Table 1.

Characteristics of Ecological Studies on Air Pollution from Motorized Traffic and Childhood Leukemia.

Reference	Region	Cases	Time frame	Type of cancera	Age	Assessment methods	Main resultsa
Alexander et al. 1996 [15]	United Kingdom	438	1984-1989	ALL	0-14 1-7	Car ownership: number of cars per household within electoral wards (area unit)	Inverse correlation between car ownership and total ALL, which was also present in stratified analysis of ALL among children ages 1-7 years
Nordlinder et al. 1997 [16]	Sweden	1528	1975-1985	ALL, AML, NHL, CML	0-24	Car density in km2: continuous scale and <5 vs. >20 cars/km2	In municipalities with more than 20 cars/km2 the incidence of AML was 5.5 (95% CI 4.4 – 6.8) as compared with 3.4 (1.9 – 5.7) cases per 1 million person-years in those with less than 5 cars/km2 (p=0.05)
Reynolds et al. 2002 [17]	California	7143	1988-1994	All cancers, all leukemias, gliomas	0-15	Ambient air monitoring data and vehicle density (vehicles per square mile), road density (miles of roads per sq mi) and traffic density (vehicle miles traveled per day per sq mi)	Traffic density indicators were strongly correlated with measures of benzene and 1,3-butadiene, carbon monoxide and nitrogen dioxides and slightly with particulate matter. Rate ratios at the 90th percentiles of traffic density were 1.08 (95% CI 0.98 – 1.20) for all cancers, 1.15 (0.97 – 1.97) for leukemia and 1.14 (0.90 – 1.45) for gliomas.
Reynolds et al. 2003 [18]	California	6989	1988-1994	All cancers, all leukemias, gliomas	0-15	25 Hazardous air pollutant (HAPs) exposure by census tract modeled by US EPA and divided into percentiles	Elevated rate ratios with increasing exposure levels in tracts ranked highest for exposure to HAPs: for all leukemias they were 1.21 (95% CI 1.03 – 1.42) for of the combination of 25 HAPs grouped together and 1.32 (1.11 – 1.57) for point-source HAP exposure.
Whitworth et al. 2008 [19]	Texas	977	1995-2004	All leukemias, ALL, AML, HL	0-20	Benzene and 1,3-butadiene exposure level: lowest vs. highest quartile US EPA ASPEN estimates of ambient air pollutants	Elevated rate ratios for benzene were 1.37 (95% CI 1.05 – 1.78, p=0.019), 2.02 (1.03 – 3.96, p=0.153) and 1.24 (0.92 – 1.66, p=0.040) for all leukemias, AML and ALL respectively; for 1,3-butadiene rate ratios were 1.40 (1.07 – 1.81, p=0.013), 1.68 (0.84 – 3.35, p=0.064), and 1.32 (0.98-1.77, p=0.142) for all leukemias, AML and ALL respectively
Senkayi et al. 2014 [20]	Texas	2134	1995-2005	All leukemias	0-9	Benzene modeled emissions from airports (aircraft exhaust and auxiliary power units) and roads	Beta regression coefficients (β) showed strong relations between benzene emissions from roads with β=0.497 (95% CI 0.358 – 0.637, p<0.001) and from airports with β=0.230 (0.149 – 0.311, p<0.001)

^aALL, acute lymphoblastic leukemia; AML, acute myeloid leukemia; ANLL, acute non-lymphoblastic leukemia; NL, Hodgkin lymphoma; NHL, non-Hodgkin lymphomas; CML, chronic myeloid leukemia; CI, confidence intervals.