Formaldehyde and cancer risk: a quantitative review of cohort studies through 2006

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.

Formaldehyde and leukemia: epidemiology, potential mechanisms, and implications for risk assessment

Formaldehyde is widely used in the United States and other countries. Occupational and environmental exposures to formaldehyde may be associated with an increased risk of leukemia in exposed individuals. However, risk assessment of formaldehyde and leukemia has been challenging due to inconsistencies in human and animal studies and the lack of a known mechanism for leukemia induction. Here, we provide a summary of the symposium at the Environmental Mutagen Society Meeting in 2008, which focused on the epidemiology of formaldehyde and leukemia, potential mechanisms, and implication for risk assessment, with emphasis on future directions in multidisciplinary formaldehyde research. Updated results of two of the three largest industrial cohort studies of formaldehyde-exposed workers have shown positive associations with leukemia, particularly myeloid leukemia, and a recent meta-analysis of studies to date supports this association. Recent mechanistic studies have shown the formation of formaldehyde-induced DNA adducts and characterized the essential DNA repair pathways that mitigate formaldehyde toxicity. The implications of the updated findings for the design of future studies to more effectively assess the risk of leukemia arising from formaldehyde exposure were discussed and specific recommendations were made. A toxicogenomic approach in experimental models and human exposure studies, together with the measurement of biomarkers of internal exposure, such as formaldehyde-DNA and protein adducts, should prove fruitful. It was recognized that increased communication among scientists who perform epidemiology, toxicology, biology, and risk assessment could enhance the design of future studies, which could ultimately reduce uncertainty in the risk assessment of formaldehyde and leukemia.

Epidemiological studies of formaldehyde exposure and risk of leukemia and nasopharyngeal cancer: a meta-analysis

The authors conducted meta-analyses of the epidemiological literature on formaldehyde exposure and risk of leukemia and risk of nasopharyngeal cancer. The authors abstracted study results and confounder information from cohort and case-control studies, and used quantile plots and regression models to evaluate heterogeneity and possible publication bias. No evidence of serious heterogeneity or publication bias was seen. For leukemias, the summary relative risk (RR) was 1.05 (95% confidence interval [CI]: 0.93, 1.20) for cohort studies, and the summary odds ratio (OR) was 0.99 (95% CI: 0.71, 1.37) for case-control studies. Based on cohort and case-control studies, no significant differences were seen by leukemia subtype, job type, publication period, or region. Summary estimates for nasopharyngeal cancers were not elevated after excluding a single plant with an unexplained cluster of nasopharyngeal cancers (cohort RR = 0.72, 95% CI: 0.40, 1.28). The summary estimate was increased for case-control studies overall, but the summary OR for smoking-adjusted studies was 1.10 (95% CI: 0.80, 1.50). Previous meta-analyses showed elevated summary estimates for leukemia; however, these analyses included results from proportionate mortality studies and did not explore other factors that could influence or confound results. By limiting analyses to stronger case-control and cohort study designs, considering the effects of smoking and ignoring anomalous results from a single plant, our meta-analyses provide little support for a causal relationship between formaldehyde exposure and leukemia or nasopharyngeal cancer.

Commentary: mechanistic considerations for associations between formaldehyde exposure and nasopharyngeal carcinoma

Occupational exposure to formaldehyde has been linked to nasopharyngeal carcinoma. To date, mechanistic explanations for this association have primarily focused on formaldehyde-induced cytotoxicity, regenerative hyperplasia and DNA damage. However, recent studies broaden the potential mechanisms as it is now well established that formaldehyde dehydrogenase, identical to S-nitrosoglutathione reductase, is an important mediator of cGMP-independent nitric oxide signaling pathways. We have previously described mechanisms by which formaldehyde can influence nitrosothiol homeostasis thereby leading to changes in pulmonary physiology. Considering evidences that nitrosothiols govern the Epstein-Barr virus infection cycle, and that the virus is strongly implicated in the etiology of nasopharyngeal carcinoma, studies are needed to examine the potential for formaldehyde to reactivate the Epstein-Barr virus as well as additively or synergistically interact with the virus to potentiate epithelial cell transformation.

An internal reaction chamber in dimethylglycine oxidase provides efficient protection from exposure to toxic formaldehyde

We report a synthetic biology approach to demonstrate substrate channeling in an unusual bifunctional flavoprotein dimethylglycine oxidase. The catabolism of dimethylglycine through methyl group oxidation can potentially liberate toxic formaldehyde, a problem common to many amine oxidases and dehydrogenases. Using a novel synthetic in vivo reporter system for cellular formaldehyde, we found that the oxidation of dimethylglycine is coupled to the synthesis of 5,10-methylenetetrahydrofolate through an unusual substrate channeling mechanism. We also showed that uncoupling of the active sites could be achieved by mutagenesis or deletion of the 5,10-methylenetetrahydrofolate synthase site and that this leads to accumulation of intracellular formaldehyde. Channeling occurs by nonbiased diffusion of the labile intermediate through a large solvent cavity connecting both active sites. This central "reaction chamber" is created by a modular protein architecture that appears primitive when compared with the sophisticated design of other paradigm substrate-channeling enzymes. The evolutionary origins of the latter were likely similar to dimethylglycine oxidase. This work demonstrates the utility of synthetic biology approaches to the study of enzyme mechanisms in vivo and points to novel channeling mechanisms that protect the cell milieu from potentially toxic reaction products.

Chemical analysis of late stages of pheomelanogenesis: conversion of dihydrobenzothiazine to a benzothiazole structure

Pheomelanogenesis is a complex pathway that starts with the oxidation of tyrosine (or DOPA, 3,4-dihydroxyphenylalanine) by tyrosinase in the presence of cysteine, which results in the production of 5-S-cysteinyldopa and its isomers. Beyond that step, relatively little has been clarified except for a possible intermediate produced, dihydro-1,4-benzothiazine-3-carboxylic acid (DHBTCA). We therefore carried out a detailed study on the course of pheomelanogenesis using DOPA and cysteine and the physiological enzyme tyrosinase. To elucidate the later stages of pheomelanogenesis, chemical degradative methods of reductive hydrolysis with hydroiodic acid and alkaline peroxide oxidation were applied. The results show that: (1) DHBTCA accumulates after the disappearance of the cysteinyldopa isomers, (2) DHBTCA is then oxidized by a redox exchange with dopaquinone to form ortho-quinonimine, which leads to the production of pheomelanin with a benzothiazine moiety, and (3) the benzothiazine moiety gradually degrades to form a benzothiazole moiety. This latter process is consistent with the much higher ratio of benzothiazole-derived units in human red hair than in mouse yellow hair. These findings may be relevant to the (photo)toxic effects of pheomelanin.

Age-incidence curves of nasopharyngeal carcinoma worldwide: bimodality in low-risk populations and aetiologic implications

The distinct geographic variation in the global incidence of nasopharyngeal carcinoma reflects a complex etiology involving viral, environmental, and genetic components. The high to intermediate rates observed in endemic areas contrast markedly with the uniformly low rates seen in much of the world. An interesting epidemiologic observation is the early peak in age-incidence curves observed in certain geographically disparate populations, suggestive of distinct causal entities and the possible exhaustion of susceptible individuals from the population at a certain age. The aim of this study was to systematically evaluate the age-incidence profiles of NPC worldwide on partitioning populations according to level of risk, in an effort to provide clues about the importance of early-in-life factors and genetic susceptibility. Using data from 23 high-quality population-based cancer registries for the period 1983-1997, a key finding was the consistent pattern of bimodality that emerged across low-risk populations, irrespective of geographic location. Continual increases in NPC risk by age up to a first peak in late adolescence/early adulthood (ages 15-24 years) were observed, followed by a second peak later in life (ages 65-79 years). No such early peak in NPC incidence by age group was evident among the high-risk populations studied. These findings are discussed according to existing lines of biological and epidemiologic evidence related to level of population risk, age at diagnosis, and histologic subtype. A modified model for NPC tumor development is proposed on the basis of these observations.

Formaldehyde exposure and leukemia: a new meta-analysis and potential mechanisms

Formaldehyde is an economically important chemical, to which more than 2 million U.S. workers are occupationally exposed. Substantially more people are exposed to formaldehyde environmentally, as it is generated by automobile engines, is a component of tobacco smoke and is released from household products, including furniture, particleboard, plywood, and carpeting. The International Agency for Research on Cancer (IARC) recently classified formaldehyde as a human carcinogen that causes nasopharyngeal cancer and also concluded that there is "strong but not sufficient evidence for a causal association between leukemia and occupational exposure to formaldehyde". Here, we review the epidemiological studies published to date on formaldehyde-exposed workers and professionals in relation to lymphohematopoietic malignances. In a new meta-analysis of these studies, focusing on occupations known to have high formaldehyde exposure, we show that summary relative risks (RRs) were elevated in 15 studies of leukemia (RR=1.54; confidence interval (CI), 1.18-2.00) with the highest relative risks seen in the six studies of myeloid leukemia (RR=1.90; 95% CI, 1.31-2.76). The biological plausibility of this observed association is discussed and potential mechanisms proposed. We hypothesize that formaldehyde may act on bone marrow directly or, alternatively, may cause leukemia by damaging the hematopoietic stem or early progenitor cells that are located in the circulating blood or nasal passages, which then travel to the bone marrow and become leukemic stem cells. To test these hypotheses, we recommend that future studies apply biomarkers validated for other chemical leukemogens to the study of formaldehyde.