include Ballarin et al. (1992)—wood dust, no exposure- response relationship; Boysen et al. (1990)—wood dust exposure for some workers and exposures to >2 ppm formaldehyde (not further quantified) for almost a quar- ter of the cohort; Edling et al. (1988)—smoking, wood dust exposure for some workers, unknown numbers of peak exposures up to 5 ppm and no exposure-response relationship; and Holmstrom et al. (1989)—wood dust, resin exposure, smoking, and lack of exposure-response relationship. Similarly, most studies conducted in residential dwell- ings (including both conventional homes as well as man- ufactured housing such as trailers and mobile homes) with the goal of assessing potential formaldehyde-related effects on sensory irritation are also typically confounded by co-exposures to active smoking, environmental tobacco smoke, volatile organic chemicals (VOCs), smoke from wood fires, cooking fumes, house dust, pet dander, molds, fungi, etc. These co-exposures make it impossible to conclude with confidence that any results reported are due solely to formaldehyde. In addition, the results of some residential and/or community studies can also be confounded by selection bias (e.g., offers of free testing due to adverse publicity, emotional media stories, etc.), which can be substantially influenced by false-positive results (i.e., reporting of symptoms in the absence of formaldehyde or at concentrations insufficient to elicit symptoms [e.g., Main et al., 1983; Bracken et al., 1985; Kilburn et al., 1985; Imbus et al., 1985; Anderson et al., 1979; Ritchie and Lehnen, 1987]). A striking example of these issues is illustrated by a study that actually tested whether effects attributed to formaldehyde in a residential setting might be con- founded by other exposures and/or psychological fac- tors. Broder et al. (1991) investigated a large group of about 200 control homes and 600 houses that had been insulated with urea formaldehyde foam insulation (UFFI) and then, due to complaints and government- provided subsidies for UFFI removal, about half of the UFFI houses were remediated to remove the insulation. Each of the houses and occupants were investigated on two occasions separated by an interval of 12 months. In the first survey of the population, prior to remedial work, there was a moderate excess of many signs and symp- toms of irritation, including nasal problems, eye, throat discomfort, cough, headache, and dizziness. These symptoms were associated with an exposure-response relationship between formaldehyde levels in the UFFI homes (0.046 ppm), but no such relationship in control homes (0.035 ppm). In the second survey conducted in controls and houses following UFFI removal, there was an appreciable reduction in the reported incidence of irritation symptoms and the disappearance of the expo- sure-response relationship, even though the remedia- tion efforts had no effect on formaldehyde levels in the remediated homes (0.044 ppm). The authors concluded that the symptoms in the initial survey were not due to formaldehyde alone and that their observations were © 2011 Informa Healthcare USA, Inc. Indoor air exposure limit for formaldehyde 681 “…indicative of the complexities that may arise in assess- ing and understanding health risks…related to chemicals in indoor air.” Despite the problems and issues with these studies, they are still inexplicably relied upon for health-based conclusions concerning formaldehyde levels in indoor air. For example, a recent report from the Centers for Disease Control and Prevention (CDC, 2008) addressed a number of issues pertaining to formaldehyde concentra- tions in Federal Emergency Management Agency (FEMA) trailers. In reaching conclusions about the formaldehyde concentrations that might be associated with symptoms of sensory irritation, none of the studies cited were those involving controlled human exposures. Instead, this report concluded that “… formaldehyde-sensitive persons have reported symptoms at levels around 100 ppb,” citing Main and Hogan (1983) and Bender et al. (1983), and that “Additional studies have found health effects at 100 ppb in sensitive persons chronically exposed to formalde- hyde,” citing Ritchie and Lehnen (1987). It is illustrative to compare the rigor of studies relied upon by CDC to the controlled human exposure studies. Main and Hogan (1983) involved a symptom survey of 21 subjects who worked in two trailers where formal- dehyde levels between 0.12 and 1.6 ppm had been mea- sured. Because of this study’s design, it is not possible to ascertain whether any reported symptoms were the result of exposure to 0.12 ppm (or any other specific concentra- tion) and “formaldehyde-sensitive persons” are never addressed. The study byBender et al. (1983) was obviously erroneously cited as evidence that symptoms can occur in formaldehyde-sensitive persons at levels around 100 ppb. This study is one of the earliest controlled human volun- teer chamber studies, testing responses to formaldehyde at concentrations of 0.35, 0.56, 0.7, 0.9, and 1.0 ppm; 100 ppb (i.e., 0.1 ppm) was not tested. Severity of response rated above “slight” occurred only at the highest test concentration of 1.0 ppm and, as noted by the authors, “These data agree with other reported studies in which eye irritation occurred between 0.4 and 1.0 ppm rather than those which report eye irritation at extremely low levels but where other irritants may have been present.” The study by Ritchie and Lehnen (1987) was conducted in response to widespread publicity and subsequent offers of free formaldehyde testing. The study involved a survey of approximately 2000 people living in conventional and mobile homes where formaldehyde concentrations were measured in air samples taken from two rooms in each residence. The percentages of subjects with eye irritation, nose/throat irritation, headaches, and skin rash were recorded for homes with formaldehyde concentrations classified as “low” (<0.1 ppm), “medium” (0.1 ppm to <0.3 ppm), or “high” (>0.3 ppm). In both conventional and mobile homes with air concentrations >0.3 ppm, more than 60% of subjects reported eye irritation, nose/throat irritation, or headache; with air concentrations between 0.1 and 0.3 ppm, respective reporting percentages ranged from 10% to 20% for eye irritation, 15% to 20% for nose/