(n = 73); black, 10% (n = 13); Hispanic, 8% (n = 10); Asian, 20% (n = 26); other, 6% (n = 8). Subjects completed a medical history, physical examination, and standard clinical blood chemistry to rule out previous signiﬁcant occupational exposure to chemicals and the following health conditions: neurologic disease or brain injury, stroke or cardiovascular disease, serious pulmonary disease including asthma, liver or kidney disease, serious gastrointestinal disorders, known endocrine disease, pregnancy or lactation, and major psychiatric conditions, including psychoses, bipolar disorder, alco- holism or drug abuse, and multiple chemical sensitivity with signiﬁcant illness behavior or disability. Three hundred forty-one individuals were screened for the study. Fifty-four were excluded at the telephone screening because of medical conditions, 74 never came in for their appointed physical examination, 40 declined to participate after the physical examination, 19 were excluded for medical conditions dis- covered at the physical examination, 4 dropped out after their first exposure, and 6 dropped out after their second exposure (n = 197). Fourteen of the 144 subjects who completed the study were pilot subjects whose data were not included in the analysis. Dependent Measures Symptom questionnaire. Subjects rated each symptom on a ratio scale from 0 (barely detectable/no sensation) to 100 (strongest imaginable) (Green et al. 1996). We chose symptoms based on previous literature assess- ing the health effects of indoor air mixtures (Molhave et al. 1986; Prah et al. 1998). These symptoms included the cognitive and physical effects expected of VOC mixtures (Hudnell et al. 1992; Molhave et al. 1986), anxiety symptoms associated with the odor of expo- sure, eye irritation, upper respiratory and lower respiratory symptoms associated with O3 and the secondary products generated from the reactions of O3 with VOCs (Fan et al. 2003), and somatic symptoms not typically associated with VOC mixtures (Appendix 1) (Dalton et al. 1997). Neurobehavioral. This computerized divided-attention test of cognitive performance, performance on-line (POL) (Mills et al. 1996) offered ﬁve different levels of complexity. The test was validated in alcohol dosing trials and was developed explicitly for use in repeated- measures studies of alcohol and drug effects. POL included a central task in which the sub- ject was presented with two lanes of traffic, divided by a double yellow line. Four condi- tions of “headlights” and “tail lights” appeared on any one trial. The subject was instructed to press the space bar only when a “safe” condition (i.e., left lane, white headlights, and right lane, red tail lights) existed. The peripheral task required the subject to respond with one of four arrow keys (up, down, left, right) in the direc- tion of the critical stimulus (red octagon among other shapes). Task difficulty increased by increasing the number of distracting stimuli in the peripheral display to a random assortment of different colored circles, squares, and trian- gles. For the divided-attention display, the sub- ject responded to both central and peripheral critical stimuli. After practice, 10 trials of 45 displays were presented at the most complex level. A composite performance score, com- posed of seven component scores, including hits, misses, false positives, response latency, and responses to targets at varying levels of visual angle, was the performance variable measured (Badiani et al. 1995). Cortisol assays. An extensive literature documents the signiﬁcant (r ≥ 0.90) associa- tion between salivary and plasma cortisol (Kirschbaum and Hellhammer 1989, 1994). Salivary ﬂow, which may be affected by anxi- ety, is not documented to affect the concen- tration of salivary cortisol (Dirks et al. 1988; Kahn et al. 1988), although circadian rhythm affects cortisol production (Walker et al. 1984). Thus, we tested subjects at the same time of day to control for the well-known circadian effect on cortisol production. Kirschbaum and Hellhammer (1989, 1994) also reported that, although a highly signifi- cant correlation is shown between salivary and plasma cortisol, absolute values vary sig- nificantly. However, in our study, we evalu- ated relative change scores rather than absolute values. Because hormonal fluctua- tions also affect cortisol levels, we measured salivary estradiol at baseline before each expo- sure session to account for any effects of ovu- lation on salivary cortisol. Consistent with Kirschbaum and Helhammer (1989), we collected samples using the Salivette (Sarstedt Inc., Rommelsdorf, Germany) method. We asked the subject to chew on a cotton swab for 60 sec, then place the swab into a Salivette holder and afﬁx the cap. The samples were centrifuged at 3,000 rpm for 10 min, which produced 0.5–1 mL of saliva. The saliva was frozen. Samples were analyzed, blind to exposure con- dition and subject characteristics, in one of the co-investigators’ laboratories (J.O.). Samples were run in duplicate, and equal numbers of samples from each group were run in the same assays. Pulmonary function test. With the sub- jects in standing position with nose clips attached, spirometry was performed using a Multispiro SX spirometry system (Creative Biomedics, Inc., San Clemente, CA) that was calibrated daily. We used the highest value for each parameter from any one of three repro- ducible tracings before exposure for compari- son with the highest value for each parameter postexposure. Forced vital capacity (FVC), Environmental Health Perspectives • VOLUME 113 | NUMBER 11 | November 2005 Volatile organics, ozone, and stress forced expiratory volume in 1 sec (FEV1), and forced expiratory ﬂow 5–75% (FEF25–75) were parameters of interest. Independent Variables Public speaking task stressor. After a 4-min silent preparation period, subjects constructed and delivered a 4-min speech on the following controversial scenarios: a) construct a presenta- tion on the causes of gray hair from a Reader’s Digest article; b) present a position on whether homosexual men should be allowed in the mili- tary and be given special civil rights protection; c) defend herself against a shoplifting charge in front of three authority figures (al’Absi et al. 1997). In a direct comparison al’Absi et al. (1997) demonstrated that these three different topics caused a similar level of stress (cortisol response). We randomized the order of the three scenarios across subjects such that each subject delivered a speech on a different topic in each of her three exposure conditions. After the preparation period, subjects were told by the experimenter to stand, face the video camera, and begin giving the speech. To enhance the stressfulness of the procedure, we told each sub- ject that the research technician was evaluating the speech as it was given and that three staff members would evaluate a videotape of the speech later; the subject was promised an addi- tional stipend of $10 if she performed well. After completion of the three experimental ses- sions, all subjects received the added stipend regardless of performance. Controlled environment facility and expo- sure generation. The Environmental and Occupational Health Sciences Institute Controlled Environment Facility (CEF) is a stainless steel room 2.2 m × high × 4.1 m wide × 2.7 m deep with a total volume of 25 m3. The air supply is treated in a series of conditioning processes, which include air cooling/heating, humidiﬁcation/dehumidiﬁcation, and ﬁltration through carbon and high-efﬁciency particulate air (HEPA) ﬁlters. All parameter controls are computer interfaced to maintain constant con- ditions in the CEF. To simulate a typically ven- tilated ofﬁce building and to allow sufﬁcient time for the formation of O3–VOC reaction products, the air ﬂow rate through the facility was controlled at 1.8 ± 0.2 air changes/hr for all the exposures conducted in this study. The air supply enters the facility through two diffusers in the ceiling and exits through the perforated stainless steel ﬂoor to the exhaust vents. Small brushless fans (to prevent unwanted particle generation from brush degradation) were used in the CEF to ensure that the air was well mixed. During exposure sessions, the relative humidity and temperature were controlled to a range of between 24 and 49% and 73–82°F (23–28°C), respectively. A Teflon partition separated the subjects’ work stations, which were equipped with a computer and typing 1543