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 Sant et de la Recherch 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.
Benzene concentrations near Paris ranged from 0.3 to 8.5 micrograms per cubic meter.
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.
Evidence is starting to mount that exposure to traffic emissions early in life is tied to childhood leukemia, either only to AML or just more strongly to this than to other forms of leukemia, said professor Marco Vinceti of the Universita di Modena e Reggio Emilia in Italy, who has studied this question but was not part of the French paper.
"Our study as well found the same association with leukemia," Vinceti told Reuters Health by phone. "We looked at particulate matter and benzene, and the results were that there was no association for particulate matter."
The French study was large and carefully avoided selection bias by including data from all patients diagnosed in the country, he said.
There seems to be a stronger link between benzene and leukemia in Europe than in the U.S., though it is unclear why, he said.
Though benzene levels have sharply decreased in Europe in recent years, and the levels in the current study are largely well below European regulations, childhood leukemia cases have not decreased, Vinceti said. This may be due to other risk factors, like genetics, he said.
There is not much an individual can do to reduce exposure to traffic emissions in a big city, other than supporting legislation to further reduce automobile emissions, Hemon said.