Thomas W. Hesterberg

Chronic Inhalation Toxicity of Size-Separated Glass Fibers in Fischer 344 Rats

This study was initiated to determine the chronic biological effects in Fisher 344 rats of inhaled size-separated respirable fractions of fibrous glass (FG) having compositions representative of common building insulation wools. Rats were exposed using nose-only inhalation chambers, 6 hr/day, 5 days/week, for 24 months to three concentrations (3, 16, and 30 mg/m3) of two different compositions of FG (designated MMVF 10 and MMVF 11), or to filtered air (negative control). Fibrous glass findings were compared to those from a concurrent inhalation study of chrysotile asbestos and refractory ceramic fiber (RCF). The FGs used in this study were size selected to be largely respirable in the rat and the aerosol generation technique did not alter the dimensions of the fibers. Interim euthanizations took place at 3- to 6-month intervals to monitor progression of pulmonary changes. Fibers were recovered from digested lung tissue for determination of changes in fiber number and morphology. In animals exposed to 30 mg/m3 of MMVF 10 or MMVF 11, 4.2 +/- 0.9 x 10(5) and 6.4 +/- 3.1 x 10(5) fibers/mg dry lung tissue, respectively, were recovered after 24 months of exposure. Exposure to chrysotile asbestos (10 mg/m3) and to a lesser extent RCF (30 mg/m3) resulted in pulmonary fibrosis as well as mesothelioma and significant increases in lung tumors. FG exposure was associated with a nonspecific inflammatory response (macrophage response) in the lungs that did not appear to progress after 6-12 months of exposure. These cellular changes are reversible and are similar to the effects observed after inhalation of an inert dust. No lung fibrosis was observed in the FG-exposed animals. Further, FG exposure resulted in no mesotheliomas and no statistically significant increase in lung tumor incidence when compared to that of the negative control group. These findings, along with previous inhalation studies, suggest that respirable fibrous glass does not represent a significant hazard for fibrotic or neoplastic lung disease in humans.

Pulmonary Endpoints (Lung Carcinomas and Asbestosis) Following Inhalation Exposure to Asbestos

Lung carcinomas and pulmonary fibrosis (asbestosis) occur in asbestos workers. Understanding the pathogenesis of these diseases is complicated because of potential confounding factors, such as smoking, which is not a risk factor in mesothelioma. The modes of action (MOA) of various types of asbestos in the development of lung cancers, asbestosis, and mesotheliomas appear to be different. Moreover, asbestos fibers may act differentially at various stages of these diseases, and have different potencies as compared to other naturally occurring and synthetic fibers. This literature review describes patterns of deposition and retention of various types of asbestos and other fibers after inhalation, methods of translocation within the lung, and dissolution of various fiber types in lung compartments and cells in vitro. Comprehensive dose-response studies at fiber concentrations inhaled by humans as well as bivariate size distributions (lengths and widths), types, and sources of fibers are rarely defined in published studies and are needed. Species-specific responses may occur. Mechanistic studies have some of these limitations, but have suggested that changes in gene expression (either fiber-catalyzed directly or by cell elaboration of oxidants), epigenetic changes, and receptor-mediated or other intracellular signaling cascades may play roles in various stages of the development of lung cancers or asbestosis.

Chronic Inhalation Study of Size-Separated Rock and Slag Wool Insulation Fibers in Fischer 344/N Rats

AbstractThis study was designed to investigate the potential pathogenic effects in Fischer 344/N rats of two different types of man-made vitreous fibers (MMVF). Eight-week-old male rats were exposed in nose-only inhalation chambers, 6 hlday, 5 dayslwk, lor 24 mo to 3 concentrations (3, 16, and 30 mg/m1) each of the two MMVFs: a basalt-based rock wool (stone wool), and a slag wool (blast furnace). Crocidolite asbestos (10 mg/m3) was used as a positive control. The experimental groups were compared to unexposed (chamber) controls. The MMVFs used in this study were size selected to be largely respirable in rats. Interim sacrifices took place at 3- and 6-mo intervals to monitor the progression of pulmonary changes. Fibers were recovered from digested lung tissue for determination of changes in fiber number and morphology. Exposure to crocidolite asbestos was terminated after 10 mo because of increased morbidity/mortality. Exposure to rock and slag wool, while producing a dose-related nonspecific inflammatory ...

Critical review of the human data on short-term nitrogen dioxide (NO2) exposures: Evidence for NO2 no-effect levels

Nitrogen dioxide (NO2) is a ubiquitous atmospheric pollutant due to the widespread prevalence of both natural and anthropogenic sources, and it can be a respiratory irritant when inhaled at elevated concentrations. Evidence for health effects of ambient NO2 derives from three types of studies: observational epidemiology, human clinical exposures, and animal toxicology. Our review focuses on the human clinical studies of adverse health effects of short-term NO2 exposures, given the substantial uncertainties and limitations in interpretation of the other lines of evidence. We examined more than 50 experimental studies of humans inhaling NO2, finding notably that the reporting of statistically significant changes in lung function and bronchial sensitivity did not show a consistent trend with increasing NO2 concentrations. Functional changes were generally mild and transient, the reported effects were not uniformly adverse, and they were not usually accompanied by NO2-dependent increases in symptoms. The available human clinical results do not establish a mechanistic pathway leading to adverse health impacts for short-term NO2 exposures at levels typical of maximum 1-h concentrations in the present-day ambient environment (i.e., below 0.2 ppm). Our review of these data indicates that a health-protective, short-term NO2 guideline level for susceptible (and healthy) populations would reflect a policy choice between 0.2 and 0.6 ppm. Extended abstract Nitrogen dioxide (NO2) is a ubiquitous atmospheric pollutant due to the widespread prevalence of both natural and anthropogenic sources, and it can be a respiratory irritant when inhaled at elevated concentrations. Natural NO2 sources include volcanic action, forest fires, lightning, and the stratosphere; man-made NO2 emissions derive from fossil fuel combustion and incineration. The current National Ambient Air Quality Standard (NAAQS) for NO2, initially established in 1971, is 0.053 ppm (annual average). Ambient concentrations monitored in urban areas in the United States are ~0.015 ppm, as an annual mean, i.e., below the current NAAQS. Short-term (1-h peak) NO2 concentrations outdoors are not likely to exceed 0.2 ppm, and even 1-h periods exceeding 0.1 ppm are infrequent. Inside homes, 1-h NO2 peaks, typically arising from gas cooking, can range between 0.4 and 1.5 ppm. The health effects evidence of relevance to ambient NO2 derives from three lines of investigation: epidemiology studies, human clinical studies, and animal toxicology studies. The NO2 epidemiology remains inconsistent and uncertain due to the potential for exposure misclassification, residual confounding, and co-pollutant effects, whereas animal toxicology findings using high levels of NO2 exposure require extrapolation to humans exposed at low ambient NO2 levels. Given the limitations and uncertainties in the other lines of health effects evidence, our review thus focused on clinical studies where human volunteers (including asthmatics, children, and elderly) inhaled NO2 at levels from 0.1 to 3.5 ppm during short-term (½–6-h) exposures, often combined with exercise, and occasionally combined with co-pollutants. We examined the reported biological effects and classified them into (a) lung immune responses and inflammation, (b) lung function changes and airway hyperresponsiveness (AHR), and (c) health effects outside the lungs (extrapulmonary). We examined more than 50 experimental studies of humans inhaling NO2, finding that such clinical data on short-term exposure allowed discrimination of NO2 no-effect levels versus lowest-adverse-effects levels. Our conclusions are summarized by these six points: For lung immune responses and inflammation: (1) healthy subjects exposed to NO2 below 1 ppm do not show pulmonary inflammation; (2) at 2 ppm for 4 h, neutrophils and cytokines in lung-lavage fluid can increase, but these changes do not necessarily correlate with significant or sustained changes in lung function; (3) there is no consistent evidence that NO2 concentrations below 2 ppm increase susceptibility to viral infection; (4) for asthmatics and individuals having chronic obstructive pulmonary disease (COPD), NO2-induced lung inflammation is not expected below 0.6 ppm, although one research group reported enhancement of proinflammatory processes at 0.26 ppm. With regard to NO2-induced AHR: (5) studies of responses to specific or nonspecific airway challenges (e.g., ragweed, methacholine) suggest that asthmatic individuals were not affected by NO2 up to about 0.6 ppm, although some sensitive subsets may respond to levels as low as 0.2 ppm. And finally, for extra-pulmonary effects: (6) such effects (e.g., changes in blood chemistry) generally required NO2 concentrations above 1–2 ppm. Overall, our review of data from experiments with humans indicates that a health-protective, short-term-average NO2 guideline level for susceptible populations (and healthy populations) would reflect a policy choice between 0.2 and 0.6 ppm. The available human clinical results do not establish a mechanistic pathway leading to adverse health impacts for short-term NO2 exposures at levels typical of maximum 1-h concentrations in the present-day ambient environment (i.e., below 0.2 ppm).

Health risk of chrysotile revisited

Abstract This review provides a basis for substantiating both kinetically and pathologically the differences between chrysotile and amphibole asbestos. Chrysotile, which is rapidly attacked by the acid environment of the macrophage, falls apart in the lung into short fibers and particles, while the amphibole asbestos persist creating a response to the fibrous structure of this mineral. Inhalation toxicity studies of chrysotile at non-lung overload conditions demonstrate that the long (>20 µm) fibers are rapidly cleared from the lung, are not translocated to the pleural cavity and do not initiate fibrogenic response. In contrast, long amphibole asbestos fibers persist, are quickly (within 7 d) translocated to the pleural cavity and result in interstitial fibrosis and pleural inflammation. Quantitative reviews of epidemiological studies of mineral fibers have determined the potency of chrysotile and amphibole asbestos for causing lung cancer and mesothelioma in relation to fiber type and have also differentiated between these two minerals. These studies have been reviewed in light of the frequent use of amphibole asbestos. As with other respirable particulates, there is evidence that heavy and prolonged exposure to chrysotile can produce lung cancer. The importance of the present and other similar reviews is that the studies they report show that low exposures to chrysotile do not present a detectable risk to health. Since total dose over time decides the likelihood of disease occurrence and progression, they also suggest that the risk of an adverse outcome may be low with even high exposures experienced over a short duration.

Chronic Inhalation Toxicity of a Kaolin-Based Refractory Ceramic Fiber in Syrian Golden Hamsters

AbstractKaolin-based refractory ceramic fiber (RCF) is a man-made vitreous fiber used primarily in industrial high-temperature applications, especially for insulation of furnaces and kilns. Because of its increasing use and potential for human exposure, a chronic toxicity/ carcinogenicity inhalation study was conducted in Syrian golden hamsters. Two groups of 140 weanling male hamsters were exposed via nose-only inhalation to either HEPA-filtered air (chamber controls) or 30 mg/m3 (-220 fibers/cm3) of “size-selected” RCF fibers (1 µm in diameter and -25 /jm in length) for 6 h/day, 5 dayslwk for 18 mo. They were then held unexposed until -20% survival (20 mo). A positive control group of 80 hamsters was exposed to 10 mg/m3 chrysotile asbestos (0.09 µm average diameter and 2.2 µm average length). Groups of 3 hamsters were sacrificed at 3, 6, 9, 12, 15, and 18 mo to follow the progression of lesions. Additional groups of 3 hamsters were removed from exposure at 3, 6, 9, and 12 mo and were held until 18 mo (r...

A Critical Assessment of Studies on the Carcinogenic Potential of Diesel Exhaust

After decades of research involving numerous epidemiologic studies and extensive investigations in laboratory animals, a causal relationship between diesel exhaust (DE) exposure and lung cancer has not been conclusively demonstrated. Epidemiologic studies of the transportation industry (trucking, busing, and railroad) show a small elevation in lung cancer incidence (relative risks [RRs] generally below 1.5), but a dose response for DE is lacking. The studies are also limited by a lack of quantitative concurrent exposure data and inadequate or lack of controls for potential confounders, particularly tobacco smoking. Furthermore, prior to dieselization, similar elevations in lung cancer incidence have been reported for truck drivers, and in-cab diesel particulate matter (DPM) exposures of truck drivers were comparable to ambient highway exposures. Taken together, these findings suggest that an unidentified occupational agent or lifestyle factor might be responsible for the low elevations in lung cancer reported in the transportation studies. In contrast, underground miners, many of whom experience the highest occupational DPM exposures, generally do not show elevations in lung cancer. Laboratory studies must be interpreted with caution with respect to predicting the carcinogenic potential of DE in humans. Tumors observed in rats following lifetime chronic inhalation of very high levels of DPM may be attributed to species-specific overload mechanisms that lack relevance to humans. Increased tumor incidence was not observed in other species (hamsters or mice) exposed to DPM at very high levels or in rats exposed at lower levels (≤2000 μg/m3). Although DPM contains mutagens, mutagenicity studies in which cells were exposed to concentrated extracts of DPM also have limited application to human risk assessment, because such extracts can be obtained from DPM only by using strong organic solvents, agitation, and heat. Most studies have shown that whole DPM itself is not mutagenic because the adsorbed organic compounds are minimally bioavailable in aqueous-based fluids. In the past two decades, dramatic changes in diesel engine technology (e.g., low-sulfur fuel and exhaust after-treatment) have resulted in >99% reduction in DPM and other quantitative and qualitative changes in the chemical and physical characteristics of diesel exhaust. Thus, the current database, which is focused almost entirely on the potential health effects of traditional diesel exhaust (TDE), has only limited utility in assessing the potential health risks of new-technology diesel exhaust (NTDE). To overcome some of the limitations of the historical epidemiologic database on TDE and to gain further insights into the potential health effects of NTDE, new studies are underway and more studies are planned.

Non-cancer health effects of diesel exhaust: a critical assessment of recent human and animal toxicological literature.

We reviewed laboratory and clinical studies bearing on the non-cancer health effects of diesel exhaust (DE) published since the 2002 release of the US EPA Health Assessment Document for Diesel Engine Exhaust. We critically evaluated over 100 published articles on experimental research, focusing on their value for predicting the risk of non-cancer health effects in humans exposed to DE. Human controlled-exposure studies provide new evidence of lung inflammatory effects and thrombogenic and ischemic effects of inhaled DE, albeit for older-model diesel engines and concentrations that are much higher (~300 μg/m3) than typical ambient or even occupational levels. Recent animal studies provide insight into the potential mechanisms underlying observed respiratory and cardiovascular health responses; however, because of unrealistically high DE concentrations, the mechanisms elucidated in these studies may not be relevant at lower DE exposure levels. Although larger in number, and suggestive of possible mechanisms for non-cancer health effects at elevated DE levels, interpretation of this recent group of clinical-study findings and laboratory-animal results remains hindered by inconsistencies and variability in outcomes, potentially irrelevant DE-exposure compositions, limitations in exposure protocols and pathways, and uncertainties in extrapolation and generalization. A mechanism of action that allows reliable prediction of adverse health effects at DE-exposure levels typical of the present-day ambient and occupational environment has not emerged. Because of changing diesel-engine technology, inhalation studies using realistic environmental and occupational exposures of new-technology diesel exhaust are of critical importance.

Health effects research and regulation of diesel exhaust: an historical overview focused on lung cancer risk.

The mutagenicity of organic solvent extracts from diesel exhaust particulate (DEP), first noted more than 55 years ago, initiated an avalanche of diesel exhaust (DE) health effects research that now totals more than 6000 published studies. Despite an extensive body of results, scientific debate continues regarding the nature of the lung cancer risk posed by inhalation of occupational and environmental DE, with much of the debate focused on DEP. Decades of scientific scrutiny and increasingly stringent regulation have resulted in major advances in diesel engine technologies. The changed particulate matter (PM) emissions in “New Technology Diesel Exhaust (NTDE)” from today’s modern low-emission, advanced-technology on-road heavy-duty diesel engines now resemble the PM emissions in contemporary gasoline engine exhaust (GEE) and compressed natural gas engine exhaust more than those in the “traditional diesel exhaust” (TDE) characteristic of older diesel engines. Even with the continued publication of epidemiologic analyses of TDE-exposed populations, this database remains characterized by findings of small increased lung cancer risks and inconsistent evidence of exposure–response trends, both within occupational cohorts and across occupational groups considered to have markedly different exposures (e.g. truckers versus railroad shopworkers versus underground miners). The recently published National Institute for Occupational Safety and Health (NIOSH)-National Cancer Institute (NCI) epidemiologic studies of miners provide some of the strongest findings to date regarding a DE-lung cancer association, but some inconsistent exposure–response findings and possible effects of bias and exposure misclassification raise questions regarding their interpretation. Laboratory animal studies are negative for lung tumors in all species, except for rats under lifetime TDE-exposure conditions with durations and concentrations that lead to “lung overload.” The species specificity of the rat lung response to overload, and its occurrence with other particle types, is now well-understood. It is thus generally accepted that the rat bioassay for inhaled particles under conditions of lung overload is not predictive of human lung cancer hazard. Overall, despite an abundance of epidemiologic and experimental data, there remain questions as to whether TDE exposure causes increased lung cancers in humans. An abundance of emissions characterization data, as well as preliminary toxicological data, support NTDE as being toxicologically distinct from TDE. Currently, neither epidemiologic data nor animal bioassay data yet exist that directly bear on NTDE carcinogenic potential. A chronic bioassay of NTDE currently in progress will provide data on whether NTDE poses a carcinogenic hazard, but based on the significant reductions in PM mass emissions and the major changes in PM composition, it has been hypothesized that NTDE has a low carcinogenic potential. When the International Agency for Research on Cancer (IARC) reevaluates DE (along with GEE and nitroarenes) in June 2012, it will be the first authoritative body to assess DE carcinogenic health hazards since the emergence of NTDE and the accumulation of data differentiating NTDE from TDE.

Diesel exhaust particulate (DEP) and nanoparticle exposures: What do DEP human clinical studies tell us about potential human health hazards of nanoparticles?

Engineered nanoparticles (ENPs) are increasingly tested in cellular and laboratory-animal experiments for hazard potential, but there is a lack of health effects data for humans exposed to ENPs. However, human data for another source of nanoparticle (NP) exposure are available, notably for the NPs contained in diesel exhaust particulate (DEP). Studies of human volunteers exposed to diesel exhaust (DE) in research settings report DEP-NP number concentrations (i.e., >106 particles/cm3) that exceed number concentrations reported for worst-case exposure conditions for workers manufacturing and handling ENPs. Recent human DE exposure studies, using sensitive physiological instrumentation and well-characterized exposure concentrations and durations, suggest that elevated DE exposures from pre-2007 engines may trigger short-term changes in, for example, lung and systemic inflammation, thrombogenesis, vascular function, and brain activity. Considerable uncertainty remains both as to which DE constituents underlie the observed responses (i.e., DEP NPs, DEP mass, DE gases), and as to the implications of the observed short-term changes for the development of disease. Even so, these DE human clinical data do not give evidence of a unique toxicity for NPs as compared to other small particles. Of course, physicochemical properties of toxicological relevance may differ between DEP NPs and other NPs, yet overall, the DE human clinical data do not support the idea that elevated levels of NPs per se (at least in the DEP context) must be acutely toxic by virtue of their nano-sized nature alone.