Traffic pollution near the home – and specifically, benzene in the air - increases the risk of one type of childhood leukemia, according to a nationwide study in France.

Leukemia, or cancer of the blood cells, is the most common cancer among children younger than age 15, according to the U.S. National Cancer Institute.

Childhood leukemia is a very rare disease, so it is hard to have enough cases in one study to determine which risk factors play a role, but according to this and other large surveys, the evidence seems to be pointing toward an association between traffic emissions and childhood leukemia, said coauthor Denis Hemon of the Institute National de la Santé et de la Recherché Medical (INSERM) based in Paris.

“Overall I would say the balance is in favor of an association,” Hemon told Reuters Health by phone.

The researchers used a nationwide study of 2,760 childhood leukemia cases in France compared with 30,000 kids who did not have leukemia between 2002 and 2007.

They used residential addresses to estimate proximity to traffic, including distance to nearest major road and total length of roads near the home, as well as estimates of benzene concentrations specifically for the Paris metro area.

Children who lived more than 500 meters from the nearest road were used for reference, having the lowest traffic exposure, and those who lived 150 meters or less from a major road had the highest exposure.

More than 2,000 of the leukemia cases were acute lymphoblastic leukemia (ALL), while only 418 were acute myeloblastic leukemia (AML).

A 300 meter increase in major road length within 150 meters of the home appeared to increase the risk of AML by 20 percent, but did not affect the risk of ALL, when the researchers compared the leukemia groups and the comparison group.

There were similar results specifically in the Paris metro area when benzene levels were included in the analysis, as reported in the American Journal of Epidemiology.

According to the Air Quality in Europe Report 2014 by the European Environmental Agency, the limit to environmental levels of benzene should be 5 micrograms per cubic meter, although the World Health Organization has not set an air quality guideline for benzene.

Short-term benzene exposure may cause drowsiness, dizziness, and headaches, as well as eye, skin, and respiratory tract irritation, while long-term inhalation exposure in occupational settings has caused various disorders in the blood, including reduced numbers of red blood cells and aplastic anemia, according to the U.S. Environmental Protection Agency.

Though benzene is carcinogenic for adults, it is not clear how benzene exposure would cause AML in children, Hemon said.

High radiation exposure can cause leukemia, as can genetic risk factors like Down Syndrome, and there may be other risk factors we do not know about, he said.

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