TABLE 1

Incidence rate ratio (IRR) for wheezing lower respiratory tract illnesses in the first 3 years of life by 1) diesel traffic related air pollutant (D-TRAP) exposure, 2) maternal age, and 3) D-TRAP exposure by maternal age category

Exposure	Subjects n	Crude			Adjusted^#
Exposure	Subjects n	IRR	95% CI	p-value	IRR	95% CI	p-value
Model 1: D-TRAP^¶	822	1.13	1.03–1.23	0.009	1.07	0.98–1.17	0.13
Model 2: maternal age
<21 years	42	1.01	0.66–1.54	0.96	1.06	0.70–1.63	0.77
21–25 years	238	1.33	1.01–1.76	0.04	1.35	1.04–1.76	0.02
26–30 years	316	1.14	0.88–1.47	0.34	1.20	0.93–1.56	0.17
>30 years	229	Reference			Reference
Model 3: D-TRAP by maternal age^¶
<21 years	42	1.35	1.05–1.74	0.02	1.31	1.02–1.67	0.03
21–25 years	238	1.33	1.14–1.57	<0.001	1.25	1.07–1.45	0.005
26–30 years	316	0.96	0.82–1.13	0.63	0.94	0.81–1.11	0.47
>30 years	229	0.97	0.80–1.17	0.75	0.92	0.75–1.12	0.42

Results are from negative binomial regression models, and statistically significant associations are indicated in boldface. An interaction term was included in model 3 with D-TRAP exposure and maternal age, evaluated with the Wald test (chi-squared 16.94, degrees of freedom 3, p=0.0007). ^#: adjusted for child ethnicity (variance inflation factor (VIF) 1.19), older children in the household (VIF 1.10), household cooling methods (VIF 1.03) and census tract socioeconomic status (VIF 1.23). VIF was 1.11 and 1.13 for D-TRAP and maternal age, respectively. ^¶: IRR per log increase in D-TRAP exposure.