even below measured formaldehyde concentrations in normal human breath (i.e., ≈2 ppb) as well as rural air ambient formaldehyde concentrations (i.e., ≈8 ppb), it is unclear how such values should be interpreted in the context of providing realistic public health protection. Importantly, with respect to the value of 0.815 ppb estab- lished by OEHHA, a recent (2008) joint communiqué from the Department of Health and Human Services (DHHS), the Centers for Disease Control and Prevention (CDC), the Department of Homeland Security (DHS), FEMA, and US EPA noted that “Due to the fact that OEHHA determined that the CREL for formaldehyde is less than typical ambient levels, they (i.e., OEHHA) recommended an office concentration level of 23 ppb, based upon the concept of “as low as reasonably achievable.” However, even 23 ppb is far below the empirically derived value of 0.3 ppm (300 ppb) that will be protective even for sensi- tive individuals. Another example of a lower exposure limit is ATSDR’s (1999) chronic Minimum Risk Level (MRL) of 0.008 ppm (8 ppb). That level was based on an occupational study, instead of controlled studies, in which nasal histopathol- ogy was evaluated in one group of workers exposed pre- dominantly to formaldehyde and another group exposed to both formaldehyde and wood dust (Holmstrom et al., 1989). Because the subjects worked “…in a chemical industry in which formaldehyde and products based on formaldehyde were produced as resins,” they apparently were not exposed solely to formaldehyde as the only nasal irritant. In addition, workers were exposed to frequent formaldehyde peaks above 1 mg/m3 (≈1 ppm) so that the formaldehyde exposure concentrations associated with mean biopsy scores could be characterized only as >0.5 for nearly 30% of the group. The well-known confound- ing due to wood dust has already been described. ATSDR determined 0.24 ppm to be the lowest observed adverse effect level (LOAEL) from this study. An uncertainty fac- tor of 3 was then used to obtain a presumed NOAEL and an uncertainty factor of 10 was applied for human vari- ability (0.24/3 × 10 = 0.008). Such co-exposures and lack of defined exposure concentrations make it questionable that reported effects were due solely to formaldehyde. 2.Derivation of recommended value Although 0.3 ppm represents the best science-based exposure limit for formaldehyde in indoor air, an alter- native, even more health-protective approach also can be justified. Based on the substantial amount of data derived from controlled human studies and on numer- ous reviews of those data, 1.0 ppm is the LOAEL that is unequivocally associated with eye irritation and with substantial scientific certainty that false positives play no confounding role. Using the weight of evidence–based value of 1.0 ppm and dividing by a “certainty factor” of 10 would yield an indoor air concentration limit of 0.1 ppm (100 ppb). This limit is a conservative target level that provides scientifically documentable protection from formaldehyde-induced sensory irritation as well as from © 2011 Informa Healthcare USA, Inc. Indoor air exposure limit for formaldehyde 689 odor and annoyance effects for everybody, including sensitive individuals, children, and people with asthma. This value was endorsed by the expert panel assembled by Paustenbach et al. (1997), which stated, “The panel concluded that any occupational or environmental guide- line for formaldehyde should be based primarily on con- trolled studies in humans, since nearly all other studies are compromised by the presence of other contaminants. The panel also concluded that if concentrations of formalde- hyde are kept below 0.1 ppm in the indoor environment (where exposures might occur 24 h/d) this should prevent irritation in virtually all persons.” Likewise, in a NAS (2008) report prepared for the National Aeronautics and Space Administration (NASA), the issue of deriving an airborne concentration (AC) for formaldehyde for per- sonnel exposed for long durations was critically assessed. As noted in this report, “…a formaldehyde concentration of 0.1 ppm is being identified as a reasonable guideline for long-term exposure that is unlikely to result in any irritant effects, even with sensitive individuals. Instead of relying on one particular study, multiple lines of evidence support this AC as appropriate. This evidence…includes the findings from a number of controlled human studies, evaluations from several comprehensive scientific reviews, community health surveys, and practical NASA experience with formaldehyde in an enclosed environment designed to mimic conditions relevant to spacecraft exposures.” Finally, a value of 0.1 ppm is also in close agreement with the most recent comprehensive evaluation of formaldehyde in indoor air including a recommended indoor air level by Wolkoff and Nielsen (2010). This evaluation, carried out in the framework of the WHO Indoor Air Quality Guideline development (2006–2010), concluded that, “With the eye the most sensitive organ, subjective irritation is reported at 0.3–0.5 mg/m3 [0.24– 0.5 ppm], which is somewhat higher than reported odour thresholds. Objective effects in the eyes and airways occur around 0.6–1 mg/m3 [0.46–0.8 ppm]. Dose–response relationships between FA and lung function effects have not been found in controlled human exposure studies below 1 mg/m3 [0.8 ppm], and epidemiological asso- ciations between FA concentrations and exacerbation of asthma in children and adults are encumbered by com- plex exposures. Neither experimental nor epidemiologi- cal studies point to major differences in susceptibility to FA among children, elderly, and asthmatics…. An air quality guideline of 0.1 mg/m3 (0.08 ppm) is considered protective against both acute and chronic sensory irrita- tion in the airways in the general population assuming a log normal distribution of nasal sensory irritation.” Consequently, based on the weight of evidence from the numerous controlled human studies in which expo- sure to formaldehyde was the only variable, any derived “safe” concentration of formaldehyde which falls below the range of 0.08–0.1 ppm (80–100 ppb) cannot be justi- fied, since there are no un-confounded empirical data that demonstrate formaldehyde-associated sensory irritation effects below this level.