One-year inhalation toxicity study of formaldehyde in male rats with a damaged or undamaged nasal mucosa

A Comprehensive Literature Review on the Effects of Formaldehyde on the Upper Respiratory Tract

Prolonged exposure to indoor air pollutants at high concentrations can have adverse health effects on the respiratory system of individuals who spend most of their time indoors. Formaldehyde (FA) is a common indoor air pollutant because of its extensive use in household products such as cleaners, floorings, and furnishings. As a chemical, FA is highly water soluble and reactive. When its airborne form is inhaled, it is mainly absorbed in the upper airways. FA has been extensively studied for its carcinogenic effects, but it can also cause inflammation in the upper airways. The objective of the current review was to assess the secondary effects of such inflammation and how it can contribute to an increased risk for upper respiratory infections, which are mostly caused by viruses. A rigorous literature review was conducted through gathering, reading, and analyzing relevant literature, including peer-reviewed articles published after 1990 and seminal literature regardless of publication date. Findings from the review provide a greater understanding of the outcomes of FA exposure, the potential accumulative damage to the upper respiratory tract, and the associated increased risk for acute infections of the upper respiratory tract. This information can help in the development and enforcement of stricter regulations for furniture and building materials for household-related products to limit exposure to indoor pollutants such as FA.

Benchmark Dose Modeling Approaches for Volatile Organic Chemicals using a Novel Air-Liquid Interface In Vitro Exposure System

Inhalation is the most relevant route of volatile organic chemical (VOC) exposure; however, due to unique challenges posed by their chemical properties and poor solubility in aqueous solutions, in vitro chemical safety testing is predominantly performed using direct application dosing/submerged exposures. To address the difficulties in screening toxic effects of VOCs, our cell culture exposure system permits cells to be exposed to multiple concentrations at air-liquid interface (ALI) in a 24-well format. ALI exposure methods permit direct chemical-to-cell interaction with the test article at physiological conditions. In the present study, BEAS-2B and primary normal human bronchial epithelial cells (pHBEC) are used to assess gene expression, cytotoxicity, and cell viability responses to a variety of volatile chemicals including acrolein, formaldehyde, 1,3-butadiene, acetaldehyde, 1-bromopropane, carbon tetrachloride, dichloromethane, and trichloroethylene. BEAS-2B cells were exposed to all the test agents, while pHBECs were only exposed to the latter four listed above. The VOC concentrations tested elicited only slight cell viability changes in both cell types. Gene expression changes were analyzed using benchmark dose (BMD) modeling. The BMD for the most sensitive gene set was within one order of magnitude of the threshold-limit value reported by the American Conference of Governmental Industrial Hygienists, and the most sensitive gene sets impacted by exposure correlate to known adverse health effects recorded in epidemiologic and in vivo exposure studies. Overall, our study outlines a novel in vitro approach for evaluating molecular-based points-of-departure in human airway epithelial cell exposure to volatile chemicals.

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

Using mechanistic information to support evidence integration and synthesis: a case study with inhaled formaldehyde and leukemia

Formaldehyde is one of the most comprehensively studied chemicals, with over 30 years of research focused on understanding the development of cancer following inhalation. The causal conclusions regarding the potential for leukemia are largely based on the epidemiological literature, with little consideration of cancer bioassays, dosimetry studies, and mechanistic research, which challenge the biological plausibility of the disease. Recent reanalyzes of the epidemiological literature have also raised significant questions related to the purported associations between formaldehyde and leukemia. Because of this, considerable scientific debate and uncertainty remain on whether there is a causal association between formaldehyde inhalation exposure and leukemia. Further complexity in evaluating this association is related to the endogenous production of formaldehyde. Multiple modes of action (MOA) have been postulated for the development of leukemia following formaldehyde inhalation that includes unsupported hypotheses of direct or indirect toxicity to the target cell population. Herein, the available evidence relevant to evaluating the postulated MOAs for leukemia following formaldehyde inhalation exposure is organized in the IPCS MOA Framework. The integration of all the available evidence clearly highlights the limited amount of data that support any of the postulated MOAs and demonstrates a significant amount of research supporting the null hypothesis that there is no causal association between formaldehyde inhalation exposure and leukemia. These analyses result in a lack of confidence in any of the postulated MOAs, increasing confidence in the conclusion that there is a lack of biological plausibility for a causal association between formaldehyde inhalation exposure and leukemia.

Wound Healing of the Nasal and Paranasal Mucosa: A Review

Background

Wound healing is a highly coordinated process involving clot formation, inflammatory reaction, immune response, and, finally, tissue remodeling and maturation. Only few data regarding the specific healing of the nasal or sinusal mucosa are available.

Methods

After a short summary of the general principles of wound healing, the most important data regarding in vitro or in vivo models of wound healing of the nasal and paranasal mucosa are discussed. Attention is paid to clinical application.

Main Findings

First observations regarding the specific regulation of epithelial regeneration by growth factors have underlined the complex relationship between extracellular matrix and epithelium during the repair process. However, only poor and aspecific correlations can be described between endoscopically and histomorphologically defined postoperative phases.

Six years after the NRC review of EPA's Draft IRIS Toxicological Review of Formaldehyde: Regulatory implications of new science in evaluating formaldehyde leukemogenicity

Shortly after the International Agency for Research on Cancer (IARC) determined that formaldehyde causes leukemia, the United States Environmental Protection Agency (EPA) released its Draft IRIS Toxicological Review of Formaldehyde ("Draft IRIS Assessment"), also concluding that formaldehyde causes leukemia. Peer review of the Draft IRIS Assessment by a National Academy of Science committee noted that "causal determinations are not supported by the narrative provided in the draft" (NRC 2011). They offered recommendations for improving the Draft IRIS assessment and identified several important research gaps. Over the six years since the NRC peer review, significant new science has been published. We identify and summarize key recommendations made by NRC and map them to this new science, including extended analysis of epidemiological studies, updates of earlier occupational cohort studies, toxicological experiments using a sensitive mouse strain, mechanistic studies examining the role of exogenous versus endogenous formaldehyde in bone marrow, and several critical reviews. With few exceptions, new findings are consistently negative, and integration of all available evidence challenges the earlier conclusions that formaldehyde causes leukemia. Given formaldehyde's commercial importance, environmental ubiquity and endogenous production, accurate hazard classification and risk evaluation of whether exposure to formaldehyde from occupational, residential and consumer products causes leukemia are critical.

Lowest adverse effects concentrations (LOAECs) for formaldehyde exposure

In 2012 the Committee for Risk Assessment (RAC) of the European Chemicals Agency concluded that 2ppm formaldehyde represent a Lowest Observed Adverse Effect Concentration (LOAEC) for polypoid adenomas, histopathological lesions and cell proliferation. An analysis of all data shows that a LOAEC of 2ppm it is not justified for cell proliferation and polypoid adenomas. Higher values are also supported by a new statistical analysis. For histopathological lesions a NOAEC of 1ppm may be defined but the lesions at 2ppm cannot be regarded as pre-stages for tumour development. One major uncertainty exists: the description of polypoid adenomas and the lesions at 2ppm often is insufficient and diagnostic uncertainties can only be resolved by a re-evaluation according to modern histomorphological standards. Although the discrepancy between our assessment and that of RAC may seem rather small we feel the LOAECs proposed by RAC must be challenged taking into consideration the broad data base for formaldehyde and the potential impact of any published RAC opinion on the present discussions about appropriate occupational and indoor exposure limits.

Exposure to formaldehyde and its potential human health hazards

A widely used chemical, formaldehyde is normally present in both indoor and outdoor air. The rapid growth of formaldehyde-related industries in the past two decades reflects the result of its increased use in building materials and other commercial sectors. Consequently, formaldehyde is encountered almost every day from large segments of society due to its various sources. Many governments and agencies around the world have thus issued a series of standards to regulate its exposure in homes, office buildings, workshops, public places, and food. In light of the deleterious properties of formaldehyde, this article provides an overview of its market, regulation standards, and human health effects.

Identifying an indoor air exposure limit for formaldehyde considering both irritation and cancer hazards

Formaldehyde is a well-studied chemical and effects from inhalation exposures have been extensively characterized in numerous controlled studies with human volunteers, including asthmatics and other sensitive individuals, which provide a rich database on exposure concentrations that can reliably produce the symptoms of sensory irritation. Although individuals can differ in their sensitivity to odor and eye irritation, the majority of authoritative reviews of the formaldehyde literature have concluded that an air concentration of 0.3 ppm will provide protection from eye irritation for virtually everyone. A weight of evidence-based formaldehyde exposure limit of 0.1 ppm (100 ppb) is recommended as an indoor air level for all individuals for odor detection and sensory irritation. It has recently been suggested by the International Agency for Research on Cancer (IARC), the National Toxicology Program (NTP), and the US Environmental Protection Agency (US EPA) that formaldehyde is causally associated with nasopharyngeal cancer (NPC) and leukemia. This has led US EPA to conclude that irritation is not the most sensitive toxic endpoint and that carcinogenicity should dictate how to establish exposure limits for formaldehyde. In this review, a number of lines of reasoning and substantial scientific evidence are described and discussed, which leads to a conclusion that neither point of contact nor systemic effects of any type, including NPC or leukemia, are causally associated with exposure to formaldehyde. This conclusion supports the view that the equivocal epidemiology studies that suggest otherwise are almost certainly flawed by identified or yet to be unidentified confounding variables. Thus, this assessment concludes that a formaldehyde indoor air limit of 0.1 ppm should protect even particularly susceptible individuals from both irritation effects and any potential cancer hazard.

Is exposure to formaldehyde in air causally associated with leukemia?--A hypothesis-based weight-of-evidence analysis

Recent scientific debate has focused on the potential for inhaled formaldehyde to cause lymphohematopoietic cancers, particularly leukemias, in humans. The concern stems from certain epidemiology studies reporting an association, although particulars of endpoints and dosimetry are inconsistent across studies and several other studies show no such effects. Animal studies generally report neither hematotoxicity nor leukemia associated with formaldehyde inhalation, and hematotoxicity studies in humans are inconsistent. Formaldehyde's reactivity has been thought to preclude systemic exposure following inhalation, and its apparent inability to reach and affect the target tissues attacked by known leukemogens has, heretofore, led to skepticism regarding its potential to cause human lymphohematopoietic cancers. Recently, however, potential modes of action for formaldehyde leukemogenesis have been hypothesized, and it has been suggested that formaldehyde be identified as a known human leukemogen. In this article, we apply our hypothesis-based weight-of-evidence (HBWoE) approach to evaluate the large body of evidence regarding formaldehyde and leukemogenesis, attending to how human, animal, and mode-of-action results inform one another. We trace the logic of inference within and across all studies, and articulate how one could account for the suite of available observations under the various proposed hypotheses. Upon comparison of alternative proposals regarding what causal processes may have led to the array of observations as we see them, we conclude that the case for a causal association is weak and strains biological plausibility. Instead, apparent association between formaldehyde inhalation and leukemia in some human studies is better interpreted as due to chance or confounding.

Cancer effects of formaldehyde: a proposal for an indoor air guideline value

Formaldehyde is a ubiquitous indoor air pollutant that is classified as "Carcinogenic to humans (Group 1)" (IARC, Formaldehyde, 2-butoxyethanol and 1-tert-butoxypropanol-2-ol. IARC monographs on the evaluation of carcinogenic risks to humans, vol 88. World Health Organization, Lyon, pp 39-325, 2006). For nasal cancer in rats, the exposure-response relationship is highly non-linear, supporting a no-observed-adverse-effect level (NOAEL) that allows setting a guideline value. Epidemiological studies reported no increased incidence of nasopharyngeal cancer in humans below a mean level of 1 ppm and peak levels below 4 ppm, consistent with results from rat studies. Rat studies indicate that cytotoxicity-induced cell proliferation (NOAEL at 1 ppm) is a key mechanism in development of nasal cancer. However, the linear unit risk approach that is based on conservative ("worst-case") considerations is also used for risk characterization of formaldehyde exposures. Lymphohematopoietic malignancies are not observed consistently in animal studies and if caused by formaldehyde in humans, they are high-dose phenomenons with non-linear exposure-response relationships. Apparently, these diseases are not reported in epidemiological studies at peak exposures below 2 ppm and average exposures below 0.5 ppm. At the similar airborne exposure levels in rodents, the nasal cancer effect is much more prominent than lymphohematopoietic malignancies. Thus, prevention of nasal cancer is considered to prevent lymphohematopoietic malignancies. Departing from the rat studies, the guideline value of the WHO (Air quality guidelines for Europe, 2nd edn. World Health Organization, Regional Office for Europe, Copenhagen, pp 87-91, 2000), 0.08 ppm (0.1 mg m(-3)) formaldehyde, is considered preventive of carcinogenic effects in compliance with epidemiological findings.

Effects of formaldehyde inhalation on the junctional proteins of nasal respiratory mucosa of rats

Exposure to formaldehyde, which is an organic compound, disturbs the integrity of nasal mucosa. In this study, we aimed to clarify the protein changes in the junctional complex of nasal mucosa of Wistar rats exposed to formaldehyde inhalation. The study was performed in 20 female Wistar rats. Rats were divided into two groups randomly. Control rats were allowed free access to standard rat chaw and tap water (n:10). Experimental group was exposed to formaldehyde vapor at 15ppm, 6h/day, 5 days/week for 12 weeks (n:10). Histological evaluation of the experimental model was determined by hematoxylin-eosin (HE) and periodic acid Schiff (PAS) stainings of paraffin-embedded nasal mucosa tissues and by electron microscopy. The effects of formaldehyde inhalation on the distribution of occludin, E-cadherin, and gamma-catenin were assessed by immunohistochemistry. The nasal mucosa of the experimental group was correlated with hypertrophy in goblet cell, degeneration in basal lamina, stratification of epithelium, and proliferation. Thickness of basal lamina and also local degenerative regions, vacuole increase in cytoplasmic areas, irregular forms of kinocilium and loss of sharpness in the kinocilium membrane were the findings at the ultrastructural level. The expressions of E-cadherin, occludin, gamma-catenin proteins in intercellular junctional complexes of rat nasal mucosa were also decreased in experimental group compared to control group. The findings of the present study indicated that formaldehyde vapor inhalation in the concentrations and duration of exposure used in the present experiment significantly decreased the density of structural proteins of the junctional complex in the nasoepithelium. It was suggested that, the formaldehyde inhalation could cause complete impairment of intercellular junctional complexes and disturb the tissue integrity in nasal mucosa at higher concentrations.

Formaldehyde as a potential human leukemogen: an assessment of biological plausibility

The International Agency for Research on Cancer (IARC, 2004) recently reevaluated the epidemiological data on formaldehyde and concluded that there was "strong but not sufficient evidence for a causal association between leukaemia and occupational exposure to formaldehyde." This conclusion was tempered since a mechanism for leukemia induction could not be identified. Chemically induced leukemia is a well-studied phenomenon with benzene and a number of cancer chemotherapeutic drugs recognized as capable of causing this effect. Abundant in vitro and in vivo data in animals and humans demonstrate that exposure to sufficient doses of these recognized leukemogens can initiate a cascade of events leading to hematopoietic toxicity and the subsequent development of leukemia. This review addresses the biological plausibility that formaldehyde might be capable of causing any type of leukemia by providing a broad overview of the scientific data that must be considered in order to support or refute a conclusion that a particular substance might be leukemogenic. Data on benzene and selected chemotherapeutic cancer drugs are used as examples and are briefly summarized to demonstrate the similar biological events thought to result in leukemogenesis. These data are compared and contrasted with the available data on formaldehyde in order to judge whether they fulfill the criteria of biological plausibility that formaldehyde would be capable of inducing leukemia as suggested by the epidemiological data. Based on the epidemiological data, it is reasonable to expect that if formaldehyde was capable of inducing leukemia, in vivo and in vitro data would offer supporting evidence for biological plausibility. In particular, there is (1) no evidence to suggest that formaldehyde reaches any target organ beyond the site of administration including the bone marrow, (2) no indication that formaldehyde is toxic to the bone marrow/hematopoietic system in in vivo or in vitro studies, and (3) no credible evidence that formaldehyde induces leukemia in experimental animals. As discussed in this review, based on the key biological events that occur in the process of chemically induced leukemia, there is inadequate biological evidence currently available to corroborate existing weak epidemiological associations. This provides an insufficient database to conclude that there is a causal relationship for formaldehyde and leukemia risk.

Inhaled formaldehyde: evaluation of sensory irritation in relation to carcinogenicity

OBJECTIVES

The critical health effects of formaldehyde exposure include sensory irritation and the potential to induce tumours in the upper respiratory tract. In literature, a concentration as low as 0.24 ppm has been reported to be irritating to the respiratory tract in humans. Nasal tumour-inducing levels in experimental animals seem to be 1-2 orders of magnitude larger. In this paper, the subjectively measured sensory irritation threshold levels in humans are discussed in line with findings obtained in animal experiments. In addition, a Benchmark dose (BMD) analysis of sensory irritation was used to estimate response incidences at different formaldehyde concentrations.

METHODS

Data on respiratory irritation and carcinogenicity of formaldehyde were retrieved from public literature and discussed. BMD analysis was carried out on human volunteer studies using the US-EPA BMD software.

RESULTS

Subjective measures of irritation were the major data found in humans to examine sensory (eye and nasal) irritation; only one study reported objectively measured eye irritation. On a normalized scale, mild/slight eye irritation was observed at levels 1 ppm, and mild/slight respiratory tract irritation at levels 2 ppm. With the BMD software, it was estimated that at a level of 1 ppm, only 9.5% of healthy volunteers experience 'moderate' (i.e., annoying) eye irritation (95% upper confidence limit). An important factor modulating the reported levels of irritation and health symptoms most probably includes the perception of odour intensity. In several studies, the 0-ppm control condition was missing. From the results of the long-term inhalation toxicity studies in experimental animals, a level of 1 ppm formaldehyde has been considered a NOAEL for nasal injury.

CONCLUSIONS

Sensory irritation is first observed at levels of 1 ppm and higher. From both human and animal studies, it was concluded that at airborne levels for which the prevalence of sensory irritation is minimal both in incidence and degree (i.e., <1 ppm), risks of respiratory tract cancer are considered to be negligibly low.

Risk assessment of formaldehyde for the general population in Japan

Formaldehyde is used in the production of resins, molding compounds, photographic film, bactericide, and tissue preservative. The purpose of this study was to provide an up-to-date critical review of the information to the toxicological profile of formaldehyde, and to assess the risk of formaldehyde for the general population in Japan. Inhaled formaldehyde is an effective sensory irritant at a dosage of 0.5 ppm in mice. Following inhalation in laboratory animals, more than 6 ppm formaldehyde causes degenerative non-neoplastic effects in mice and monkeys and nasal tumors in rats. It is considered that formaldehyde induces genotoxic effects directly in vitro and secondarily in vivo. Sensory irritation of the eyes and respiratory tract in response to inhalation exposure to formaldehyde has been reported at 0.08 ppm and above in human study. Formaldehyde is carcinogenic at the site of contact as a consequence of epithelial cell regenerative proliferation resulting from cytotoxicity and mutation, based on studies in both animals and humans. Levels of formaldehyde in atmosphere detected in rural, suburban, and urban areas in Japan were 2.5-3.2 ppb from 1998 to 2003. The majority of the population is exposed to atmosphere concentrations of formaldehyde less than those associated with sensory irritation. The reference concentration of formaldehyde in atmosphere for the Japanese general population is recommended to be 0.01 ppm.

Local effects in the respiratory tract: relevance of subjectively measured irritation for setting occupational exposure limits

OBJECTIVES

Chemosensory effects of stimulation by a chemical can either be irritating (trigeminal stimulation) or odorous (olfactory stimulation) or both. For odorous irritants, a clear-cut distinction between odour and irritation is difficult to make. The differences in the lowest concentration found to be irritating to the respiratory tract in humans when compared to experimental animals has brought much debate in the process of setting occupational exposure limits (OELs) for such chemicals. In this paper it will be discussed as to how far subjectively measured sensory irritation threshold levels can be used to establish OELs.

METHODS

Data on respiratory irritation of four odorous irritants were retrieved from public literature and discussed, viz. acetone, formaldehyde, furfural and sulphur dioxide.

RESULTS

Objective measures of irritation yielded results that differed from subjective evaluations. Important factors modulating the reported levels of irritation and health symptoms include the perception of odour intensity, exposure history and the individual's bias to report irritation on the basis of his/her prejudice or knowledge of the compound.

CONCLUSIONS

Subjective measures alone are less appropriate for establishing sensory irritation thresholds of odorous irritants and are, therefore, less suitable to establish OELs without supporting evidence. Objectively measured irritation in humans, the Alarie assay (an experimental animal test assessing the concentration that results in a 50% reduction of the breathing frequency) and repeated exposure studies in animals may be of help to study objective irritation. If subjective measurements are used to study sensory irritation, the study design should at least include: measurement of both incidence and severity determined at several concentrations, an appropriate (0 ppm) control condition, preferably a non-irritant odorant control exposure, validated questionnaires and correct concentration measurements.

Toxicologic evaluation of licorice extract as a cigarette ingredient

Licorice extract (block, powder or liquid) may be applied to cigarette tobacco at levels of about 1-4% to enhance and harmonize the flavor characteristics of smoke, improve moisture holding characteristics of tobacco, and act as a surface active agent for ingredient application. Neat material pyrolysis studies, and smoke chemistry and biological activity studies (bacterial mutagenicity, cytotoxicity, micronucleus, and sub-chronic inhalation) with mainstream smoke, or mainstream smoke preparations from cigarettes containing various target levels (1.5-12%) of the licorice extracts were performed to provide data for an assessment of the use of licorice extract as a cigarette tobacco ingredient. At simulated tobacco burning temperatures up to 900 degrees C all forms of neat licorice extract pyrolyzed extensively, yielding small amounts of benzene, toluene, phenol and acetaldehyde with no indication that licorice extracts would transfer intact to mainstream smoke. As a single ingredient added to cigarette tobacco, block licorice extract at a target level of 12.5% increased smoke constituents including selected PAH, arsenic, lead, phenol and formaldehyde (on a TPM basis), while licorice extract powder (target level of 8% tobacco) increased select PAH, phenol and formaldehyde (on a TPM basis). Lower target application levels (including typical application levels) of block, powder or liquid licorice extract did not significantly alter the smoke chemistry profile. Biological tests indicated no relevant difference in the genotoxic or cytotoxic potential of either mainstream smoke (or smoke preparations) from cigarettes with added licorice extracts compared to control cigarettes. In sub-chronic 90-day rat inhalation studies, the mainstream smoke from cigarettes with 12.5% added block and 8% added powder licorice extract contained higher formaldehyde concentrations compared to control cigarette smoke. Female rats in the 12.5% block licorice extract exposure group displayed an increased incidence and severity of epithelial hyperplasia in the nose (level 2), with no relevant respiratory tract changes in the 8% powder licorice extract exposed rats. At the lower licorice extract application levels (1.25-5%), there was no indication of increased formaldehyde concentration in the smoke atmosphere and no relevant changes in respiratory tract tissues. Mineralcorticoid-like effects which have been associated with excess licorice ingestion were not found in any of the smoke inhalation studies. The results of these studies with various forms of licorice extract applied to cigarette tobacco suggest that adding licorice extract to cigarette tobacco at levels of < or =5% does not discernibly alter the smoke chemistry or biological effects normally associated with mainstream cigarette smoke.

The implausibility of leukemia induction by formaldehyde: a critical review of the biological evidence on distant-site toxicity

Formaldehyde is a naturally occurring biological compound that is present in tissues, cells, and bodily fluids. It is also a potent nasal irritant, a cytotoxicant at high doses, and a nasal carcinogen in rats exposed to high airborne concentrations. The normal endogenous concentration of formaldehyde in the blood is approximately 0.1 mM in rats, monkeys, and humans, and it is 2- to 4-fold higher in the liver and nasal mucosa of the rat. Inhaled formaldehyde enters the one-carbon pool, and the carbon atom is rapidly incorporated into macromolecules throughout the body. Oxidation to formate catalyzed by glutathione-dependent and -independent dehydrogenases in nasal tissues is a major route of detoxication and generally precedes incorporation. The possibility that inhaled formaldehyde might induce various forms of distant-site toxicity has been proposed, but no convincing evidence for such toxicity has been obtained in experimental studies. This review summarizes the biological evidence that pertains to the issue of leukemia induction by formaldehyde, which includes: (1) the failure of inhaled formaldehyde to increase the formaldehyde concentration in the blood of rats, monkeys, or humans exposed to concentrations of 14.4, 6, or 1.9 ppm, respectively; (2) the lack of detectable protein adducts or DNA-protein cross-links (DPX) in the bone marrow of normal rats exposed to [3H]- and [14C]formaldehyde at concentrations as high as 15 ppm; (3) the lack of detectable protein adducts or DPX in the bone marrow of glutathione-depleted (metabolically inhibited) rats exposed to [3H]- and [14C]formaldehyde at concentrations as high as 10 ppm; (4) the lack of detectable DPX in the bone marrow of Rhesus monkeys exposed to [14C]formaldehyde at concentrations as high as 6 ppm; (5) the failure of formaldehyde to induce leukemia in any of seven long-term inhalation bioassays in rats, mice, or hamsters; and (6) the failure of formaldehyde to induce chromosomal aberrations in the bone marrow of rats exposed to airborne concentrations as high as 15 ppm or of mice injected intraperitoneally with formaldehyde at doses as high as 25 mg/kg. Biological evidence that might be regarded as supporting the possibility of leukemia induction by formaldehyde includes: (1) the detection of cytogenetic abnormalities in circulating lymphocytes in seven studies of human subjects exposed to ambient concentrations in the workplace (but not in seven other studies of human subjects or in rats exposed to 15 ppm); (2) the induction of leukemia in rats in a single questionable drinking water study with formaldehyde concentrations as high as 1.5 g/L (but not in three other drinking water studies with concentrations as high as 1.9 or 5 g/L); (3) the detection of chromosomal aberrations in the bone marrow of rats exposed to very low concentrations of formaldehyde (0.4 or 1.2 ppm) (but not in another study at concentrations as high as 15 ppm); and (4) an apparent increase in the fraction of protein-associated DNA (assumed to be due to DPX) in circulating lymphocytes of humans exposed to ambient concentrations in the workplace (1-3 ppm). This evidence is regarded as inconsequential for several reasons, including lack of reproducibility, inadequate reporting of experimental methods, inconsistency with other data, or insufficient analytical sensitivity, and therefore, it provides little justification for or against the possibility that inhaled formaldehyde may be a leukemogen. In contrast to these inconclusive findings, the abundance of negative evidence mentioned above is undisputed and strongly suggests that there is no delivery of inhaled formaldehyde to distant sites. Combined with the fact that formaldehyde naturally occurs throughout the body, and that multiple inhalation bioassays have not induced leukemia in animals, the negative findings provide convincing evidence that formaldehyde is not leukemogenic.

Inhaled formaldehyde: exposure estimation, hazard characterization, and exposure-response analysis

Formaldehyde has been assessed as a Priority Substance under the Canadian Environmental Protection Act. Probabilistic estimates of exposure of the general population in Canada to formaldehyde in ambient and indoor air are presented. Critical health effects include sensory irritation and the potential to induce tumors in the upper respiratory tract (the nasal region in rodents and potentially the lungs of humans). The majority of the general population is exposed to airborne concentrations of formaldehyde less than those typically associated with sensory irritation (i.e., 0.1 mg/m3). Based primarily upon data derived from laboratory studies, the inhalation of formaldehyde under conditions that induce cytotoxicity and sustained regenerative proliferation within the respiratory tract is considered to present a carcinogenic hazard to humans. At airborne levels for which the prevalence of sensory irritation is minimal (i.e., 0.1 mg/m3), risks of respiratory-tract cancers for the general population estimated on the basis of a biologically motivated case-specific model are exceedingly low. This biologically motivated case-specific model incorporates two-stage clonal expansion and is supported by dosimetry calculations from computational fluid dynamics analyses of formaldehyde flux in various regions of the nose and single-path modeling for the lower respiratory tract. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

A brief review of formaldehyde carcinogenesis in relation to rat nasal pathology and human health risk assessment

The 1980 report that inhaled formaldehyde induced nasal squamous cell carcinomas in rats had a significant societal impact and resulted in extensive research in the fields of rodent nasal pathology and human cancer risk assessment. This article presents an overview of the evolution of these events. It is concluded that the nasal passages of humans and rats are fundamentally identical biological target organs. Nevertheless, in the case of human health risk assessment, minor differences between these species may be critically important. Special attention should be paid to interspecies differences in nasal dosimetry and local metabolism; thus, chemical toxicity data derived from rats require careful interpretation when used for human risk assessments. In the case of formaldehyde, it is recommended that low-concentration (< or = 2 ppm airborne exposure) extrapolation, where no tissue damage is observed, be uncoupled from the responses at high concentrations (> or = 6 ppm), where epithelial degeneration, regenerative cell replication, and inflammation appear to be essential driving forces in formaldehyde carcinogenesis. The presence of treatment-related nasal lesions in rats following exposure to chemicals should always be treated as an indication of a potential human health risk, whether exposure is by the inhalation, oral, or dermal route.

Formaldehyde mechanistic data and risk assessment: endogenous protection from DNA adduct formation

Exposures of rodents to airborne formaldehyde (FA) produce dose-related toxicity, enhanced cell proliferation and squamous cell carcinomas in the nasal passages. The mechanism of FA-induced tumor formation involves DNA-protein crosslink formation and enhanced cell proliferation secondarily to cytotoxicity. The mucociliary apparatus and glutathione protect against low-dose FA-induced effects. Consequently, the mechanistic information is consistent with a very sublinear dose-response curve for tumor formation. The sublinear dose-response of nasal DNA-protein crosslinks levels in rodents and monkeys has been used in the risk assessment of FA.

Formaldehyde and cancer: a critical review

Formaldehyde is a naturally occurring chemical found in every human cell. It has been in widespread use for over a century as a disinfectant and preservative agent, and more recently in a number of industrial products. Animal studies indicate that formaldehyde is a rat carcinogen at high levels (> or = 10 ppm) of exposure. Results for lower levels of exposure show less clear-cut carcinogenic effects, and some species, such as mice and hamsters, appear much less sensitive to any carcinogenic potential of formaldehyde. Epidemiologic studies of the effects of formaldehyde exposure among humans provide inconsistent results. In general, these nonexperimental studies suffer from a number of biases and flaws. The epidemiologic studies fall into three categories: formaldehyde industry workers, case-control studies, and studies of professionals who use formaldehyde. Studies of industry workers with known exposure to formaldehyde report little evidence of an excess cancer risk. Nasopharyngeal cancer, the one cancer considered most strongly linked to formaldehyde among humans, appears after close examination to be likely a result of multiple subgroup analyses and misclassification. The case-control studies usually lack any direct measure of formaldehyde exposure and rely instead on hypothetical exposure based on occupational exposure matrices. Most of these studies, after adjustment for confounding factors, fail to find a significant association with putative formaldehyde exposure. The studies that do report a significant association suffer from methodologic problems limiting their interpretation.(ABSTRACT TRUNCATED AT 250 WORDS)

Subchronic (12-week) inhalation toxicity study of methanol-fueled engine exhaust in rats

To evaluate the inhalation toxicity to rats of exhaust at low concentration for longer periods, Fischer 344 rats were exposed to 3 concentrations of exhaust generated by an M85 methanol-fueled engine (methanol with 15% gasoline) without catalyst for 8 h/d, 6 d/wk for 4, 8, or 12 wk. Concentration- and time-dependent increase carboxyhemoglobin in the erythrocytes and decrease in cytochrome P-450 in the lungs were observed in all treated groups. Furthermore, significant increases in plasma formaldehyde were observed in all treated groups. Furthermore, significant increases in plasma formaldehyde were observed in the group exposed to the highest concentration of exhaust (carbon monoxide, 89.8 ppm; formaldehyde, 2.3 ppm; methanol, 8.1 ppm; nitrogen oxides, 22.9 ppm; nitrogen dioxide, 1.1 ppm) for 8 or 12 wk. No change of plasma folic acid was observed in any group, and no methanol or formic acid was detected in the plasma in any animals. Histopathologically, exposure-related changes were found only in the nasal cavity of the high-concentration group. Slight hyperplasia/squamous metaplasias of the respiratory epithelium lining the nasoturbinate and maxilloturbinate were observed after 4 wk of exposure, and the incidences and degrees of these lesions increased slightly with the exposure time. No changes were found in the olfactory epithelium of the nasal cavity. As judged by optical microscopy, the exhaust concentration with no effect on the nasal cavity under the experimental conditions was concluded to be the medium concentration level containing 0.55 ppm formaldehyde. In the present study, however, concentration- and time-dependent increase of carboxyhemoglobin in the erythrocytes and decrease of the lung P-450 level were observed. Therefore, further study on more long-term inhalation of lower concentrations of exhaust might be needed.

Recovery from changes in the blood and nasal cavity and/or lungs of rats caused by exposure to methanol-fueled engine exhaust

One group of male, pathogen-free, Fischer 344 rats was exposed to about 17-fold diluted exhaust generated by an M85 methanol-fueled engine (methanol with 15% gasoline) without catalyst for 8 h, and then the rates of recovery from the resulting increased levels of plasma formaldehyde and carboxyhemoglobin in their erythrocytes were measured. The carboxyhemoglobin level in the erythrocytes was restored within 4 h, whereas the plasma formaldehyde level was still elevated after 4 h but was restored to the normal level within 8 h. No methanol or formic acid was detected in the plasma. Another group of rats was exposed to the same dilution of exhaust for 8 h/d for 7 d, and then the recovery from histopathological damage of the nasal cavity and lungs was also examined. Hyperplasia/squamous metaplasia and erosion of the respiratory epithelium lining the nasoturbinate, maxilloturbinate, or nasal septum, and infiltration of neutrophils into the submucosa at level 1 (level of the posterior edge of the upper incisor teeth) were observed immediately after the exposure period. Lesions of the respiratory epithelium at level 2 (incisive papilla) were less than those at level 1. Slight lesions at levels 1 or 2 were still noticed 1 wk after exposure, but not 4 wk after exposure. Just after exposure, decreases of Clara cells in the terminal bronchiolus and of cilia in the bronchial/bronchiolar epithelium were also observed. Moreover, focal hypertrophy of alveolar walls and increase of macrophages were observed in parts adjacent to respiratory bronchiolus. One week after the exposure period, these changes were no longer seen. These results indicate that changes in the blood and in the nasal cavity and lungs caused by methanol-fueled engine exhaust are reversible. However, complete recovery from damage of the nasal cavity caused by 7-d exposure to the exhaust takes 4 wk, and recovery from elevated plasma formaldehyde and erythrocyte carboxyhemoglobin levels caused by a single 8-h exposure takes 4-8 h.

Toxicity to rats of methanol-fueled engine exhaust inhaled continuously for 28 days

Fischer 344 rats were exposed to three concentrations of exhaust generated by an M85 methanol-fueled engine (methanol with 15% gasoline) without catalyst for 8 h/d, 7 d/wk for 7, 14, 21, or 28 d. Concentration- and time-dependent yellowing of the fur was prominent in all treated groups. Concentration-dependent increases in the erythrocyte count, hematocrit, hemoglobin concentration, formaldehyde in plasma, and carboxyhemoglobin in the erythrocytes, and decrease in serum alkaline phosphatase activity were seen after all exposure periods. Histopathologically, lesions were found in the nasal cavity and lungs after 7 d of exposure. Squamous metaplasia of the respiratory epithelium of level 1 (level of the posterior edge of the upper incisor teeth) lining of the nasoturbinate and/or maxilloturbinate and infiltration of neutrophils into the submucosa, and decreases of Clara cells in the terminal bronchiolus and of cilia in the bronchiolar epithelium, were observed in the high-concentration group (carbon monoxide, 94 ppm; formaldehyde, 6.9 ppm; methanol, 17.9 ppm; nitrogen oxides, 52.7 ppm; nitrogen dioxide, 10.6 ppm). The histopathological extents of several lesions increased slightly with the exposure time. Slight squamous metaplasia and hyperplasia of the respiratory epithelium at level 1 were also observed in the medium-concentration group (one in three of the high-concentration group). No histopathological changes were found in the olfactory epithelium of the nasal cavity. In the low-concentration group (one in nine of the high-concentration group), no marked histopathological changes in these organs were observed. These results may suggest that the lesions observed in the nasal cavity of rats exposed to methanol-fueled engine exhaust were mainly caused by formaldehyde, although other components in the exhaust also may have affected nasal cavity and/or lungs to less extent.

Nasal tumours in rats after severe injury to the nasal mucosa and prolonged exposure to 10 ppm formaldehyde

To study the significance of damage to the nasal mucosa for the induction of nasal tumours by formaldehyde in rats, a long-term inhalation study was conducted in which male rats with severely damaged or undamaged nose were exposed 6 h/day for 5 days/week to 0, 0.1, 1.0 or 10 ppm formaldehyde vapour for 28 months, or for 3 months followed by a 25-month observation period. The damage to the nasal mucosa was induced by bilateral intranasal electrocoagulation. The total number of rats used was 720, 480 with damaged and 240 with intact nose. Compound-related degenerative, inflammatory and hyperplastic changes of the nasal respiratory and olfactory mucosa were invariably observed when rats with intact nose were exposed to 10 ppm but not when exposed to 1.0 or 0.1 ppm formaldehyde. Nasal electrocoagulation increased the incidences of formaldehyde-induced rhinitis, hyper- and metaplasia of the respiratory epithelium, and degeneration and hyper- and metaplasia of the olfactory epithelium. In addition, exposure to 10 ppm formaldehyde for 28 months produced nasal squamous cell carcinomas in rats with damaged nose (15/58) but not in rats with intact nose. Three months of exposure to 10 ppm formaldehyde or exposure to 0.1 or 1.0 ppm formaldehyde for 28 months had no such effect. It was concluded that severe damage to the nasal mucosa may contribute to the induction of nasal tumours by formaldehyde.