coded as reliable and reproducible. A reliable maneuver was a maximal exhalation without cough, excessive hesitation, leak, obstructed mouthpiece, variable effort, or early termina- tion (NCHS 1996). Reproducible maneuvers were recorded for FVC and FEV1 and were deﬁned as the largest FVC and second largest FVC within 5%, and the largest FEV1 and second largest FEV1 within 5%. Of 1,018 participants with VOC measures, 953 met our pulmonary function inclusion criteria. VOCs. In the Priority Toxicant Reference Range Study, 32 VOCs were measured in blood, using purge and trap gas chromatogra- phy/mass spectrometry as previously described (Ashley et al. 1992, 1994). We analyzed the 11 VOCs with median values above the limit of detection [1,1,1-trichloroethane (1,1,1-TCE), 1,4-dichlorobenzene (1,4-DCB), 2-butanone, acetone, benzene, ethylbenzene, m,p-xylene, o-xylene, styrene, tetrachloroethene, toluene]. Of the 953 participants who had a value for at least 1 of 11 VOCs and who had acceptable pulmonary function data, sample sizes varied across VOCs (range, 513–953), because results for some VOCs were not available for all par- ticipants (NCHS 2001). Although the reasons for different sample sizes are not given in the NHANES III documentation, it is possible that some blood samples failed to meet accept- ability standards or were invalid due to clot- ting, or that the laboratory experienced problems with instruments or quality control parameters (Sexton et al. 2005a). Statistical analyses. We used ordinary least-squares regression models to evaluate the association between each VOC and each pul- monary function outcome. For samples with VOC measures below the limit of detection, a value equal to the detection limit divided by the square root of 2 was assigned (NCHS 2000). We used natural log transformations of VOC concentrations to reduce the inﬂuence of their skewed distributions on the regression model estimates. We used Wilcoxon rank-sum tests to compare VOC median values between two groups and Kruskal-Wallis tests to com- pare values among three or more groups. We performed tests for linear trends across deciles using one-way analysis of variance. We con- ducted all analyses using SAS (version 9.0; SAS Institute Inc., Cary, NC). Weighting is not recommended for analysis of data from the Priority Toxicant Reference Range Study (NCHS 2000). For descriptive purposes, we ﬁrst analyzed VOCs in relation to pulmonary function with- out adjustment for smoking. However, because smoking and environmental tobacco smoke are sources of VOCs and also affect pulmonary function, we then added terms for smoking status (current, quit within the previous 12 months, quit more than 12 months previously, never), number of cigarettes smoked per day Environmental Health Perspectives • VOLUME 114 | NUMBER 8 | August 2006 VOCs and pulmonary function in NHANES III (continuous), years smoked (continuous), and serum cotinine level (continuous). Smoking was a confounder for most VOCs. We then used a change-in-estimate method to evaluate additional variables as con- founders for the VOCs still related to pul- monary function after adjustment for smoking (Greenland 1989). Our cutoff criterion was a 10% change in the VOC β-coefﬁcient in rela- tion to pulmonary function. In this manner, we assessed the following potential con- founders: socioeconomic status (education, poverty:income ratio, use of food stamps within the previous 12 months), self-reported doctor diagnosis of emphysema, use of ﬁreplace within the previous 12 months or wood or gas stove for heating or cooking, age of the house (construction year before 1946, 1946–1973, 1974 to present), presence of furred pets at home, and occupational exposure. Occupational exposure (yes, no) was indicated by a variable denoting occupations associated with chronic obstructive pulmonary disease (COPD) in this population (Hnizdo et al. 2002). The only fac- tor that met the criterion for confounding was self-reported doctor diagnosis of emphysema, and this was included in the ﬁnal models. We repeated analyses excluding people with self- reported doctor diagnosis of asthma, and the results were not changed appreciably. Results Characteristics of the study population are shown in Table 1. The mean age was 36.6 years (range, 20–59), 43.1% were female, and 26.3% were current smokers. Table 2 shows distributions of the 11 VOCs with median values above the limit of Table 1. Selected characteristics of participants in NHANES III Priority Toxicant Reference Range Study (1988–1994). Males (n = 542) Females (n = 411) Totala (n = 953) Race/ethnicity (%) Non-Hispanic white African American Mexican American Other Smoking status (%) Current smokers Former smokers1b Former smokers2c Never smokers Potential confounders (%) Diagnosed asthmad Diagnosed emphysemad Presence of furred pets Occupation with COPD risk Pulmonary function measures (mean ± SD) FEV1 (mL) FVC (mL) PEFR (mL/sec) MMEFR (mL/sec) Age [years (mean ± range)] 39.9 32.3 25.3 2.6 29.5 1.3 15.3 53.9 7.0 1.1 34.3 31.5 3,875 ± 732 4,858 ± 826 9,374 ± 1,834 3,812 ± 1,344 36.9 ± 20–59 38.2 31.1 26.3 4.4 22.1 1.2 9.3 67.4 9.0 0.7 35.8 12.2 2,867 ± 544 3,516 ± 612 6,731 ± 1,292 3,023 ± 1,016 36.2 ± 20–59 aSubjects in this table were included in at least one analysis of VOC blood concentration and pulmonary function. bFormer smokers who quit smoking within the previous 12 months. cFormer smokers who quit smoking > 12 months previously. dSelf-reported doctor’s diagnosis of asthma or emphysema. Table 2. Values of selected VOCsa (µg/L) measured in participants in NHANES III Priority Toxicant Reference Range Study, 1988–1994, limited to participants with pulmonary function data. VOC 1,1,1-TCE 1,4-DCB 2-Butanone Acetone Benzene Ethylbenzene m,p-Xylene o-Xylene Styrene Tetrachloroethene Toluene LOD 0.09 0.07 0.50 200 0.03 0.02 0.03 0.04 0.02 0.03 0.09 Total No.b < LODc No. Median Males 10th 513 854 908 852 743 570 953 593 589 539 540 122 38 0 0 113 33 362 24 74 133 4 292 0.14 491 0.33 515 5.59 479 1,945 421 0.07 325 0.07 542 0.13 343 0.11 336 0.05 306 0.07 308 0.33 0.06 0.11 2.41 801 0.02 0.03 0.02 0.06 0.01 0.02 0.14 Females Median 10th 90th 0.54 3.89 13.72 7,187 0.42 0.22 0.47 0.21 0.16 0.38 1.32 No. 221 0.13 363 0.30 393 5.36 373 1,788 322 0.06 245 0.05 411 0.11 250 0.10 253 0.04 233 0.06 232 0.25 0.06 0.10 2.26 769 0.02 0.02 0.02 0.06 0.01 0.02 0.13 LOD, limit of detection. 10th and 90th are percentiles. aCompounds were selected if median values were above the limit of detection. VOCs not meeting inclusion criterion: 1,1,2,2-tetrachloroethane, 1,1,2-TCE, 1,1-dichloroethane, 1,1-dichloroethene, 1,2-DCB, 1,2-dichloroethane, 1,2-dichloro- propane, 1,3-DCB, bromodichloromethane, bromoform, carbon tetrachloride, chlorobenzene, chloroform, cis-1,2- dichloroethene, dibromochloromethane, dibromomethane, methylene chloride, trans-1,2-dichloroethene, and trichloroethene. bNumber of available samples for each VOC. Not all VOCs were measured in every individual, resulting in different sample sizes. cNumber of participants with samples below the limit of detection. 39.1 31.8 25.7 3.4 26.3 1.3 12.7 59.7 7.9 0.9 34.9 22.3 3,440 ± 825 4,279 ± 996 8,234 ± 2,085 3,472 ± 1,275 36.6 ± 20–59 90th 0.41 4.83 11.28 6,109 0.26 0.16 0.34 0.17 0.10 0.32 0.88 1211