PM characteristics and acute changes in respiratory function Then, to study the individual associations of different pollutants, we specified two-pollutant models with all possible combinations of mea- sured pollutants. Here, we primarily report and discuss the results of the two-pollutant models. Models in which two pollutants had a Spearman’s rank correlation coefficient (rS) > 0.7 were considered highly correlated and were not interpreted. We defined a large num- ber of models, so we focused on the consis- tency of significant associations and not on single isolated significant associations. Because there was a substantial difference in some exposure parameters between outdoor loca- tions and the underground location, we also analyzed the data separately for the outdoor locations and the underground location. To offset potential confounders, we adjusted for temperature and relative humidity measured at the location during sampling, the season in which the sampling day occurred (before or after the start date of the calendar summer), FVC (L)c Table 1. Population characteristics and baseline (t = 0) FENO and lung function. Characteristic Value Age (years) 22 (19–26) Sex [n (%)] Female 21 (68) Male 10 (32) Nasal allergy [n (%)]a 5 (16) Former smoker [n (%)] 3 (10) Body mass index (kg/m2) 22.3 (17.0–32.0) FENO (ppb)b 15.9 (5–61) FEV1 (L)c 3.86 (2.57–5.51) 4.68 (2.73–6.70) FEF25–75 (L/sec)c 3.94 (2.06–6.43) PEF (L/sec)c 8.71 (5.38–14.68) Unless otherwise stated, values are mean (range). aIncludes hay fever. bN = 151–169. cN = 165–170. an indicator variable for low/high grasses and birch pollen counts, and an indicator variable for respiratory infection. Pollen counts were obtained from the station in Leiden, located within 50 km of our sampling locations. We selected grass and birch pollen as highly aller- genic indicator pollen, with good spatial cor- relation with pollen counts at another station. Because the distribution was highly skewed, we included pollen as a low/high variable (Brunekreef et al. 2000). For the lung func- tion measurements, we investigated possible technician and instrument effects, but neither had an effect. Sensitivity analysis assessed participants who a) did not report nasal allergies, b) were not former smokers, and c) did not take anti- inflammatory medication in the 24-hr period before the start of the sampling day. We assessed the impact of influential values on the regression results by comparing effect esti- mates with and without the 1% of observa- tions with the highest Cook’s distance value. Effect estimates and their confidence intervals (CI) are presented as percentage increases over a study population mean of the baseline (t = 0) values. We express these values as percentage increases per change in interquartile ranges (IQR) for the outdoor locations. We calculated IQRs for endotoxin concentrations for all locations, but without the levels measured at the farm location. We used these IQRs in the analysis of the com- plete data set and the outdoor-only data set to allow direct comparison of effect estimates. Statistical significance was defined as p < 0.05 and borderline significance as p < 0.10. All data analyses were carried out using SAS, ver- sion 9.2 (SAS Institute Inc., Cary, NC, USA). Results A total of 170 observations were obtained from 31 participants (Table 1) who were exposed at least three and at most seven times. Each participant visited the under- ground train station at least once. We did not analyze the data from the respiratory s y m p t o m s q u e s t i o n n a i r e s b e c a u s e o n l y questions about congestion/rhinorrhea and cough reported > 15% changes in scores over the sampling days. Exposure measurements. The measure- ments at the underground train station showed substantially higher concentrations of nearly every PM characteristic, especially levels of coarse PM, iron (Fe), copper (Cu) and the sum of OPAA and OPGSH (OPTOTAL) [see Supplemental Material, Table S1 (http:// dx.doi.org/10.1289/ehp.1104389)]. For exam- ple, the mean concentration of coarse PM was 252 µg/m3 at the station and 13 µg/m3 at the outdoor sites. PNC was the highest at the continuous traffic site at 66,500 particles/cm3, and substantially increased levels of endotoxin were measured at the farm. Variability of con- centrations was large at the outdoor sites but more limited within the underground-only data set. Therefore, we will not report under- ground-only air pollution effect estimates. Correlations between air pollution con- centrations are shown in Table 2. PM10 and PM2.5 were highly correlated with each other, as well as with absorbance, EC, OC, trace metals, and OPTOTAL, but not with PNC. The high correlations decreased considerably after we excluded the measurements from the underground train station. However, as a result of the exclusion, we observed a substantial increase in correlations between Table 2. Spearman’s rank correlation coefficients (rS) between PM characteristics. PM10 PM2.5 PM2.5–10 PNC Absa EC(C) EC(F) OC(C) OC(F) Fe(tot) Fe(sol) Cu(tot) Cu(sol) Ni(tot) Ni(sol) V(tot) V(sol) Endo NO3–a SO42–a OPAA OPGSH OPTOTAL O3 NO2 NOx PM10 PM2.5 PM2.5–10 PNC Absa EC(C) EC(F) OC(C) OC(F) Fe(tot) Fe(sol) Cu(tot) Cu(sol) Ni(tot) Ni(sol) V(tot) V(sol) Endo NO3–a SO42–a OPAA OPGSH OPTOTAL 0.94 0.82 0.22 0.74 0.70 0.69 0.76 0.73 0.70 0.44 0.70 0.80 0.76 0.00 0.66 –0.25 0.36 0.07 –0.15 0.88 0.82 0.89 0.88 0.67 0.15 0.68 0.66 0.64 0.68 0.79 0.62 0.43 0.65 0.74 0.71 –0.04 0.67 –0.26 0.34 0.18 –0.04 0.91 0.79 0.88 0.55 0.22 0.21 0.71 0.68 0.67 0.78 0.46 0.70 0.46 0.69 0.73 0.79 0.01 0.58 –0.27 0.37 –0.29 –0.44 0.73 0.79 0.77 0.19 0.07 0.15 0.65 0.60 0.67 0.00 –0.04 0.62 0.65 0.60 0.56 0.07 0.47 0.25 0.17 –0.32 –0.27 –0.16 0.36 0.32 0.35 0.37 0.22 0.31 0.84 0.88 0.98 0.48 0.49 0.92 0.75 0.89 0.92 0.64 0.19 0.66 –0.19 –0.01 –0.30 –0.36 0.80 0.73 0.78 0.28 0.17 0.26 0.77 0.73 0.89 0.45 0.36 0.88 0.75 0.92 0.93 0.54 0.25 0.77 –0.06 –0.07 –0.24 –0.41 0.79 0.77 0.80 0.25 0.13 0.19 0.86 0.96 0.77 0.42 0.43 0.92 0.74 0.89 0.90 0.60 0.24 0.71 –0.15 –0.02 –0.35 –0.38 0.79 0.73 0.76 0.52 0.39 0.57 –0.06 0.00 –0.04 –0.13 0.46 0.49 0.27 0.54 0.57 0.62 –0.08 0.43 –0.25 0.59 –0.07 –0.22 0.70 0.77 0.77 0.59 0.72 0.06 –0.20 0.05 –0.26 –0.07 0.08 0.37 0.20 0.37 0.53 0.62 –0.18 0.37 –0.35 0.22 0.36 0.08 0.66 0.48 0.60 0.24 0.04 0.27 0.90 0.83 0.77 0.81 0.07 –0.22 0.78 0.96 0.88 0.57 0.15 0.62 –0.26 –0.07 –0.31 –0.55 0.72 0.69 0.72 –0.05 –0.11 –0.01 0.86 0.65 0.59 0.66 –0.23 –0.27 0.80 0.79 0.74 0.40 0.31 0.48 –0.07 –0.15 –0.38 –0.44 0.51 0.43 0.45 0.28 0.12 0.26 0.82 0.76 0.82 0.77 0.12 –0.23 0.93 0.69 0.92 0.53 0.24 0.67 –0.17 –0.05 –0.24 –0.46 0.76 0.75 0.76 0.55 0.41 0.37 0.71 0.85 0.83 0.80 0.15 0.09 0.78 0.55 0.82 0.63 0.19 0.66 –0.16 0.00 –0.14 –0.31 0.82 0.80 0.83 0.40 0.27 0.49 –0.09 0.11 –0.11 –0.01 0.22 0.37 –0.10 –0.11 –0.16 0.13 0.07 0.67 –0.20 0.40 –0.10 –0.24 0.72 0.68 0.73 –0.01 –0.06 0.00 0.46 0.35 0.43 0.46 –0.22 –0.37 0.27 0.49 0.36 0.26 0.11 0.44 0.74 0.06 0.03 0.30 0.11 0.11 0.04 0.14 0.19 –0.05 0.20 0.19 0.47 0.29 –0.18 –0.18 0.04 0.06 0.21 0.22 0.16 0.75 0.20 0.27 –0.11 –0.16 0.79 0.70 0.76 0.04 0.07 0.00 0.19 0.14 0.42 0.24 –0.17 –0.30 0.00 0.11 0.13 0.13 0.16 0.81 0.96 –0.07 0.15 0.51 –0.15 –0.22 –0.20 0.22 0.22 0.22 –0.37 –0.30 –0.49 –0.31 0.40 0.13 –0.52 –0.45 –0.49 –0.42 0.20 0.08 0.05 0.14 –0.05 0.10 0.27 0.38 0.30 0.56 0.74 –0.10 –0.26 –0.12 –0.05 –0.21 0.11 0.64 –0.22 –0.27 –0.09 0.11 0.18 –0.13 0.20 0.06 0.02 0.67 0.03 –0.19 –0.05 0.50 0.72 –0.12 –0.14 0.08 –0.07 0.05 0.12 0.54 –0.32 –0.29 –0.19 0.10 0.33 0.20 0.49 0.39 0.33 0.66 –0.10 –0.22 –0.16 0.75 0.79 0.32 0.28 0.51 0.47 0.45 0.40 0.38 0.26 0.03 0.35 0.56 0.24 0.07 0.47 0.36 0.00 0.57 0.67 0.82 0.95 0.54 0.48 0.44 0.12 0.27 0.39 0.28 0.50 –0.01 0.14 –0.15 0.35 0.47 0.15 0.02 0.21 0.15 0.13 0.10 0.34 0.54 0.92 0.73 0.73 0.40 0.22 0.42 0.50 0.35 0.56 0.24 0.23 –0.11 0.38 0.59 0.28 –0.07 0.37 0.26 –0.02 0.42 0.54 0.88 0.80 –0.21 –0.15 –0.18 –0.35 –0.57 –0.33 –0.57 0.07 –0.06 –0.18 –0.14 –0.26 –0.35 –0.20 –0.54 –0.52 –0.48 –0.21 –0.03 –0.47 –0.46 –0.21 –0.27 0.49 0.45 0.28 0.56 0.74 0.60 0.67 0.06 0.26 0.52 0.34 0.52 0.71 0.28 0.28 0.36 0.27 –0.19 0.26 0.32 0.69 0.28 0.58 0.32 0.21 0.25 0.75 0.87 0.71 0.87 –0.11 0.01 0.70 0.53 0.66 0.72 0.14 0.47 0.39 0.35 –0.23 –0.05 0.13 0.54 0.21 0.42 O3 NO2 NOx –0.67 0.26 0.37 –0.65 0.21 0.31 –0.65 0.14 0.35 –0.37 0.50 0.70 –0.81 0.39 0.70 –0.71 0.27 0.61 –0.81 0.36 0.67 –0.48 –0.01 0.15 –0.50 0.19 0.21 –0.67 0.27 0.54 –0.51 0.08 0.56 –0.70 0.24 0.57 –0.73 0.34 0.64 –0.67 0.11 0.24 –0.37 0.26 0.37 –0.80 0.16 0.35 –0.01 0.23 0.21 –0.32 –0.15 –0.19 0.17 0.27 –0.17 0.12 0.29 –0.07 –0.78 0.32 0.47 –0.69 0.14 0.35 –0.72 0.28 0.41 –0.36 –0.57 –0.62 0.65 –0.68 0.91 Abbreviations: Abs, absorbance; C, coarse PM fraction; Endo, endotoxin; F, fine PM fraction; sol, water-soluble metal extraction; tot, total. Values in the light-blue area represent correlations in the outdoor-only data set. aMeasured in PM2.5. Environmental Health Perspectives • volume 120 | number 8 | August 2012 1185