Children's exposure to air traffic pollution could increase their risk of insulin resistance, which can lead to diabetes in adults, suggests a study in Diabetologia.
German research on 397 10-year-olds found that living close to a major road increased resistance by 7% per 500m.
Air pollutants are known to be oxidisers that can impact on lipids and proteins in the blood.
But some experts say the results should be treated with caution.
The children in the study were invited for blood sampling at the age of 10, and glucose and insulin measurements were taken.
Their level of exposure to traffic pollution was estimated using air pollution figures from 2008-09 for their birth address neighbourhood.
The results were adjusted to take into account birth weight, body mass index (BMI) and exposure to second-hand smoke at home.
The study concluded that levels of insulin resistance were greater in children with higher exposure to air pollution, such as nitrogen dioxide and fine particulate matter.
It also found a larger effect in children with higher BMIs.
Elisabeth Thiering and Joachim Heinrich, who led the research at the German Research Centre for Environmental Health in Neuherberg, said the link between traffic pollution and insulin resistance could be explained.
"Although toxicity differs between air pollutants, they are all considered potent oxidisers that act either directly on lipids and proteins, or indirectly through the activation of intracellular oxidant pathways," said Dr Heinrich.
"Oxidative stress caused by exposure to air pollutants may therefore play a role in the development of insulin resistance."
But Prof Jon Ayres, an expert in environmental and respiratory medicine, of the University of Birmingham, said the results were not clear-cut.
"As the authors point out, their measurements of fasting blood insulin levels and estimations of air pollution levels were not taken at the same time.
"Therefore, these results should be regarded with caution, and a larger and methodologically more secure study needs to be done to confirm the possible link between air pollution from traffic emissions and insulin resistance in children."
Previous studies have shown links between air pollution and other chronic conditions, such as atherosclerosis and heart disease.
Yet to date, epidemiological studies that have examined associations between long-term exposure to traffic-related air pollution and type 2 diabetes in adults are inconsistent, and studies on the effect of air pollution on insulin resistance in children are scarce.
Frank Kelly, professor of environmental health at King's College London, said children were particularly vulnerable.
"They have a larger lung-to-body volume ratio, their airway epithelium is more permeable to air pollutants, and the lung defence mechanisms against particulate matter pollution and gaseous pollution are not fully evolved."
"Breathing the same pollutant concentrations, children may have a two to fourfold higher dose reaching the lung compared with adults.
"It is of interest that this new study demonstrates that both particulate matter and nitrogen dioxide are linked to increased risk of insulin resistance in children. This finding is especially relevant for cities in the UK such as London, which regularly exceeds current EU limit values for nitrogen dioxide."