Table 3.

Summary risk ratios (RRs) for association of childhood leukemia with particulate matter (${\mathrm{PM}}_{2.5}/{\mathrm{PM}}_{10}$) and 1,3-butadiene comparing the highest versus the lowest exposure categories 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	$I^2$ (%)	${\mathrm{\tau }}^2$
${\mathrm{PM}}_{2.5}$
All leukemia	3	1.05	0.94, 1.16	0.0	0.000	3	1.04	0.94, 1.16	0.0	0.000	—
Subtype
ALL	2	1.11	0.95, 1.31	0.0	0.000	2	1.11	0.95, 1.31	0.0	0.000	—
AML	2	1.00	0.87, 1.13	0.0	0.000	2	1.00	0.87, 1.13	0.0	0.000	—
Exposure timing
At birth	3	1.05	0.94, 1.16	0.0	0.000	3	1.04	0.94, 1.16	0.0	0.000	—
At diagnosis	—					—					—
Region
Europe	1	1.00	0.72, 1.39	—	0.000	1	0.94	0.62, 1.43	—	0.000	—
North America	2	1.05	0.94, 1.17	0.0	0.000	2	1.05	0.94, 1.17	0.0	0.000	—
${\mathrm{PM}}_{10}$
All leukemia	2	1.20	0.70, 2.04	43.5	0.075	2	1.09	0.66, 1.80	12.3	0.028	1	1.50	0.48, 4.70	—	0.000
Subtype
ALL	1	1.45	0.73, 2.87	—	0.000	1	1.50	0.52, 4.33	—	0.000	1	1.39	0.54, 3.57	—	0.000
AML	1	1.30	0.41, 4.14	—	0.000	1	1.21	0.18, 8.16	—	0.000	1	1.18	0.25, 5.56	—	0.000
Exposure timing
At birth	1	1.00	0.70, 1.42	—	0.000	1	0.97	0.62, 1.51	—	0.000	—
At diagnosis	1	1.80	0.82, 3.97	—	0.000	1	1.90	0.60, 6.01	—	0.000	1	1.50	0.48, 4.70	—	0.000
Region
Europe	2	1.20	0.70, 2.04	43.5	0.075	2	1.09	0.66, 1.80	12.3	0.028	1	1.50	0.48, 4.70	—	0.000
North America	—					—					—
1,3—Butadiene
All leukemia	2	1.45	1.08, 1.95	28.0	0.025	2	1.45	1.08, 1.95	28.0	0.025	—
Subtype
ALL	2	1.31	1.11, 1.54	0.0	0.000	2	1.31	1.11, 1.54	0.0	0.000	—
AML	1	2.35	1.02, 5.40	—	0.000	1	2.35	1.02, 5.40	—	0.000	—
Exposure timing
At birth	2	1.45	1.08, 1.95	28.0	0.025	2	1.45	1.08, 1.95	28.0	0.025
At diagnosis	—					—
Region
Europe	—					—					—
North America	2	1.45	1.08, 1.95	28.0	0.025	2	1.45	1.08, 1.95	28.0	0.025	—

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.