Exposure, epidemiology and human cancer incidence of naphthalene

Genotoxicity assessment in HepaRG™ cells as a new approach methodology follow up to a positive response in the human TK6 cell micronucleus assay: Naphthalene case study

We are evaluating the use of metabolically competent HepaRG™ cells combined with CometChip® for DNA damage and the micronucleus (MN) assay as a New Approach Methodology (NAM) alternative to animals for follow up genotoxicity assessment to in vitro positive genotoxic response. Naphthalene is genotoxic in human TK6 cells inducing a nonlinear dose-response for the induction of micronuclei in the presence of rat liver S9. of naphthalene. In HepaRG™ cells, naphthalene genotoxicity was assessed using either 6 (CometChip™) or 12 concentrations of naphthalene (MN assay) with the top dose used for assessment of genotoxicity for the Comet and MN assay was 1.25 and 1.74 mM respectively, corresponding to approximately 45% cell survival. In contrast to human TK6 cell with S9, naphthalene was not genotoxic in either the HepaRG™ MN assay or the Comet assay using CometChip®. The lack of genotoxicity in both the MN and comet assays in HepaRG™ cells is likely due to Phase II enzymes removing phenols preventing further bioactivation to quinones and efficient detoxication of naphthalene quinones or epoxides by glutathione conjugation. In contrast to CYP450 mediated metabolism, these Phase II enzymes are inactive in rat liver S9 due to lack of appropriate cofactors causing a positive genotoxic response. Rat liver S9-derived BMD10 over-predicts naphthalene genotoxicity when compared to the negative genotoxic response observed in HepaRG™ cells. Metabolically competent hepatocyte models like HepaRG™ cells should be considered as human-relevant NAMs for use genotoxicity assessments to reduce reliance on rodents.

Direct Mass Spectrometric Analysis of Semivolatiles Derived from Real Samples at Atmospheric Pressure

This study demonstrated a facile ionization method with the use of real samples for the ionization of their main compositions at ambient conditions for mass spectrometric analysis. Analyte ions derived from the real samples were readily observed in the mass spectrum when placing the samples close (≤1 mm) to the inlet of the mass spectrometer applied with a high voltage. No additional accessories such as an ionization emitter, a plasma generator, or a high voltage power supply were required for this approach. Ionization of semivolatiles derived from the samples occurred between the samples and the inlet of the mass spectrometer presumably owing to the dielectric breakdown induced by the electric field provided by the mass spectrometer. Real samples including plants, medicine tablets, and gloves with contaminants were used as the model samples. The putative ionization mechanisms are also discussed in this study.

Computer aided pharmacokinetic profiling and toxicity analysis of naphthalene

Naphthalene is an aromatic hydrocarbon used as room freshner. Therefore, it is of interest to document the computer aided pharmacokinetic profiling and toxicity analysis data of naphthalene.

Investigation by mass spectrometry and 32P post-labelling of DNA adducts formation from 1,2-naphthoquinone, an oxydated metabolite of naphthalene

Naphthalene is the simplest representative of polycyclic aromatic hydrocarbons (PAHs). It is detected as major pollutant in the different compartments of the environment. This compound is considered by the international agency for research on cancer (IARC), the specialized cancer agency of the World Health Organisation (WHO), as a possible carcinogenic (group 2B) since 2002, mainly based on studies on chronic inhalation in rodent by the national toxicology program of the U.S. department of health and human services. In humans, its main metabolites correspond to derivatives substituted in position and 1 and 2 as 1,2-naphthoquinone (1,2-NphQ). Based on previous studies, 1,2-NphQ is supposed to react with DNA to form mostly depurinating adducts, a possible initiating step of carcinogenicity. To confirm this potentiality, adducts were synthetized by the reaction of 1,2-NphQ with 2'-deoxyguanosine (2'-dG) in N,N-dimethylformamide (DMF), water and calf thymus DNA. 2'-dG adducts were analyzed by ³²P post-labelling, HPLC with ultra-violet detection and ultra-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). We found stable DNA adducts detected in DNA. We proposed a formation mechanism by a 1,4-Michael addition with 2'-dG. Adducts with 2'-deoxyxanthosine are formed after a spontaneous deamination of 2'-dG. These adducts are good candidates as biomarkers allowing evaluation of exposure to naphthalene and its derivatives in the development of pathologies such as cancer.

Mapping metabolome changes in Luffa aegyptiaca Mill fruits at different maturation stages via MS-based metabolomics and chemometrics

Introduction

Luffa aegyptiaca Mill, sponge gourd or Egyptian cucumber, is grown worldwide for its edible fruit consumed as a vegetable like cucumber. Unlike young fruit (YF), the fully mature ripened fruit (MF) is strongly fibrous and is used as a cleanser to make scrubbing bath sponges. YF undergoes a complex series of physiological and biochemical changes during fruit ripening. However, the chemical compositional differences between YF and MF in Luffa aegyptiaca have not been distinguished to date.

Objectives

Comprehensively compare the metabolites profile of YF and MF to give insight on how maturation stage affects chemical composition.

Methods

Mass-based metabolomics comprising GC/MS and UHPLC/MS were adopted in this study targeting its volatile and non-volatile metabolites coupled with chemometrics to rationalize for the differences.

Results

A total of 53 volatile metabolites were identified via headspace solid phase microextraction (SPME) comprising 66.2% aldehydes/furans, 51.6% alcohols, 38.2% ketones, 15.1% acids and 10.1% aromatics of which aldehydes/ furans were dominant at both fruit stages. Young fruit was in general more erniched in metabolites as revealed from UHPLC/MS and GC/MS analyses. The YF group encompassed higher levels of short chain alcohols (1-octen-3-ol) and aldehydes ((E)-2-hexenal and cucumber aldehyde) in addition to terpenoids (linalool). In contrast, fatty acids (octanoic acid) predominated MF specimens. UHPLC/MS analysis revealed for several oleanene triterpene glycosides as major secondary bioactive compounds, dihydroxy-oxo-oleanenoic acid glycoside found more abundant in YF versus MF as revealed from multivariate data analyses.

Conclusions

Our results reveal for the distinct metabolite changes in L. aegyptiaca fruit in its different stages and to rationalize for its different usage.

Toxicokinetic Interaction between Hepatic Disposition and Pulmonary Bioactivation of Inhaled Naphthalene Studied Using Cyp2abfgs-Null and CYP2A13/2F1-Humanized Mice with Deficient Hepatic Cytochrome P450 Activity

Previous studies using Cyp2abfgs-null (lacking all genes of the Cyp2a, 2b, 2f, 2g, and 2s subfamilies), CYP2A13/2F1-humanized, and liver-Cpr-null (LCN) mice showed that although hepatic cytochrome P450 (P450) enzymes are essential for systemic clearance of inhaled naphthalene (a possible human carcinogen), both hepatic and extrahepatic P450 enzymes may contribute to naphthalene-induced lung toxicity via bioactivation. Herein, we aimed to further understand the toxicokinetics of inhaled naphthalene in order to provide a basis for predicting the effects of variations in rates of xenobiotic disposition on the extent of target tissue bioactivation. We assessed the impact of a hepatic deficit in naphthalene metabolism on the toxicokinetics of inhaled naphthalene using newly generated Cyp2abfgs-null-and-LCN and CYP2A13/2F1-humanized-and-LCN mice. We determined plasma, lung, and liver levels of naphthalene and naphthalene-glutathione conjugate, a biomarker of naphthalene bioactivation, over time after naphthalene inhalation. We found that the loss of hepatic naphthalene metabolism severely decreased naphthalene systemic clearance and caused naphthalene to accumulate in the liver and other tissues. Naphthalene release from tissue, as evidenced by the continued increase in plasma naphthalene levels after termination of active inhalation exposure, was accompanied by prolonged bioactivation of naphthalene in the lung. In addition, transgenic expression of human CYP2A13/2F1 in the respiratory tract caused a reduction in plasma naphthalene levels (by 40%, relative to Cyp2abfgs-null-and-LCN mice) and corresponding decreases in naphthalene-glutathione levels in the lung in mice with hepatic P450 deficiency, despite the increase in local naphthalene-bioactivating P450 activity. Thus, the bioavailability of naphthalene in the target tissue has a significant effect on the extent of naphthalene bioactivation in the lung. SIGNIFICANCE STATEMENT: In this study, we report several novel findings related to the toxicokinetics of inhaled naphthalene, the ability of which to cause lung carcinogenesis in humans is a current topic for risk assessment. We show the accumulation of naphthalene in the liver and lung in mice with compromised hepatic cytochrome P450 (P450) activity; the ability of tissue-stored naphthalene to redistribute to the circulation after termination of active inhalation exposure, prolonging exposure of target tissues to naphthalene; and the ability of non-CYP2ABFGS enzymes of the lung to bioactivate naphthalene. These results suggest potentially large effects of deficiencies in hepatic P450 activity on naphthalene tissue burden and bioactivation in human lungs.

Excretion of polycyclic aromatic hydrocarbon metabolites (OH-PAHs) in cattle urine in Ghana

Previous studies of polycyclic aromatic hydrocarbons (PAHs) in particulate matter, soils and livers of wild rats indicated that the city centre of Kumasi, Ghana has been severely polluted with high cancer potency. Cattle urine were therefore collected from Kumasi (urban) and Offinso (rural), Ghana: to determine concentrations of urinary PAH metabolites (OH-PAHs); and find their association with sex; and to estimate exposure of cattle to PAHs from the different sites. From the results, geometric mean concentrations (adjusted by specific gravity), GM_SG, showed that 2-OHNaphthalene (2-OHNap) was the most abundant OH-PAH in cattle urine from all study sites, and naphthalene-containing-mothballs might have contributed significantly to the levels. There was no significant difference between urinary OH-PAHs concentrations in cattle from urban and rural sites except for 2-OHPhe and 4-OHPhe, and similar to urban areas, rural sites could also be polluted with PAHs. GM_SG of 2-OHNap in cattle urine in Kokote (21.9 ± 6.51 ng/mL; a rural area), was significantly higher compared to the other sites followed by Oforikrom (4.15 ± 4.37 ng/mL; urban). The GM_SG concentration (ng/mL) of the sum of OH-PAHs decreased in the order, Kokote (44.7) > Oforikrom (7.87) > Saboa (6.98) > Santasi (6.68) > and Twumasen Estate (5.23). The high concentrations of urinary 2-OHNap, 2-3-OHFlu, 2-OHPhe, 3-OHPhe and 4-OHPhe in Kokote indicated high PAHs exposure to cattle in this area or different/specific source of PAHs exposure. GM_SG of 2-OHNap was significantly higher in male cattle compared to females while 1-9-OHPhe was significantly higher in females.

Protective effect of diallyl trisulfide against naphthalene-induced oxidative stress and inflammatory damage in mice

The aim of this study was to investigate the possible protective effects of diallyl trisulfide (DATS) against naphthalene-induced oxidative and inflammatory damage in the livers and lungs of mice. Elevated serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels showed significant hepatic damage after the challenge with 100 mg/kg naphthalene. Hepatic malondialdehyde (MDA) contents and the activity of myeloperoxidase (MPO) increased significantly, accompanying a decrease in the hepatic activity of total superoxide dismutase (SOD) and glutathione (GSH) levels after the naphthalene damage. In addition, the serum levels of nitric oxide (NO), tumor necrosis factor α (TNF-α), and interleukin 8 (IL-8) increased significantly in the groups damaged with naphthalene. The main parameters related to oxidative stress and inflammatory responses in the lungs, including the NO, MPO, and GSH contents, were determined, and the histopathological and immunohistochemical changes in the lung and liver tissues were also observed. In the DATS-treated groups, all of the oxidative and inflammatory damage in the serum, liver, and lung tissues were significantly prevented.

Hypothesis-based weight-of-evidence evaluation and risk assessment for naphthalene carcinogenesis

Inhalation of naphthalene causes olfactory epithelial nasal tumors in rats (but not in mice) and benign lung adenomas in mice (but not in rats). The limited available human data have not identified an association between naphthalene exposure and increased respiratory cancer risk. Assessing naphthalene's carcinogenicity in humans, therefore, depends entirely on experimental evidence from rodents. We evaluated the respiratory carcinogenicity of naphthalene in rodents, and its potential relevance to humans, using our Hypothesis-Based Weight-of-Evidence (HBWoE) approach. We systematically and comparatively reviewed data relevant to key elements in the hypothesized modes of action (MoA) to determine which is best supported by the available data, allowing all of the data from each realm of investigation to inform interpretation of one another. Our analysis supports a mechanism that involves initial metabolism of naphthalene to the epoxide, followed by GSH depletion, cytotoxicity, chronic inflammation, regenerative hyperplasia, and tumor formation, with possible weak genotoxicity from downstream metabolites occurring only at high cytotoxic doses, strongly supporting a non-mutagenic threshold MoA in the rat nose. We also conducted a dose-response analysis, based on the likely MoA, which suggests that the rat nasal MoA is not relevant in human respiratory tissues at typical environmental exposures. Our analysis illustrates how a thorough WoE evaluation can be used to support a MoA, even when a mechanism of action cannot be fully elucidated. A non-mutagenic threshold MoA for naphthalene-induced rat nasal tumors should be considered as a basis to determine human relevance and to guide regulatory and risk-management decisions.

Cytotoxicity of naphthalene toward cells from target and non-target organs in vitro

Chronic inhalation exposure to high concentrations of naphthalene produced nasal tumors in rats and lung tumors in female mice. Naphthalene bioactivation is required for target organ toxicity and cytotoxicity in target organs may be involved in tumor development. The present studies characterized the dose-response relationships for naphthalene-induced glutathione (GSH) depletion, effects on cellular ATP, and cytotoxicity in cells from both target (lung, nasal epithelium) and non-target (liver) organs in vitro using cells from F-344 rats, B6C3F1 mice and humans. The cells were incubated with various concentrations of naphthalene in sealed glass flasks for 3h, then placed in monolayer culture in fresh media for 24h to examine the repair or progression of damage. Naphthalene was a low potency cytotoxicant in vitro, with 500 μM frequently observed as a no-observed adverse effect concentration or lowest observed adverse effect concentration. Naphthalene exposure produced dose-dependent decreases in cellular GSH, ATP and viability in rat, mouse and human hepatocytes at concentrations >500 μM. Human nasal respiratory epithelial cells exhibited greater naphthalene cytotoxicity than rat or mouse nasal respiratory epithelial cell preparations. Rat nasal respiratory epithelial cell preparations metabolized naphthalene through pathways leading to the preferential formation of 1,2-naphthoquinone GSH conjugates rather than 1,4-naphthoquinone GSH conjugates observed in rat hepatocytes or mouse nasal respiratory epithelial cells, consistent with the suggestion that this bioactivation pathway may be involved in rat nasal tumor development. Naphthalene exposures of ≥500 μM decreased cellular GSH and ATP in rat, mouse and human lung cell preparations. The variability of the responses of the human lung cell preparations was consistent with the known variability of CYP activities in human lung tissue. The results of these studies can be used as the basis for future studies of the mechanisms involved in naphthalene-induced cytotoxicity and the relevance of the bioactivation pathways for human exposure to naphthalene.

Volatile aromatic hydrocarbons (VAHs) in residential indoor air in Brisbane, Australia

Volatile aromatic hydrocarbons (VAHs: benzene, toluene, ethylbenzene, mp-xylene, o-xylene, styrene, naphthalene) in residential indoor air in Brisbane, Australia were measured in 32 houses. The total VAHs (TVAHs) levels ranged between 2 and 137μg/m(3) and were lower than the most of the houses in the literature data. The VAHs were believed to originate from heat insulation systems, building material products as well motor vehicles but naphthalene and styrene originated from other sources. Internal garages had concentrations which are higher than the indoor air by 25-50% due to the presence of motor vehicles and may be a major source of indoor VAHs. However indoor concentrations are higher than that in the outdoor ambient air. The age of the house was found to be negatively related to VAHs concentrations in the houses with the half-life of TVAH at approximately 13years. The concentration levels of benzene, toluene, ethylbenzene and styrene are well below the guideline values set by agencies from Hong Kong, Japan, Germany and the WHO while the concentration level of naphthalene in one house exceeded the guideline value from Germany.

Sources, concentrations, and risks of naphthalene in indoor and outdoor air

Naphthalene is a ubiquitous pollutant, and very high concentrations are sometimes encountered indoors when this chemical is used as a pest repellent or deodorant. This study describes the distribution and sources of vapor-phase naphthalene concentrations in four communities in southeast Michigan, USA. Outdoors, naphthalene was measured in the communities and at a near-road site. Indoors, naphthalene levels were characterized in 288 suburban and urban homes. The median outdoor concentration was 0.15 μg/m(3), and a modest contribution from rush-hour traffic was noted. The median indoor long-term concentration was 0.89 μg/m(3), but concentrations were extremely skewed and 14% of homes exceeded 3 μg/m(3), the chronic reference concentration for non-cancer effects, 8% exceeded 10 μg/m(3), and levels reached 200 μg/m(3). The typical excess individual lifetime cancer risk was about 10(-4) and reached 10(-2) in some homes. Important sources include naphthalene's use as a pest repellent and deodorant, migration from attached garages and, to lesser extents, cigarette smoke and vehicle emissions. Excessive use as a repellent caused the highest concentrations. Naphthalene presents high risks in a subset of homes, and policies and actions to reduce exposures, for example, sales bans or restrictions, improved labeling, and consumer education, should be considered.

PRACTICAL IMPLICATIONS

Long-term average concentrations of naphthalene in most homes fell into the 0.2-1.7 μg/m(3) range reported as representative in earlier studies. The highly skewed distribution of concentrations results in a subset of homes with elevated concentrations and health risks that greatly exceed US EPA and World Health Organization (WHO) guidelines. The most important indoor source is the use of naphthalene as a pest repellant or deodorant; secondary sources include presence of an attached garage, cigarette smoking, and outdoor sources. House-to-house variation was large, reflecting differences among the residences and naphthalene use practices. Stronger policies and educational efforts are needed to eliminate or modify indoor usage practices of this chemical.

Urinary naphthol metabolites and chromosomal aberrations in 5-year-old children

BACKGROUND

Exposure to naphthalene, an International Agency for Research on Cancer (IARC)-classified possible carcinogen and polycyclic aromatic hydrocarbon (PAH), is widespread, though resulting health effects are poorly understood. Metabolites of naphthalene, 1- and 2-naphthol, are measurable in urine and are biomarkers of personal exposure. Chromosomal aberrations, including translocations, are established markers of cancer risk and a biodosimeter of clastogenic exposures. Although prenatal (maternal) PAH exposure predicts chromosomal aberrations in cord blood, few studies have examined chromosomal aberrations in school-age children and none has examined their association with metabolites of specific PAHs.

METHODS

Using Whole Chromosome Paint Fluorescent in situ Hybridization, we documented chromosomal aberrations including translocations, in 113 five-year-old urban minority children and examined their association with concurrent concentrations of PAH metabolites measured in urine.

RESULTS

We report that in lymphocytes, the occurrence and frequency of chromosomal aberrations including translocations are associated with levels of urinary 1- and 2-naphthol. When doubling the levels of urinary naphthols, gender-adjusted OR for chromosomal aberrations are 1.63 [95% confidence interval (CI), 1.21-2.19] and 1.44 (95% CI, 1.02-2.04) for 1- and 2-naphthol, respectively; and for translocations OR = 1.55 (95% CI, 1.11-2.17) and 1.92 (95% CI, 1.20-3.08) for 1- and 2-naphthol, respectively.

CONCLUSION

Our results show that markers of exposure to naphthalene in children are associated with translocations in a dose-related manner, and that naphthalene may be a clastogen.

IMPACT

Indoor exposure to elevated levels of naphthalene is prevalent in large regions of the world. This study is the first to present an association between a marker of naphthalene exposure and a precarcinogenic effect in humans.

Dose-response assessment of naphthalene-induced genotoxicity and glutathione detoxication in human TK6 lymphoblasts

The dose-response relationship for the induction of micronuclei (MN) and the impact of glutathione (GSH) detoxication on naphthalene-induced cytotoxicity and genotoxicity were investigated in human TK6 cells. TK6 cells were exposed to 10 concentrations ranging from 0.0625 to 30μM naphthalene in the presence of β-naphthoflavone- and phenobarbital (βNP/PB)-induced rat liver S9 with a nicotinamide adenine dinucleotide phosphate-generating system. Three approaches were used to identify a no-observed-effect level (NOEL) for naphthalene-induced genotoxicity: (1) laboratory criteria of ≥ twofold increase over the concurrent solvent controls (NOEL = 10μM), (2) ANOVA with Bonferroni correction (NOEL = 2.5μM), and (3) the benchmark dose approach (BMCL(10) = 3.35μM). The NOEL and point of departure micronucleus frequency for naphthalene-induced MN are between the tested naphthalene concentrations of 2.5-10.0μM in this experimental system. Supplementation of the exposure system with physiological relevant concentrations of 5mM GSH eliminated naphthalene-induced cytotoxicity and genotoxicity; no increased cytotoxicity or genotoxicity was observed at concentrations of up to 500μM naphthalene in the presence of GSH compared with 2.5-10.0μM in the absence of GSH. Naphthalene bioactivation by βNP/PB-induced rat liver S9 exhibits a nonlinear dose-response for the induction of MN in TK6 cells with a NOEL of 2.5-10μM that in the presence of GSH is shifted upward greater than 50- to 200-fold. These data demonstrate a nonlinear dose-response for naphthalene-induced genotoxicity that is eliminated by GSH, and both observations should be considered when assessing human risk from naphthalene exposures.

Naphthalene animal carcinogenicity and human relevancy: overview of industries with naphthalene-containing streams

Bioassays in mice and rats exposed via inhalation to naphthalene show incidences of lung and nasal cancer, respectively. To address the question of human relevancy, a literature search for cancer case reports in workers exposed to naphthalene was performed, along with an evaluation of major studies from industries with naphthalene-containing streams having the highest naphthalene exposures and/or most extensive epidemiology data. Although no epidemiologic studies of workers exposed solely to naphthalene were found, a population-based case-control study of oral and oropharynx cancer found no relation with naphthalene exposure. Limited case reports of laryngeal and colorectal cancer and naphthalene exposure exist, but these data are inadequate for evaluating human cancer risk. No case reports of nasal tumors were found, which is informative because case reports have historically identified several occupational carcinogens. Combined with anatomic and metabolic differences between rodent and human upper airways and data suggesting that cancer potency based on the rat bioassay is overestimated, relevancy of rat nasal tumors to humans is questionable. For lung cancer, existing human studies are insufficient to make firm conclusions about the presence or absence of a potential naphthalene-related risk, although no occupationally-related lung cancer risks were identified in the industries evaluated.

p53 codon 271 CGT to CAT mutant fraction does not increase in nasal respiratory and olfactory epithelia of rats exposed to inhaled naphthalene

A 2-year rat tumor bioassay testing whole body exposure to naphthalene (NA) vapor found a significant increase in nasal respiratory epithelial adenomas in male rats and in olfactory epithelial neuroblastomas in female rats. To obtain mechanistic insight into NA-induced nasal carcinogenesis, NA dose-response was characterized in nasal epithelium using a tumor-relevant endpoint. Specifically, levels of p53 codon 271 CGT to CAT mutation were measured in nasal respiratory and olfactory epithelium of NA-exposed male and female rats by allele-specific competitive blocker-PCR (ACB-PCR). Male and female, 8-9 week-old F344 rats (5 rats/group) were exposed to 0, 0.1, 1.0, 10, and 30ppm NA vapor for 13 weeks (6h/day, 5 days/week). The geometric mean p53 mutant fraction (MF) levels in nasal epithelium of control treatment groups ranged between 2.05 × 10(-5) and 3.05 × 10(-5). No significant dose-related changes in p53 mutant fraction (MF) were observed in the olfactory or respiratory epithelia of female rats. However, statistically significant treatment-related differences were observed in male respiratory and olfactory epithelium, with the p53 MF in the respiratory epithelium of male rats exposed to 30ppm NA significantly lower than that in controls. Further, a significant trend of decreasing p53 MF with increasing dose was observed in the male respiratory epithelium. Of the tissue types analyzed, respiratory epithelium is the most sensitive to the cytotoxic effects of NA, suggesting cytotoxicity may be responsible for the loss of p53 mutation. Because ACB-PCR has been used successfully to detect the effects of known mutagenic carcinogens, the absence of any significant increases in p53 MF associated with NA exposure adds to the weight of evidence that NA does not operate through a directly mutagenic mode of action.

A critical review of naphthalene sources and exposures relevant to indoor and outdoor air

Both the recent classification of naphthalene as a possible human carcinogen and its ubiquitous presence motivate this critical review of naphthalene's sources and exposures. We evaluate the environmental literature on naphthalene published since 1990, drawing on nearly 150 studies that report emissions and concentrations in indoor, outdoor and personal air. While naphthalene is both a volatile organic compound and a polycyclic aromatic hydrocarbon, concentrations and exposures are poorly characterized relative to many other pollutants. Most airborne emissions result from combustion, and key sources include industry, open burning, tailpipe emissions, and cigarettes. The second largest source is off-gassing, specifically from naphthalene's use as a deodorizer, repellent and fumigant. In the U.S., naphthalene's use as a moth repellant has been reduced in favor of para-dichlorobenzene, but extensive use continues in mothballs, which appears responsible for some of the highest indoor exposures, along with off-label uses. Among the studies judged to be representative, average concentrations ranged from 0.18 to 1.7 microg m(-3) in non-smoker's homes, and from 0.02 to 0.31 microg m(-3) outdoors in urban areas. Personal exposures have been reported in only three European studies. Indoor sources are the major contributor to (non-occupational) exposure. While its central tendencies fall well below guideline levels relevant to acute health impacts, several studies have reported maximum concentrations exceeding 100 microg m(-3), far above guideline levels. Using current but draft estimates of cancer risks, naphthalene is a major environmental risk driver, with typical individual risk levels in the 10(-4) range, which is high and notable given that millions of individuals are exposed. Several factors influence indoor and outdoor concentrations, but the literature is inconsistent on their effects. Further investigation is needed to better characterize naphthalene's sources and exposures, especially for indoor and personal measurements.

Screening-level population risk assessment of nasal tumors in the US due to naphthalene exposure

Several naphthalene Unit Risk Factors (URFs) were proposed by the US Environmental Protection Agency in 2004 using data on the development of olfactory epithelial neuroblastomas and nasal respiratory epithelial adenomas in rats, but these URFs may be inappropriate and unnecessarily conservative for estimating human cancer risks. The purpose of the present exercise was to perform a screening-level population risk assessment of the US population to compare the observed number of naphthalene-induced nasal tumors in the US to the number that would be predicted if the URFs for naphthalene were as proposed. Nine scenarios were evaluated to represent the range of exposures individuals have typically experienced. Results indicate that the total predicted burden of naphthalene-induced nasal tumors per year in the US (65,905 rare nasal tumors, of which 29,121 are olfactory epithelial neuroblastomas) is much greater than the number of these tumors actually observed per year (910 total nasal tumors, of which 66 are olfactory neuroblastomas) over the period 1973-2006. This suggests that using rat nasal tumor data to derive a naphthalene URF for humans should be re-evaluated.

Available data on naphthalene exposures: strengths and limitations

Exposures to naphthalene occur from a number of sources. Data on these exposures have been reported by a number of researchers; however, these data have not been organized into a consistent framework. In addition, while naphthalene has been identified as an animal inhalation carcinogen there has been no attempt to define the ranges of long-term average inhalation exposures for the general population or sub populations that are relevant to the assessment of the risk of the occurrence of chronic effects. This paper attempts to organize the available data on naphthalene levels in air and provide ranges for the long-term average concentrations experienced by different populations. The paper also discusses the limitations of available data and identifies areas for future research.

Naphthalene metabolism in relation to target tissue anatomy, physiology, cytotoxicity and tumorigenic mechanism of action

This report provides a summary of deliberations conducted under the charge for members of Module C Panel participating in the Naphthalene State-of-the-Science Symposium (NS(3)), Monterey, CA, October 9-12, 2006. The panel was charged with reviewing the current state of knowledge and uncertainty about naphthalene metabolism in relation to anatomy, physiology and cytotoxicity in tissues observed to have elevated tumor incidence in these rodent bioassays. Major conclusions reached concerning scientific claims of high confidence were that: (1) rat nasal tumor occurrence was greatly enhanced, if not enabled, by adjacent, histologically related focal cellular proliferation; (2) elevated incidence of mouse lung tumors occurred at a concentration (30 ppm) cytotoxic to the same lung region at which tumors occurred, but not at a lower and less cytotoxic concentration (tumorigenesis NOAEL=10 ppm); (3) naphthalene cytotoxicity requires metabolic activation (unmetabolized naphthalene is not a proximate cause of observed toxicity or tumors); (4) there are clear regional and species differences in naphthalene bioactivation; and (5) target tissue anatomy and physiology is sufficiently well understood for rodents, non-human primates and humans to parameterize species-specific physiologically based pharmacokinetic (PBPK) models for nasal and lung effects. Critical areas of uncertainty requiring resolution to enable improved human cancer risk assessment were considered to be that: (1) cytotoxic naphthalene metabolites, their modes of cytotoxic action, and detailed low-dose dose-response need to be clarified, including in primate and human tissues, and neonatal tissues; (2) mouse, rat, and monkey inhalation studies are needed to better define in vivo naphthalene uptake and metabolism in the upper respiratory tract; (3) in vivo validation studies are needed for a PBPK model for monkeys exposed to naphthalene by inhalation, coupled to cytotoxicity studies referred to above; and (4) in vivo studies are needed to validate a human PBPK model for naphthalene. To address these uncertainties, the Panel proposed specific research studies that should be feasible to complete relatively promptly. Concerning residual uncertainty far less easy to resolve, the Panel concluded that environmental, non-cytotoxic exposure levels of naphthalene do not induce tumors at rates that can be predicted meaningfully by simple linear extrapolation from those observed in rodents chronically exposed to far greater, cytotoxic naphthalene concentrations.

A review of whole animal bioassays of the carcinogenic potential of naphthalene

This report provides a summary of deliberations conducted under the charge for members of Module A participating in the Naphthalene State-of-the-Science Symposium (NS3), Monterey, CA, October 9-12, 2006. Whole animal bioassays have been performed by the National Toxicology Program in mice and rats to ascertain the carcinogenic potential of naphthalene by inhalation exposure. A statistically significant increased incidence of pulmonary alveolar/bronchiolar adenoma (a benign lesion), was observed among female mice; an observed increase among the males did not reach statistical significance. No nasal tumors were observed in either sex. A tumorigenic response was observed in both sexes of rats, in males an increased incidence of nasal respiratory epithelium adenoma (a benign rather than malignant lesion) and in females, olfactory epithelial neuroblastoma. Interpretations of these studies vary. On the one hand, evidence of extensive non-neoplastic response in both sexes of both species indicates cytotoxicity occurred at all doses, and strongly suggests that cytotoxicity played a significant role in the tumor responses observed in the target tissues. On the other hand, olfactory epithelial neuroblastoma has rarely been observed in NTP bioassays. This review seeks to develop a consensus understanding of the scientific evidence provided by these studies, taking into account that they have been used as the basis for quantitative human cancer risk assessment, and suggests scientific studies that, if performed, could resolve scientific uncertainties.