NO2, and PM10) variables. In addition, cor- relations between adjacent particle size classes were high. R e s p i r a t o r y E R V i n c r e a s e d b y 5 % [RR = 1.05 (95% CI, 1.02–1.08)] with an IQR increase (4,400 cm–3) in PNC100–300 with a 1-day lag (PDL model) [Table 3; see also Supplemental Material, Table 2 (doi:10.1289/ehp.1002203)]. Effect estimates for an IQR increase in NO2 were similar in magnitude to estimates for IQR increases in particle number and surface area concentra- tions of particles > 100 nm. Some inverse associations were observed with UFP and particle number concentrations of fractions < 50 nm. In most cases PNC50–100 was posi- tively associated with respiratory ERV based on PDL models, although some single lag model associations were inverse. Effect estimates for PNC, PSC, and NO2 were mostly higher in magnitude after adjustment for PM10 [Table 4; see also Supplemental Material, Table 3 (doi:10.1289/ ehp.1002203)]. Associations with PNC and PSC were comparable in magnitude but less precise after adjustment for NO2, with borderline p-values (p < 0.09) only for PNC100–300 and PSC100–300 cumulative effects over 3 days (present and previous 2 days). In two-pollutant models, associations between respiratory ERV and NO2 were more consis- tent than those for other pollutants. A s s o c i a t i o n s w i t h P N C 100–300 w e r e robust to variations in model parameters [see Supplemental Material, Table 4 (doi:10.1289/ ehp.1002203)] in the sensitivity analyses. Discussion We observed adverse associations between respiratory ERV and NO2 and particle num- ber and surface area concentrations in sev- eral size ranges. Effects estimates for IQR increases in both particle number and surface area concentrations of 100–300 nm and 300– 1,000 nm particles were comparable with estimated effects for IQR increases in NO2 concentrations, and effect estimates for NO2 were more consistent than those for other exposures in two-pollutant models. In most cases, particles < 50 nm were not positively associated with respiratory ERV, whereas par- ticles in the size range of 50–100 nm were adversely associated with respiratory ERV, both being fractions of UFP. Our findings are consistent with results from other studies reporting that levels of air pollution, often represented by PM10, PM2.5, NO2, SO2, O3, or UFP, are associated with short-term increases in ERV for respiratory complaints (Knol et al. 2009; Peel et al. 2005; Stieb et al. 2009; Tolbert et al. 2007). Interestingly, we did not observe adverse associations of respiratory ERV with small particles (< 50 nm) but did observe positive Particles and respiratory health Table 3. Overview of RR (95% CIs) for respiratory ERV per IQR increment of air pollutant in single-pollutant models.a Pollutant Time delay SO2 NO2 PM10 UFP PNCtotal PNC3–10 PNC10–30 PNC30–50 PNC50–100 PNC100–300 PNC300–1,000 PSCtotal PSC50–100 PSC100–300 PSC300–1,000 Same day 5 Same day 5 Same day 5 Same day 5 Same day 1 2 3 4 5 Same day 5 Same day 5 Same day 1 2 3 4 5 Same day 1 2 3 4 5 Same day 1 2 3 4 5 Same day 5 Same day 1 2 3 4 5 Same day 1 2 3 4 5 Same day 1 2 3 4 5 Same day 1 5 IQRb 100 100 40 40 90 90 11,000 11,000 12,600 12,600 12,600 12,600 12,600 12,600 3,880 3,880 4,300 4,300 2,300 2,300 2,300 2,300 2,300 2,300 3,600 3,600 3,600 3,600 3,600 3,600 4,400 4,400 4,400 4,400 4,400 4,400 830 830 960 960 960 960 960 960 60 60 60 60 60 60 440 440 440 440 440 440 490 490 490 Cumulative effects modelc RR (95% CI) 1.01 (0.97–1.05) 1.04 (0.97–1.12) 1.02 (0.98–1.05) 1.06 (1.00–1.12)* 1.01 (0.98–1.05) 1.00 (0.94–1.06) 1.01 (0.95–1.07) 0.99 (0.89–1.09) 1.03 (0.98–1.09) 1.04 (0.97–1.12) 1.07 (0.99–1.16) 1.05 (0.96–1.15) 1.04 (0.94–1.14) 1.03 (0.93–1.15) 0.97 (0.93–1.01) 0.94 (0.86–1.02) 0.98 (0.93–1.04) 0.98 (0.88-1.10) 1.03 (0.99–1.08) 1.03 (0.97–1.09) 1.05 (0.98–1.12) 1.04 (0.96–1.12) 1.03 (0.94–1.12) 1.01 (0.92–1.11) 1.03 (0.99–1.07) 1.03 (0.98–1.09) 1.07 (1.00–1.15)* 1.08 (1.00–1.17)* 1.05 (0.96–1.14) 1.06 (0.97–1.17) 1.04 (1.00–1.08) 1.05 (0.99–1.11) 1.09 (1.02–1.16)* 1.08 (1.00–1.17)* 1.05 (0.96–1.14) 1.08 (0.99–1.18) 1.04 (1.00–1.08) 1.03 (0.96–1.10) 1.05 (1.00–1.09)* 1.04 (0.99–1.10) 1.05 (0.99–1.11) 1.04 (0.98–1.11) 1.02 (0.95–1.09) 1.04 (0.97–1.12) 1.03 (0.99–1.07) 1.03 (0.98–1.09) 1.07 (1.01–1.15)* 1.09 (1.01–1.17)* 1.05 (0.96–1.14) 1.07 (0.97–1.17) 1.04 (1.00–1.09)* 1.05 (0.99–1.11) 1.08 (1.01–1.15)* 1.07 (1.00–1.15) 1.04 (0.96–1.13) 1.07 (0.98–1.17) 1.04 (1.00–1.08) 1.03 (0.99–1.08) 1.02 (0.96–1.09) PDL modeld RR (95% CI) 1.00 (0.96–1.04) 1.01 (0.98–1.04) 1.01 (0.97–1.05) 1.01 (0.98–1.04) 1.01 (0.97–1.05) 1.02 (0.99–1.06) 1.00 (0.94–1.07) 0.93 (0.88–0.99)* 1.01 (0.95–1.08) 1.06 (1.01–1.11)* 1.04 (1.00–1.08) 0.99 (0.95–1.03) 0.96 (0.92–1.00)* 0.97 (0.92–1.03) 0.98 (0.93–1.03) 0.96 (0.92–0.99)* 0.99 (0.93–1.06) 0.95 (0.90–1.00)* 1.02 (0.98–1.07) 1.04 (1.00–1.07)* 1.02 (0.99–1.05) 0.99 (0.96–1.02) 0.97 (0.94–1.00) 0.96 (0.92–1.01) 1.01 (0.97–1.06) 1.05 (1.01–1.08)* 1.03 (1.00–1.06)* 0.99 (0.97–1.02) 0.98 (0.95–1.01) 1.01 (0.97–1.05) 1.04 (0.99–1.08) 1.05 (1.02–1.08)* 1.02 (0.99–1.04) 0.99 (0.97–1.01) 0.98 (0.96–1.01) 1.04 (1.00–1.08)* 1.04 (0.99–1.08) 1.04 (1.00–1.07)* 1.04 (1.00–1.08) 1.03 (1.00–1.05)* 1.00 (0.98–1.02) 0.98 (0.96–1.00) 0.99 (0.97–1.01) 1.04 (1.01–1.08)* 1.01 (0.97–1.06) 1.05 (1.02–1.08)* 1.03 (1.00–1.06)* 1.00 (0.97–1.02) 0.98 (0.95–1.01) 1.01 (0.97–1.05) 1.04 (1.00–1.09) 1.04 (1.01–1.07)* 1.01 (0.99–1.04) 0.99 (0.96–1.01) 0.99 (0.96–1.01) 1.04 (1.00–1.08)* 1.04 (0.99–1.08) 1.01 (0.99–1.04) 1.04 (1.01–1.07)* Single lag modele (95% CI) 1.01 (0.97–1.05) 1.01 (0.98–1.05) 1.02 (0.98–1.05) 1.02 (0.98–1.05) 1.01 (0.98–1.05) 1.02 (0.99–1.05) 1.01 (0.95–1.07) 0.93 (0.88–0.97)* 1.03 (0.98–1.09) 1.02 (0.96–1.08) 1.03 (0.98–1.10) 0.96 (0.91–1.02) 0.95 (0.90–1.00) 0.94 (0.89–0.99)* 0.97 (0.93–1.01) 0.96 (0.93–0.99)* 0.98 (0.93–1.04) 0.95 (0.90–0.99)* 1.03 (0.99–1.08) 1.00 (0.95–1.04) 1.01 (0.96–1.05) 0.98 (0.93–1.02) 0.95 (0.91–0.99)* 0.96 (0.91–1.00) 1.03 (0.99–1.07) 1.00 (0.96–1.04) 1.03 (0.98–1.07) 1.00 (0.96–1.04) 0.96 (0.92–1.00)* 0.99 (0.95–1.03) 1.04 (1.00–1.08) 1.01 (0.97–1.06) 1.03 (0.99–1.07) 0.99 (0.96–1.03) 0.98 (0.94–1.01) 1.02 (0.98–1.05) 1.04 (1.00–1.08) 1.02 (0.99–1.05) 1.05 (1.00–1.09)* 1.01 (0.98–1.05) 1.01 (0.98–1.05) 0.99 (0.96–1.03) 0.99 (0.96–1.02) 1.02 (0.99–1.06) 1.03 (0.99–1.07) 1.00 (0.96–1.04) 1.03 (0.99–1.07) 1.00 (0.96–1.04) 0.96 (0.93–1.00) 0.99 (0.95–1.03) 1.04 (1.00–1.09)* 1.02 (0.98–1.06) 1.02 (0.99–1.06) 0.99 (0.95–1.03) 0.98 (0.95–1.02) 1.02 (0.98–1.06) 1.04 (1.00–1.08) 1.01 (0.98–1.05) 1.02 (0.99–1.06) aFor a complete table, see Supplemental Material, Table 2 (doi:10.1289/ehp.1002203). bUnits for IQR: SO2, NO2, and PM10 (µg/m3); PNCx and UFP (1/cm3); PSCx (µm2/cm3). cCumulative-effects models represent time-delayed effects with moving averages up to 6 days (mean of the same day and 5 previous days). dPDL models with the lag coefficients follow a third- degree polynomial of the lag number and a maximum lag of 5 days. eSingle lag models represent time-delayed effects with lagged effect up to 5 days. *p < 0.05 (p-values for the hypothesis that the corresponding parameter is zero). Environmental Health Perspectives • volume 119 | number 4 | April 2011 511