Children whose mothers were occupationally exposed to benzene face a higher risk of leukemia, according to a national Swiss cohort study published in the November issue of Environmental International. The study was conducted by the Swiss Paediatric Oncology Group and the Swiss National Cohort Study Group.
The association was strongest among children younger than age 5 years and was not due to socioeconomic factors, the authors reported. It was most pronounced for acute lymphoblastic leukemia (ALL).
“Maternal exposure was associated with an increased risk of childhood leukemia (hazard ratio [HR], 1.73; 95% CI, 1.12–2.67) and ALL (HR, 1.88; 95% CI, 1.16–3.04),” reported lead author Ben Daniel Spycher, PhD, of the Institute of Social and Preventive Medicine, University of Bern, in Bern, Switzerland, and coauthors. “This nationwide cohort study suggests an increased risk of leukemia among children whose mothers were exposed to benzene at work.”
In a multivariate analysis that controlled for potential confounding variables, the HR between leukemia risk and maternal benzene exposure rose to 2.31 (95% CI, 1.4–3.7).
Previous epidemiologic studies have tied childhood household exposures to benzene-containing paints and solvents to childhood leukemia, and scattered, small previous studies have suggested a possible link between parental occupational benzene exposure and childhood leukemia. However, benzene’s leukemogenicity is best established for adult acute myeloid leukemia (AML) following patients’ own occupational exposures.
The authors included children younger than age 16 years in the analysis of census data from 1990 and 2000. Parental occupations data were used to estimate benzene exposures and compared with data from the Swiss Childhood Cancer Registry. The analyzed data represented 9 million person-years for maternal occupational risk and 13.2 million person-years at risk for fathers.
“We found no evidence of an association for other cancer types,” Dr. Spycher and colleagues reported. “We found no evidence of an association for paternal exposure.”
Adjusting statistical analyses for potential confounders such as sex, birth year (in 5-year categories), and census year “did not materially affect the results.” However, secular trends in incidence or cancer registration coverage could confound the association, they cautioned. Parental education level, household density (persons per room), neighborhood socioeconomic quintile, birth order, birth weight, and urban, suburban or rural household were also controlled in multivariate analysis.
“The findings of our cohort study are in line with recent reviews of studies on the risk of childhood leukemia and parental exposure to benzene,” the authors noted. In a 2016 meta-analysis, associations between childhood leukemia and parental occupational benzene exposures also suggested a stronger association for maternal exposures than paternal exposures (relative risks of 1.96 and 1.23, respectively).
“A plausible explanation for our findings is that maternal exposure to benzene during pregnancy can initiate leukemia in the developing child,” the authors concluded.
However, benzene exposures are clearly not the only cause of childhood leukemias. Assuming the reported association was causal, the findings suggest that perhaps 3% of childhood leukemia cases recorded in Switzerland during the study period were attributable to maternal benzene exposures.