Table 2.

Meta-analyses of associations between air pollutants and traffic indicators and the risk for asthma incidence.

Exposure	Increase	OR (95% CI)			Heterogeneity between cohorts (model 3)
		Model 1	Model 2	Model 3	I2(%)	p-Value
NOx, no. of participants		23,693	23,693	22,814
NO2	10 μg/m3	1.11 (1.00,1.23)	1.04 (0.99,1.09)	1.10 (0.99,1.21)	46.2	0.10
NO2 back-extrapolated to follow-up	10 μg/m3	1.10 (1.00,1.21)	1.04 (0.99,1.09)	1.10 (1.00,1.20)	49.6	0.08
NOx	20 μg/m3	1.09 (1.00,1.18)	1.04 (0.99,1.08)	1.04 (0.99,1.08)	39.8	0.14
PM, no. of participants		17,798b	17,798b	17,098b
PM10	10 μg/m3	1.05 (0.89,1.24)	1.05 (0.89,1.24)	1.04 (0.88,1.23)	0.0	0.44
PM10 back-extrapolated to follow-up	10 μg/m3	1.04 (0.88,1.24)	1.04 (0.88,1.24)	1.04 (0.87,1.24)	0.0	0.78
PMcoarse	5 μg/m3	0.98 (0.86,1.12)	0.98 (0.86,1.12)	0.99 (0.87,1.14)	0.0	0.61
PM2.5	5 μg/m3	1.11 (0.80,1.54)	1.04 (0.88,1.23)	1.04 (0.88,1.23)	24.2	0.25
PM2.5absorbance	10–5/m	1.05 (0.94,1.16)	1.05 (0.94,1.17)	1.06 (0.95,1.19)	44.5	0.11
Traffic variables, no. of participantsa		22,430	22,428	21,551
Traffic intensity on nearest road	5,000 vehicles/day	1.06 (0.98,1.14)	1.05 (0.98,1.13)	1.05 (0.98,1.13)	56.4	0.04
Traffic load in a 100-m buffer	4,000,000 vehicles × m/day	1.11 (0.94,1.31)	1.09 (0.94,1.27)	1.10 (0.93,1.30)	57.4	0.04
Model 1: unadjusted; model 2: adjusted for age and sex; model 3: adjusted for age, sex, smoking, overweight, and education level. The logistic regression models were conducted with random effects per city/area for each study except for SALIA, where there was only one area, and EGEA, where family structure was taken into account. The OR corresponds to the fixed effect when the p-value for heterogeneity was > 0.1; when the p-value for heterogeneity was < 0.1, the random effect is stated. I2: variation of estimate effect attributable to heterogeneity. aFor traffic intensity on the nearest road. bFor PM10.