Table 2.

Summary risk ratios (RRs) of childhood leukemia in the highest exposure category versus the lowest one for traffic density, benzene, and nitrogen dioxide (${\mathrm{NO}}_2$) exposure, for all studies and stratified by age at diagnosis, leukemia subtype, exposure timing, and region.

	All children					Preschool children ($<6\mathrm{y}\text{of}\text{age}$)					$\text{Children}\ge 6\mathrm{y}\text{of}\text{age}$
Indicator	$n$	RR	95% CI	$I^2$ (%)	${\mathrm{\tau }}^2$	$n$	RR	95% CI	$I^2$ (%)	${\mathrm{\tau }}^2$	$n$	RR	95% CI	$I^2$ (%)	${\mathrm{\tau }}^2$
Traffic density
All leukemia	16	1.09	1.00, 1.20	56.2	0.012	7	1.00	0.93, 1.09	67.0	0.006	3	1.05	0.96, 1.15	10.0	0.002
Subtype
ALL	9	1.05	0.96, 1.16	34.7	0.005	3	1.02	0.99, 1.05	0.0	0.000	1	1.00	0.92, 1.09	—	0.000
AML	5	1.09	0.86, 1.38	70.0	0.034	2	1.03	0.77, 1.38	87.7	0.039	1	1.25	1.02, 1.53	—	0.000
Exposure timing
At birth	5	0.98	0.90, 1.06	30.9	0.003	4	0.95	0.85, 1.05	52.4	0.006	1	1.15	0.78, 1.70	—	0.000
At diagnosis	14	1.32	1.12, 1.55	50.7	0.033	3	1.27	0.95, 1.71	81.0	0.049	2	1.05	0.94, 1.17	24.6	0.004
Region
Asia	1	1.27	0.51, 3.17	—	0.000	—					—
Europe	9	1.25	1.05, 1.49	50.4	0.026	3	1.05	0.87, 1.25	63.8	0.017	1	1.05	0.95, 1.17	—	0.000
North America	6	1.02	0.89, 1.16	65.4	0.014	4	0.98	0.84, 1.15	74.9	0.017	2	1.09	0.72, 1.64	17.4	0.032
Benzene
All leukemia	7	1.27	1.03, 1.56	52.4	0.043	4	1.39	1.03, 1.87	27.9	0.035	2	1.08	0.64, 1.82	0.0	0.000
Subtype
ALL	7	1.09	0.88, 1.36	51.8	0.034	3	1.19	1.00, 1.40	0.0	0.000	1	0.69	0.27, 1.78	—	0.000
AML	5	1.84	1.31, 2.59	0.0	0.000	2	3.21	1.39, 7.42	0.0	0.000	1	0.43	0.04, 4.79	—	0.000
Exposure timing
At birth	3	1.21	1.04, 1.41	0.0	0.000	3	1.22	1.03, 1.43	0.0	0.000	1	1.14	0.63, 2.08	—	0.000
At diagnosis	4	1.36	0.92, 2.00	65.2	0.125	1	3.30	1.03, 10.59	—	0.000	1	0.90	0.31, 2.60	—	0.000
Region
Asia	—					—					—
Europe	4	1.36	0.92, 2.00	65.2	0.125	1	3.30	1.03, 10.59	—	0.000	1	0.90	0.31, 2.60	—	0.000
North America	3	1.21	1.04, 1.41	0.0	0.000	3	1.22	1.03, 1.43	0.0	0.000	1	1.14	0.63, 2.08	—	0.000
${\mathrm{NO}}_2$
All leukemia	8	1.04	0.90, 1.19	55.5	0.023	4	1.03	0.90, 1.18	14.9	0.004	1	0.89	0.42, 1.89	—	0.000
Subtype
ALL	4	1.02	0.89, 1.18	55.6	0.011	2	1.10	0.92, 1.32	46.2	0.008	—
AML	4	0.97	0.79, 1.19	0.0	0.000	2	0.86	0.60, 1.23	0.0	0.000	—
Exposure timing
At birth	4	1.07	0.96, 1.19	0.0	0.000	4	1.03	0.90, 1.18	14.9	0.004	1	0.89	0.42, 1.89	—	0.000
At diagnosis	4	1.17	0.82, 1.67	74.9	0.093	—					—
Region
Asia	1	2.29	1.44, 3.64	—	0.000	—					—
Europe	4	0.91	0.82, 1.00	0.0	0.000	1	0.79	0.52, 1.20	—	0.000	—
North America	3	1.06	0.95, 1.18	0.0	0.000	3	1.06	0.94, 1.19	1.7	0.000	1	0.89	0.42, 1.89	—	0.000

Note: —, data not available; ALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; CI, confidence interval; $I^2$, I-squared statistic; RR, risk ratio; ${\mathrm{\tau }}^2$, tau-squared statistic.