Table 5.

Traffic-Related Air Pollution and Repeated Hospital Encounters for Children With Asthma by Socioeconomic Status and Race/Ethnicity

Model	Patients, No.	Exposure	HR (95% CI)a	P value	Product term P valueb
Povertyc
Median or less	1,384	NOx	1.078 (0.999–1.163)	.05	Reference
		CO	1.054 (0.979–1.134	.16	Reference
Greater than the median	1,384	NOx	1.116 (1.026–1.214)	.01	.39 (.70)
		CO	1.094 (1.006–1.190)	.04	.49 (.71)
Median household incomed
Greater than the median	1,383	NOx	1.145 (1.054–1.244)	.001	Reference
		CO	1.120 (1.034–1.213)	.005	Reference
Median or less	1,385	NOx	1.068 (0.983–1.160)	.12	.23 (.09)
		CO	1.041 (0.959–1.129)	.34	.20 (.07)
Insurance status
Private	1,094	NOx	1.136 (1.036–1.247)	.007	Reference
		CO	1.102 (1.006–1.206)	.04	Reference
Government sponsored or self-pay	1,413	NOx	1.080 (1.005–1.160)	.04	.38
		CO	1.061 (0.989–1.138)	.10	.51
Unknown	261	NOx	0.886 (0.569–1.379)	.59	.28
		CO	0.913 (0.591–1.412)	.68	.41
Race/ethnicity
White	1,237	NOx	1.145 (1.055–1.243)	.001	Reference
		CO	1.113 (1.027–1.205)	.009	Reference
Hispanic	1,221	NOx	1.097 (1.008–1.193)	.03	.46
		CO	1.081 (0.996–1.173)	.06	.62
Non-Hispanic nonwhite	310	NOx	0.829 (0.624–1.102	.20	.03
		CO	0.804 (0.601–1.074)	.14	.03

Abbreviations: CI, confidence interval; CO, carbon monoxide; HR, hazard ratio; NO_x, nitrogen oxide.

^aValues are for an interquartile range increase in the air pollutant (NO_x, 4.00 ppb; CO, 0.056 ppm) adjusted for sex, age group, race, health insurance status, residence distance to hospital, and poverty. Stratified results are from product term models.

^bThe P value for interaction of air pollution with socioeconomic variables and race/ethnicity. The P value for trend from continuous poverty and median household income is given in parentheses.

^cThe median of Census 2000 block group percentage below the federal poverty level was 14%.

^dThe median of Census 2000 block group of median household income was $45,000.