocysteine independently of behavioral experience.DiscussionCollectively, these findings show the ability of behavioral experience to unmask later life residual consequences of developmental exposures to FeS in males and of latent emerging effects of Fe in females. The collective findings may have relevance to later life neurodegenerative diseases and disorders now increasingly associated with air pollution exposures, and also underscore how understanding how various components of air pollution influence brain is critical to regulatory decisions for public health protection.

Air pollution (AP) exposures have been associated with autism (ASD), schizophrenia (SCZ), and attention deficit hyperactivity disorder (ADHD), male-biased neurodevelopmental disorders that are linked to alterations in brain fronto-striatal neurotransmitter systems. The current study sought to assess how developmental exposures of mice to inhaled ambient ultrafine particle (UFP) air pollution, considered its most reactive component, alters fronto-striatal functional correlations. Mice were exposed via inhalation to concentrated ambient UFPs from postnatal days (PND) 4–7 and 10–13. Frontal cortex, striatum, and serum were collected at PND14 and PND50 to evaluate both acute and persistent effects. UFP-induced changes, more extensive and persistent in males, included elimination of frontal cortical kynurenine correlations with striatal neurotransmitter function, persistent immunosuppression of approximately 50%, and striatal neurotransmitter turnover correlations with serum corticosterone. More limited effects in females did not show persistence. Collectively, these findings depict an apparently physiologically-integrated UFP-induced persistent male-biased vulnerability to brain fronto-striatal system dysfunction that could contribute to behavioral deficits associated with neurodevelopmental disorders. Further studies are needed to ascertain the interactive physiological mechanisms of male fronto-striatal vulnerability and their relation to behavioral impairments, mechanisms of apparent female compensation, and specific contaminants of AP that underlie this vulnerability.

Abstract Background Toxicokinetics of nanomaterials, including studies on the absorption, distribution, metabolism, and elimination of nanomaterials, are essential in assessing their potential health effects. The fate of nanomaterials after inhalation exposure to multiple nanomaterials is not clearly understood. Methods Male Sprague–Dawley rats were exposed to similar sizes of silver nanoparticles (AgNPs, 10.86 nm) and gold nanoparticles (AuNPs, 10.82 nm) for 28 days (6-h/day, 5-days/week for four weeks) either with separate NP inhalation exposures or with combined co-exposure in a nose-only inhalation system. Mass concentrations sampled from the breathing zone were AuNP 19.34 ± 2.55 μg/m3 and AgNP 17.38 ± 1.88 μg/m3 for separate exposure and AuNP 8.20 μg/m3 and AgNP 8.99 μg/m3 for co-exposure. Lung retention and clearance were previously determined on day 1 (6-h) of exposure (E-1) and on post-exposure days 1, 7, and 28 (PEO-1, PEO-7, and PEO-28, respectively). In addition, the fate of nanoparticles, including translocation and elimination from the lung to the major organs, were determined during the post-exposure observation period. Results AuNP was translocated to the extrapulmonary organs, including the liver, kidney, spleen, testis, epididymis, olfactory bulb, hilar and brachial lymph nodes, and brain after subacute inhalation and showed biopersistence regardless of AuNP single exposure or AuNP + AgNP co-exposure, showing similar elimination half-time. In contrast, Ag was translocated to the tissues and rapidly eliminated from the tissues regardless of AuNP co-exposure. Ag was continually accumulated in the olfactory bulb and brain and persistent until PEO-28. Conclusion Our co-exposure study of AuNP and AgNP indicated that soluble AgNP and insoluble AuNP translocated differently, showing soluble AgNP could be dissolved into Ag ion to translocate to the extrapulmonary organs and rapidly removed from most organs except the brain and olfactory bulb. Insoluble AuNPs were continually translocated to the extrapulmonary organs, and they were not eliminated rapidly.

Exposures to ambient ultrafine particle (UFP) air pollution (AP) during the early postnatal period in mice (equivalent to human third trimester brain development) produce male-biased changes in brain structure, including ventriculomegaly, reduced brain myelination, alterations in neurotransmitters and glial activation, as well as impulsive-like behavioral characteristics, all of which are also features characteristic of male-biased neurodevelopmental disorders (NDDs). The purpose of this study was to ascertain the extent to which inhaled Cu, a common contaminant of AP that is also dysregulated across multiple NDDs, might contribute to these phenotypes. For this purpose, C57BL/6 J mice were exposed from postnatal days 4-7 and 10-13 for 4 hr/day to inhaled copper oxide (CuxOy) nanoparticles at an environmentally relevant concentration averaging 171.9 ng/m3. Changes in brain metal homeostasis and neurotransmitter levels were determined following termination of exposure (postnatal day 14), while behavioral changes were assessed in adulthood. CuxOy inhalation modified cortical metal homeostasis and produced male-biased disruption of striatal neurotransmitters, with marked increases in dopaminergic function, as well as excitatory/inhibitory imbalance and reductions in serotonergic function. Impulsive-like behaviors in a fixed ratio (FR) waiting-for-reward schedule and a fixed interval (FI) schedule of food reward occurred in both sexes, but more prominently in males, effects which could not be attributed to altered locomotor activity or short-term memory. Inhaled Cu as from AP exposures, at environmentally relevant levels experienced during development, may contribute to impaired brain function, as shown by its ability to disrupt brain metal homeostasis and striatal neurotransmission. In addition, its ability to evoke impulsive-like behavior, particularly in male offspring, may be related to striatal dopaminergic dysfunction that is known to mediate such behaviors. As such, regulation of air Cu levels may be protective of public health.

This paper summarizes recent insights into causal biological mechanisms underlying the carcinogenicity of asbestos. It addresses their implications for the shapes of exposure-response curves and considers recent epidemiologic trends in malignant mesotheliomas (MMs) and lung fiber burden studies. Since the commercial amphiboles crocidolite and amosite pose the highest risk of MMs and contain high levels of iron, endogenous and exogenous pathways of iron injury and repair are discussed. Some practical implications of recent developments are that: (1) Asbestos-cancer exposure-response relationships should be expected to have non-zero background rates; (2) Evidence from inflammation biology and other sources suggests that there are exposure concentration thresholds below which exposures do not increase inflammasome-mediated inflammation or resulting inflammation-mediated cancer risks above background risk rates; and (3) The size of the suggested exposure concentration threshold depends on both the detailed time patterns of exposure on a time scale of hours to days and also on the composition of asbestos fibers in terms of their physiochemical properties. These conclusions are supported by complementary strands of evidence including biomathematical modeling, cell biology and biochemistry of asbestos-cell interactions in vitro and in vivo, lung fiber burden analyses and epidemiology showing trends in human exposures and MM rates.

Background Air pollution has been associated with neurodevelopmental disorders in epidemiological studies. In our studies in mice, developmental exposures to ambient ultrafine particulate (UFP) matter either postnatally or gestationally results in neurotoxic consequences that include brain metal dyshomeostasis, including significant increases in brain Fe. Since Fe is redox active and neurotoxic to brain in excess, this study examined the extent to which postnatal Fe inhalation exposure, might contribute to the observed neurotoxicity of UFPs. Mice were exposed to 1 µg/m 3 Fe oxide nanoparticles alone, or in conjunction with sulfur dioxide (Fe (1 µg/m 3 ) + SO 2 (SO 2 at 1.31 mg/m 3 , 500 ppb) from postnatal days 4–7 and 10–13 for 4 h/day. Results Overarching results included the observations that Fe + SO 2 produced greater neurotoxicity than did Fe alone, that females appeared to show greater vulnerability to these exposures than did males, and that profiles of effects differed by sex. Consistent with metal dyshomeostasis, both Fe only and Fe + SO 2 exposures altered correlations of Fe and of sulfur (S) with other metals in a sex and tissue-specific manner. Specifically, altered metal levels in lung, but particularly in frontal cortex were found, with reductions produced by Fe in females, but increases produced by Fe + SO 2 in males. At PND14, marked changes in brain frontal cortex and striatal neurotransmitter systems were observed, particularly in response to combined Fe + SO2 as compared to Fe only, in glutamatergic and dopaminergic functions that were of opposite directions by sex. Changes in markers of trans-sulfuration in frontal cortex likewise differed in females as compared to males. Residual neurotransmitter changes were limited at PND60. Increases in serum glutathione and Il-1a were female-specific effects of combined Fe + SO2. Conclusions Collectively, these findings suggest a role for the Fe contamination in air pollution in the observed neurotoxicity of ambient UFPs and that such involvement may be different by chemical mixture. Translation of such results to humans requires verification, and, if found, would suggest a need for regulation of Fe in air for public health protection.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Background Inhalation exposure to nanomaterials in workplaces can include a mixture of multiple nanoparticles. Such ambient nanoparticles can be of high dissolution or low dissolution in vivo and we wished to determine whether co-exposure to particles with different dissolution rates affects their biokinetics. Methods and Results Rats were exposed to biosoluble silver nanoparticles (AgNPs, 10.86 nm) and to biopersistent gold nanoparticles (AuNPs, 10.82 nm) for 28 days (6-h/day, 5-days/week for 4 weeks) either with separate NP inhalation exposures or with combined co-exposure. The separate NPs mass concentrations estimated by the differential mobility analyzer system (DMAS) were determined to be 17.68 ± 1.69 μg/m 3 for AuNP and 10.12 ± 0.71 μg/m 3 for AgNP. In addition, mass concentrations analyzed by atomic absorption spectrometer (AAS) via filter sampling were for AuNP 19.34 ± 2.55 μg/m 3 and AgNP 17.38 ± 1.88 μg/m 3 for separate exposure and AuNP 8.20 ± 1.05 μg/m 3 and AgNP 8.99 ± 1.77 μg/m 3 for co-exposure. Lung retention and clearance were determined on day 1 (6-h) of exposure (E-1) and on post-exposure days 1, 7, and 28 (PEO-1, PEO-7, and PEO-28, respectively). While the AgNP and AuNP deposition rates were determined to be similar due to the similarity of NP size of both aerosols, the retention half-times and clearance rates differed due to the difference in dissolution rates. Thus, when comparing the lung burdens following separate exposures, the AgNP retention was 10 times less than the AuNP retention at 6-h (E-1), and 69, 89, and 121 times lower less than the AuNP retention at PEO-1, PEO-7, and PEO-28, respectively. In the case of AuNP+AgNP co-exposure, the retained AgNP lung burden was 14 times less than the retained AuNP lung burden at E-1, and 26, 43, and 55 times less than the retained AuNP lung burden at PEO-1, PEO-7, and PEO-28, respectively. The retention of AuNP was not affected by the presence of AgNP, but AgNP retention was influenced in the presence of AuNP starting at 24 h after the first day of post day of exposure. The clearance of AgNPs of the separate exposure showed 2 phases; fast (T 1/2 3.1 days) and slow (T 1/2 48.5 days), while the clearance of AuNPs only showed one phase (T 1/2 .81.5 days). For the co-exposure of AuNPs+AgNPs, the clearance of AgNPs also showed 2 phases; fast (T 1/2 2.2 days) and slow (T 1/2 28.4 days), while the clearance of AuNPs consistently showed one phase (T 1/2 54.2 days). The percentage of Ag lung burden in the fast and slow clearing lung compartment was different between separate and combined exposure. For the combined exposure, the slow and fast compartments were each 50% of the lung burden. For the single exposure, 1/3 of the lung burden was cleared by the fast rate and 2/3 of the lung burden by the slow rate. Conclusions The clearance of AgNPs follows a two- phase model of fast and slow dissolution rates while the clearance of AuNPs could be described by a one- phase model with a longer half-time. The co-exposure of AuNPs+AgNPs showed that the clearance of AgNPs was altered by the presence of AuNPs perhaps due to some interaction between AgNP and AuNP affecting dissolution and/or mechanical clearance of AgNP in vivo.

Epidemiological and experimental studies have associated oral and systemic exposures to the herbicide paraquat (PQ) with Parkinson's disease. Despite recognition that airborne particles and solutes can be directly translocated to the brain via olfactory neurons, the potential for inhaled PQ to cause olfactory impairment has not been investigated. This study sought to determine if prolonged low-dose inhalation exposure to PQ would lead to disposition to the brain and olfactory impairment, a prodromal feature of Parkinson's disease. Adult male and female C57BL/6J mice were exposed to PQ aerosols in a whole-body inhalation chamber for 4h/day, 5 days/week for 4 weeks. Subsets of mice were sacrificed during and after exposure and PQ concentrations in various brain regions (olfactory bulb, striatum, midbrain, and cerebellum) lung, and kidney were quantified via mass spectrometry. Alterations in olfaction were examined using an olfactory discrimination paradigm. PQ inhalation resulted in an appreciable burden in all examined brain regions, with the highest burden observed in the olfactory bulb, consistent with nasal olfactory uptake. PQ was also detected in the lung and kidney, yet PQ levels in all tissues returned to control values within 4 weeks post exposure. PQ inhalation caused persistent male-specific deficits in olfactory discrimination. No effects were observed in females. These data support the importance of route of exposure in determination of safety estimates for neurotoxic pesticides, such as PQ. Accurate estimation of the relationship between exposure and internal dose is critical for risk assessment and public health protection.

Background Information on particle deposition, retention, and clearance is important when evaluating the risk of inhaled nanomaterials to human health. The revised Organization Economic Cooperation and Development (OECD) inhalation toxicity test guidelines now require lung burden measurements of nanomaterials after rodent subacute and sub-chronic inhalation exposure (OECD 412, OECD 413) to inform on lung clearance behavior and translocation after exposure and during post-exposure observation (PEO). Lung burden measurements are particularly relevant when the testing chemical is a solid poorly soluble nanomaterial. Previously, the current authors showed that total retained lung burden of inhaled soluble silver nanoparticles (AgNPs) could be effectively measured using any individual lung lobe. Methods and results Accordingly, the current study investigated the evenness of deposition/retention of poorly soluble gold nanoparticles (AuNPs) after 1 and 5 days of inhalation exposure. Rats were exposed nose-only for 1 or 5 days (6 h/day) to an aerosol of 11 nm well-dispersed AuNPs. Thereafter, the five lung lobes were separated and the gold concentrations measured using an inductively coupled plasma-mass spectrophotometer (ICP-MS). The results showed no statistically significant difference in the AuNP deposition/retention among the different lung lobes in terms of the gold mass per gram of lung tissue. Conclusions Thus, it would seem that any rat lung lobe can be used for the lung burden analysis after short or long-term NP inhalation, while the other lobes can be used for collecting and analyzing the bronchoalveolar lavage fluid (BALF) and for the histopathological analysis. Therefore, combining the lung burden measurement, histopathological tissue preparation, and BALF assay from one rat can minimize the number of animals used and maximize the number of endpoints measured.

Increasing evidence links air pollution (AP) exposure to effects on the central nervous system structure and function. Particulate matter AP, especially the ultrafine (nanoparticle) components, can carry numerous metal and trace element contaminants that can reach the brain in utero and after birth. Excess brain exposure to either essential or non-essential elements can result in brain dyshomeostasis, which has been implicated in both neurodevelopmental disorders (NDDs; autism spectrum disorder, schizophrenia, and attention deficit hyperactivity disorder) and neurodegenerative diseases (NDGDs; Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and amyotrophic lateral sclerosis). This review summarizes the current understanding of the extent to which the inhalational or intranasal instillation of metals reproduces in vivo the shared features of NDDs and NDGDs, including enlarged lateral ventricles, alterations in myelination, glutamatergic dysfunction, neuronal cell death, inflammation, microglial activation, oxidative stress, mitochondrial dysfunction, altered social behaviors, cognitive dysfunction, and impulsivity. Although evidence is limited to date, neuronal cell death, oxidative stress, and mitochondrial dysfunction are reproduced by numerous metals. Understanding the specific contribution of metals/trace elements to this neurotoxicity can guide the development of more realistic animal exposure models of human AP exposure and consequently lead to a more meaningful approach to mechanistic studies, potential intervention strategies, and regulatory requirements.

Barium sulfate (BaSO4) was considered to be poorly-soluble and of low toxicity, but BaSO4 NM-220 showed a surprisingly short retention after intratracheal instillation in rat lungs, and incorporation of Ba within the bones. Here we show that static abiotic dissolution cannot rationalize this result, whereas two dynamic abiotic dissolution systems (one flow-through and one flow-by) indicated 50% dissolution after 5 to 6 days at non-saturating conditions regardless of flow orientation, which is close to the in vivo half-time of 9.6 days. Non-equilibrium conditions were thus essential to simulate in vivo biodissolution. Instead of shrinking from 32 nm to 23 nm (to match the mass loss to ions), TEM scans of particles retrieved from flow-cells showed an increase to 40 nm. Such transformation suggested either material transport through interfacial contact or Ostwald ripening at super-saturating conditions and was also observed in vivo inside macrophages by high-resolution TEM following 12 months inhalation exposure. The abiotic flow cells thus adequately predicted the overall pulmonary biopersistence of the particles that was mediated by non-equilibrium dissolution and recrystallization. The present methodology for dissolution and transformation fills a high priority gap in nanomaterial hazard assessment and is proposed for the implementation of grouping and read-across by dissolution rates.

In their Commentary Saber et al. (Part Fibre Toxicol 16: 44, 2019) argue that chronic inhalation studies in rats can be used for assessing the lung cancer risk of insoluble nanomaterials. The authors make several significant errors in their interpretation and representation of the underlying science. In this Letter to the Editor we discuss these inaccuracies to correct the scientific record. When the science is recounted accurately it does not support Saber et al's statements and conclusions.