to release free formaldehyde to adversely affect blood cells or the bone marrow. Because explanations about how this might occur conflicts with the well-established biological and chemical behavior of formaldehyde, the hypothesized leukemogenic events appear to be unlikely in biological systems. There is also another practical aspect of this issue that needs to be addressed. Any explanation of how inhaled formaldehyde might increase endogenous concentrations (e.g., methanediol releasing free formaldehyde at distant sites) would have to overcome a large body of data dem- onstrating that due to the prodigious metabolic capacity of animals and humans, exogenous formaldehyde quite simply cannot raise endogenous levels. As explained by Heck and Casanova (2004), “An adult man…would absorb 30 μ g formaldehyde per minute if the formaldehyde con- centration were 2 ppm. Assuming that 93% of the inhaled formaldehyde is eliminated by saturable metabolism in the respiratory tract as calculated for both rats and monkeys… the maximum amount of residual formaldehyde that would be available for distribution to other tissues would be 7%.... If the residual formaldehyde were unmetabolized and distributed to total body water (41 L), its maximum concentration after 8 h would be less than 0.001 mM, which is well below the concentration of endogenous formalde- hyde in human blood (≈0.1 mM). Of course, metabolism in the blood and tissues would greatly reduce the actual con- centration of residual formaldehyde in total body water. Therefore, inhaled formaldehyde would not be expected to increase the formaldehyde concentration in the blood in accordance with the empirical results.” The empirical results referenced refer to studies in rats, monkeys, and humans demonstrating that inhaled formaldehyde does not change endogenous concentrations (Casanova et al., 1988; Heck et al., 1985. The above description was con- firmed by Franks (2004) who developed a mathematical model for the absorption and metabolism of formalde- hyde vapor by humans. This model, which accounted for numerous physiological parameters, including interfac- ing between air/mucus, mucous/epithelial tissues and blood, the calculations indicated that inhalation of form- aldehyde at 1.9 ppm would lead to a predicted increase of 0.00044 mg/L in blood concentrations, which is well below measured endogenous levels. Consequently, based on measurement as well as dosimetry modeling, there is no evidence that inhaled formaldehyde can increase the endogenous levels found in the blood. A tissue-based pharmacokinetic (PK) model esti- mated various forms of tissue formaldehyde (i.e., free and formaldehyde acetal [i.e., methanediol]) and tis- sue glutathione (GSH) in conjunction with a 13-week toxicogenomic study in rats exposed to formaldehyde at 0, 0.7, 2, 6, 10, or 15 ppm for 6 hours/day for 1, 4, or 13 weeks (Andersen et al., 2010). The pharmacokinetic analysis showed that the lower two inhaled formalde- hyde concentrations (0.7 and 2 ppm) would result in only minor changes in cellular GSH and formaldehyde acetal. However, at exposures above 4 ppm, formaldehyde acetal © 2011 Informa Healthcare USA, Inc. Indoor air exposure limit for formaldehyde 677 increased with a much steeper dose-response, whereas free GSH was significantly reduced. The model was also used to estimate the dose-response of formaldehyde acetal in relation to inhaled formaldehyde accounting for endogenous formaldehyde and showed that exposures in the range of 1–2 ppm would not be sufficient to cause significant increases in tissue formaldehyde compared to background in the normal tissues of the nose. This model is the first to describe background production of formaldehyde by normal physiological processes, the associated GSH status, and increases in tissue formal- dehyde acetal that would be expected following inhala- tion of various formaldehyde concentrations. As noted by Andersen et al. (2010), “Research on [formaldehyde] histopathology, gene expression, and now with PK mod- eling of endogenous [formaldehyde] is consistent with a threshold for carcinogenicity and tissue responses to this endogenous aldehyde.” This PK tissue model further dem- onstrates the substantial degree to which the kinetics and dose-response characteristics of inhaled formaldehyde can be modeled and understood in biological systems. 2.Stable isotope studies with inhaled 13CD2-formaldehyde The unlikelihood of exogenous formaldehyde entering the blood with transport to distant sites is now further supported by a recent study by Lu et al. (2010a) in which male F344 rats were exposed to 10 ppm of the stable iso- tope 13CD2-formaldehyde for 1 or 5 days (6 hours/day). Following the 1- or 5-day exposures, blood was collected for lymphocyte isolation as well as tissue samples from nasal respiratory epithelium, spleen, thymus, lung, and liver; bone marrow was collected from both femurs. DNA adducts from all tissues were subsequently prepared for analysis. Because of the slight mass differences between DNA adducts derived from exogenous (i.e., 13CD2- formaldehyde) compared to DNA adducts derived from endogenous (i.e., 12C-formaldehyde), the source of form- aldehyde-DNA adducts detected in each tissue could be determined. Whereas formaldehyde-DNA adducts from both endogenous and exogenous formaldehyde were detected in nasal epithelium after either 1 or 5 days of exposure, no 13CD2-formaldehyde-DNA adducts were detected in any tissue distal to the nasal epithe- lium, including the lung, spleen, liver, thymus, bone marrow, or lymphocytes. As described by the authors, “The absence of exogenous formaldehyde-induced DNA adducts and crosslinks in other tissues supports the con- clusion that genotoxic effects of inhaled formaldehyde are implausible at sites remote to the portal-of-entry.” In addi- tion, with respect to the issue concerning methanediol as an explanation for transport of inhaled formaldehyde to distant sites, Lu et al. (2010a) addressed this as well, “Furthermore, by monitoring the transitions that would occur if there was any hydrogen-deuterium exchange, we have demonstrated that neither inhaled formaldehyde, nor methanediol derived from inhaled formaldehyde reaches sites distant to the portal of entry.” These findings, particularly the inability to detect 13CD2-formaldehyde-