Strak et al. (95% CI: –3.8, –1.0%), which decreased and became nonsignificant after adjusting for O3 (and PNC after adjusting for NO2) [see Supplemental Material, Table S4 (http:// dx.doi.org/10.1289/ehp.1104389)]. A posi- tive association with O3 in the single-pollutant models was consistent. For FEV1, the only consistent pattern was a positive association with OC(F) in the complete data set (see Strak et al. 2012, Tables R12–R13). Two hours after exposure, the most consistent associations for FVC were found with NO2 and NOx, respectively, at –1.5% (95% CI: –2.8, –0.3%) and –1.1% (95% CI: –2.1, –0.1%), although the latter decreased somewhat after adjusting for PNC (see Strak et al. 2012, Table R8). We did not interpret models with NO2 and NOx further because NO2 is a part of NOx. In the outdoor-only models, the significant associations with NOx disappeared after adjusting for O3 and EC(C) (see Strak et al. 2012, Table R9). The positive association with O3 was weaker after adjusting for NOx. The association of NO2 with FEV1 at –1.6% (95% CI: –2.8, –0.3%) observed in single-pollutant models remained stron- ger than other associations in two-pollutant models (see Strak et al. 2012, Table R14). In the outdoor-only data set, NOx was most con- sistently associated with FEV1 (see Strak et al. 2012, Table R15). The morning after exposure, NO2 and NOx were consistently associated with a drop in FVC (see Strak et al. 2012, Table R10) with the effect estimates of –1.9% (95% CI: –3.2, –0.6%) and –1.4% (95% CI: –2.4, –0.4%), respectively. In the outdoor models, the associations of NOx with FVC were insensitive to adjusting for other pol- lutants except for O3 (see Strak et al. 2012, Table R11), for which there was a fairly con- sistent positive association. For FEV1, NOx had a fairly consistent negative association (see Strak et al. 2012, Table R16), whereas sulfate had a fairly consistent positive association. In the outdoor data set, the fairly consistent asso- ciation with NOx remained (see Strak et al. 2012, Table R17) with an effect estimate of –1.3% (96% CI: –2.5, –0.2%). Additional analyses. Exposure of partici- pants to PNC during transport to and from the sampling sites was not associated with changes in respiratory health and did not affect associations with the experimental exposures. Pollen counts were the only variables not mea- sured on-site. Analyses with and without pollen counts resulted in similar associations. Exclusion of the three former smokers and the five participants with reported nasal allergy showed similar associations between air pollutants and FENO, FVC, and FEV1. Exclusion of the 12 observations with anti- inflammatory medication did not change the effect estimates. 1188 Discussion We investigated acute respiratory health effects in a panel of healthy, young volunteers semiexperimentally exposed to ambient air pollution with contrasting PM characteris- tics. We found associations of PNC, NO2, NOx, absorbance, EC, and trace metals with changes in FENO, FVC, and FEV1 immedi- ately after, 2 hr after, and the morning after exposure. The most consistent associations in two-pollutant models were between PNC and FENO and between NO2/NOx and lung func- tion. Changes in those parameters were not consistently related to PM mass concentration, sulfate/nitrate content, or OP of particles. We used a semiexperimental design to study the independent associations between respiratory function and a large number of PM characteristics. That design allowed us to define two-pollutant models to investigate independent associations of single pollutants with fewer problems than observational studies. Because we observed participants in a semi- experimental setting, exposure measurement error was largely due to instrumental errors; therefore, issues such as representativeness of outdoor central monitoring for personal expo- sure do not affect our study. Instrumental pre- cision of measurements was between 5% and 10%, which is very low compared with the range of measured concentrations. Furthermore, PNC and NOx were not more precisely measured than the other components [with the possible exception of total Cu and Ni; see Supplemental Material, Table S3 (http://dx.doi.org/10.1289/ ehp.1104389)]. Difference in instrumental pre- cision is thus an unlikely explanation for their stronger associations. Correlations between PM characteristics were reduced by performing repeated measurements at multiple locations with different source characteristics, although some correlations remained too high to interpret in two-pollutant models. In the present study, we observed a consis- tent association between PNC and increased FENO immediately after and 2 hr after expo- sure. This association was insensitive to adjust- ment by any of the 25 other pollutants. PNC was not highly correlated with other pollutants in the whole data set. In the outdoor-only models, we could not disentangle the associa- tions with PNC, absorbance, EC, Fe, and Cu because they were highly correlated. However, the latter components were dominant at the underground site but their associations were insignificant in the data set including under- ground, and the PNC association was consis- tently strong in both the whole data set and the outdoor-only data set, thus suggesting that the associations with absorbance, EC, Fe, and Cu for the outdoor sites are likely explained by those with PNC. PNC especially reflects the ultrafine par- ticles for which respiratory health effects have been documented in previous epidemiological and controlled exposure studies (Ibald-Mulli et al. 2002). The consistency of PNC effects is notable when we take into account the low correlation of FENO with FVC and FEV1. Our results for lung function are contrary to findings of two experimental studies with higher PNC concentrations than in our study (Larsson et al. 2007; Samet et al. 2009). The shorter duration of these studies (2 hr) may explain the lack of a lung function response. In a study in Arnhem, the Netherlands, Zuurbier et al. (2011) reported a 5% increase in FENO in response to an 18,000-particles/cm3 (IQR) increase in PNC measured 6 hr after a 2-hr exposure in a car or bus. This increase corre- sponds to a 9% increase in FENO if expressed per our IQRs, roughly comparable with our findings. Similarly, Strak et al. (2009) reported a 13% increase in FENO in cyclists after a 1-hr commute, as expressed per the IQRs in our study. McCreanor et al. (2007) reported that a 2-hr walk near heavy diesel traffic resulted in an approximately 4% increase in FENO expressed per our IQR. Similarly consistent associations were observed between NO2 and NOx and lung function parameters 2 hr after and the morn- ing after the exposure. PNC was most con- sistently related to FVC immediately after exposure. There is still a debate whether the associations observed between respiratory health and NO2 at the concentrations cur- rently found in western European countries are due to direct effects of NO2 or other PM com- ponents co-varying with NO2 (WHO 2006). In the present study, we measured a detailed set of PM characteristics including PNC as a proxy for ultrafine particles and metals, but we still observed associations with NO2. For some components that were higher at the underground station than at other locations, we observed associations only in the outdoor-only models. This likely argues against a causal role, although saturation of biological parameters after exposure to very high air pollution concentrations could also provide an explanation. Exposure to PM mass, irrespective of the size fraction, was not associated with acute respiratory health changes. The lack of asso- ciation between PM10 and PM2.5 and acute changes in respiratory function is consistent with the results from two Dutch studies on bicycle commuting (Strak et al. 2009; Zuurbier et al. 2011) in which increases in FENO were not associated with mass-based PM metrics but PNC and absorbance were. In contrast to our hypothesis, OP did not display a strong and consistent relation- ship with acute respiratory health effects. In the single-pollutant models, associations of OP metrics with increased FENO 2 hr after exposure were evident in the outdoor-only ovlume 120 | number 8 | August 2012 • Environmental Health Perspectives