There are various reports of different odor thresholds (i.e., the minimum level that can be detected), with a typ- ical range of 0.5 to 1 ppm (ATSDR, 1999; US EPA, 2006c). Although some documents list odor thresholds at lower levels, these are likely to be obfuscated by the same accu- racy issues (i.e., false positives) as for sensory irritation. For example, a guidance document (US Coast Guard, 2001) notes that the odor threshold for formaldehyde is 0.8 ppm, but also states that persons with sensitive noses can detect it at levels as low as 0.1 ppm. Although it is likely that some highly sensitive individuals can detect the odor of formaldehyde at levels below 0.5 ppm, there are no empirical data (e.g., human volunteer chamber studies) documenting that humans can reliably detect formaldehyde at levels of 0.1 ppm or less. However, it is possible that people with a diminished ability to detect odors could still have symptoms of sensory irritation without detecting the pungent odor of formaldehyde. Finally, it is important to note that olfaction of formal- dehyde does not imply that any adverse effects are trig- gered, including the onset of sensory irritation (Gaffney and Paustenbach, 2007; Paustenbach and Gaffney, 2006; Dalton, 2002, 2003). Most of the early studies that investigated the irritant properties of formaldehyde did not properly account for the characteristic acrid or pungent odor. The ability of most individuals to detect the odor of formaldehyde is typically more sensitive than the lowest exposure level that produces the symptoms of sensory irritation. Therefore, studies designed to evaluate the irritation threshold must account for the odor of formaldehyde by masking it with an odoriferous but non-irritating sub- stance, such as methylmercaptan, in order for the study results to be valid. Failure to do so raises the likelihood that subjects will confuse the odor with symptoms of sensory irritation (Lang et al., 2008). Without account- ing for the confounding effects of odor, some early, largely uncontrolled, studies suggested that sensory irritation from formaldehyde was on the order of less than 0.01 ppm (10 ppb). Those studies did not control for the misinterpretation of odor detection as sensory irritation (Abraham, 2001) or for behavioral sensitiza- tion to the odor of formaldehyde (and other highly odoriferous compounds), which confounds the study of other properties of these compounds such as sensory irritation (Shusterman et al., 1988). Properly conducted studies, using appropriate controls, have more accu- rately determined the ability of humans to detect the odor of formaldehyde. These studies have failed to con- firm the ability of even the most sensitive individual to detect exceedingly low concentrations of formaldehyde. For example, ATSDR (2008) lists the lower level of odor detection as 0.5 ppm. This level has been confirmed by a number of human volunteer chamber studies, which have placed significant emphasis on removing the odor bias associated with formaldehyde exposure (Iwasaki and Ishiguro, 1978; Leonardos et al., 1969; Hellman and Small, 1974). © 2011 Informa Healthcare USA, Inc. Indoor air exposure limit for formaldehyde 679 Like the majority of other odoriferous compounds, formaldehyde has an odor threshold that is typically less than its irritant threshold. This has been confirmed by a number of studies that have examined the ability of sev- eral groups to detect irritation (e.g., Arts et al., 2008). The conjunctiva is the most sensitive structure in the upper airway, so it is generally used as the test organ. In those studies in which both the detection and irritant levels were evaluated, the detection (odor) was always lower than the irritant level. For example, Noisel et al. (2007) reported an odor detection level of 0.75 ppm, with a minimum irritant level of 1.0 ppm, whereas an US EPA (2005) study reported an odor detection level of 0.5 ppm (consistent with ASTDR), with a minimum irritant level of 1.5 ppm. B.Formaldehyde-induced tissue damage in the upper respiratory tract Although there is a rich literature of rodent studies that have investigated sensory irritation, the numerous con- trolled human exposure chamber studies on formalde- hyde-induced sensory irritation renders the animal data of limited importance. However, it is important to distin- guish the documented differences between the sensory irritation caused by formaldehyde from its potential adverse or pathological effects on tissues. Unlike form- aldehyde, some chemicals (e.g., acrolein) can produce corrosive effects or physically damage respiratory tis- sues, even under short-term exposure conditions, at air concentrations that are not greatly above their irritation thresholds (ACGIH, 1992). In contrast, the scientific lit- erature demonstrates that sensory irritation of the eyes and upper respiratory tract caused by formaldehyde does not result in any histopathological evidence of adverse cellular effects. Rather, it is a physiological adaptation to trigeminal nerve stimulation, along with reduced breath- ing rate, bradycardia, and vasoconstriction (Alarie and Luo, 1986; Barrow et al., 1986; Paustenbach et al., 1997). Controlled studies in humans report that short-term exposure (i.e., less than 1 hour) to formaldehyde air concentrations below 2 ppm produces no toxicological effects on the eyes or on tissues in the upper respira- tory tract (ATSDR, 1999; Paustenbach et al., 1997). This finding has been extensively confirmed in studies with rats and monkeys, which show that even with long-term exposure, formaldehyde does not cause pathological lesions or changes in respiratory tract tissues at air con- centrations below 3 ppm (Paustenbach et al., 1997). For example, F344 rats exposed to formaldehyde at 0, 0.7, 2, 6, 10, and 15 ppm for 6 hours/day, 5 days/week for 6 months showed no metaplasia in any region of the nasal cavity at 0, 0.7, or 2 ppm (Kimbell et al., 1997); in another study F344 rats were exposed to the same formaldehyde concentrations for 6 hours/day, 5 days/week for 24 months with no formaldehyde-induced nasal histopa- thology observed in the 0.7 or 2 ppm groups (Monticello et al., 1996). Similarly, 12 male cynomolgus monkeys were exposed to 0, 0.2, 0.98, or 2.95 ppm formaldehyde