a change between symptoms reported at base- line (minute 15) and either minute 60 or 125. Based on 130 subjects receiving all three expo- sures, we calculated the minimum actual symptom difference between the VOC and VOC + O3 exposures and the MCA exposure that would be needed to attain a power of 90%. In these power calculations, we assumed that the size of the exposure effect, relative to MCA, was the same for VOCs and VOCs + O3. To simplify the calculations, we used changes from baseline as the response variable in a repeated-measures analysis of variance. We calculated the variance components for each response and time point from the exist- ing data. To account for multiple testing, the Bonferroni corrected significance level was used corresponding to the level used for test- ing in our present study. The results indicate that in all cases, mini- mum average increases of between 0.6 and 3.0 points in the average score for each cate- gory of symptoms would have to exist in order to detect a difference with a power of 90% (Table 4). With symptom severity scored on a scale of 0 to 100, this indicates that very small changes were detectable with the sample size used in our study. We performed similar power calculations for the main effect of exposure on neuro- behavioral performance, cortisol, and lung function. The minimum detectable difference in neurobehavioral performance between the VOC, VOC + O3, and MCA conditions for a power of 90% was calculated at 1.30 using a 0.05 signiﬁcance level. Because of a within- session learning effect, the MCA exposure resulted in a mean increase from baseline (base- line mean = 32.72; SD = 9.1) of 2.10 points on the neurobehavioral task. With 130 subjects we could detect a relative reduction of 1.30 points resulting in 0.80 point improve- ment for the VOC or VOC + O3 exposure conditions (i.e., 2.10 – 1.30 = 0.80). For corti- sol, the minimum detectable difference in changes from baseline needed for a power of 90% was 0.066 gµ/dL using a 0.05 signiﬁcance level. The MCA condition resulted in a mean decrease from baseline (the average baseline = 0.2627; SD = 0.1831) of 0.1148 gµ/dL. Thus, a decrease of no more than 0.0488 µg/dL in the VOC or VOC + O3 conditions was needed to detect a difference. Finally, for lung function, the minimum detectable differences in changes from baseline are 109.7 mL, 122.8 mL, and 0.917 L/sec, respectively, for changes in FEV1, FVC, and FEF25–75. These were calculated with a signiﬁcance level of p = 0.0167 (0.05/3). Thus, for all dependent meas- ures and the present sample size, relatively small changes were needed to detect a differ- ence between exposure conditions. Conclusions In conclusion, relatively brief, one-time expo- sures to mixtures of VOCs or VOCs and their oxidation products, at the upper bound of typical indoor concentration range, did not appear to cause significant acute changes in symptoms, neurobehavioral performance, or lung function in healthy women. In contrast, the psychological stressor was effective in pro- ducing increased autonomic arousal, as indi- cated by salivary cortisol, and in causing increased symptoms of anxiety, both well- documented effects of psychological stress. However, stress and exposure were neither synergistic nor additive in their effects on symptoms or neurobehavioral performance. The effect of stress, however, was isolated to symptoms of anxiety and did not generalize to other more typical symptoms associated with poor indoor air such as nasal irritation or headache. Although the irritation potency of complex and variable mixtures of VOCs and their oxidation products in buildings is difﬁ- cult to predict, reported air concentrations of VOCs in buildings with poor indoor air qual- ity are typically an order of magnitude lower than the VOC concentrations used in this study. Thus, our results suggest that the VOC concentration alone may not be the most salient factor to account for acute health com- plaints. Our present results support the con- clusion that 3-hr exposures to VOCs or to the reaction products of VOCs and O3 at concen- trations typically found in nonindustrial buildings are unlikely to be a signiﬁcant cause of acute health complaints or effects for most occupants. Several caveats need to be considered for our present study. This study included the largest number of subjects to date and inten- tionally selected only women for study because of their hypothesized vulnerability to report indoor air quality symptoms. However, the extent to which our results apply to indoor air problems experienced in a work environment with many other demands and chronic expo- sures is problematic. Thus, the lack of health effects observed may simply be a function of the necessarily acute exposure paradigm with Table 4. Effect sizes for symptom severity based on 130 subjects. Change Total (from 15 min) symptoma 60 min 125 min 0.99 1.15 General 2.69 2.98 VOC Cognitive 2.48 2.69 Eye 2.07 2.15 Anxiety 1.43 1.85 Respiratory Upper Lower 1.42 1.49 0.74 0.64 Somatic 1.04 1.13 aCalculated at a signiﬁcance level of 0.05. The effect sizes for all remaining subcategories of symptoms were calculated at a signiﬁcance level of 0.05/7 = 0.0071. Volatile organics, ozone, and stress healthy young subjects. Conversely, the expo- sure concentrations were quantitatively higher than those documented in most buildings with indoor air complaints. Furthermore, work demands were modeled in our study through use of a known stressor as well as requirements for computerized neuro- behavioral tasks, and the former was successful in causing autonomic arousal and symptoms of anxiety. Another attribute of our study was that all exposure conditions were conducted at a relatively high air exchange rate (~ 1.8 air exchanges/hr), whereas health complaints associated with indoor air quality often occur in buildings with poor ventilation (i.e., air exchange rates were an order of magnitude lower than the air exchange rate used in our study). Overall, this study suggested that for a 2-hr time period, psychological stress may be a more potent factor than ambient chemical mixtures in the complaints attributed to poor indoor environments. Appendix 1. Symptom list. VOC physical Headache Fatigue Lightheaded Drowsy Nausea VOC cognitive Difﬁculty concentrating Disoriented/confused Dizzy Eye irritation Eye irritation (burning, dryness, or itching) Runny/watery eyes Anxiety Feel jittery in body Feel nervous Heart palpitations Feel tense Worried Upper respiratory Sneeze Nasal congestion Choking Throat irritation (burning or dryness) Nose irritation, dryness, or itching Lower respiratory Short of breath Wheezy Chest tightening Chest pain Coughing Somatic control Skin irritation or dryness Stomachache Numbness/tingling Ear ringing Leg cramps Back pain Sweating Body aches Environmental Health Perspectives • VOLUME 113 | NUMBER 11 | November 2005 1547