Those studies have shown that exposure to fine particles that invade the breathing system and get into the heart and blood vessels increases inflammation, which may be linked to insulin resistance, said Joachim Heinrich of the German Research Centre for Environmental Health, one of the study authors.

"Given the ubiquitous nature of air pollution and the high incidence of insulin resistance in the general population, the associations examined here may have potentially important public health effects," Heinrich said in the published paper.

Diabetes occurs when blood-sugar levels are too high. In the Type 1 form of the disease, the body is unable to produce insulin, the hormone used to convert blood sugar into energy. In Type 2 diabetes, the body either cannot produce enough insulin or becomes resistant to its effects.

Type 2 diabetes tends to strike later in life, brought on by obesity and sedentary lifestyles. The Type 2 form accounts for 90 per cent of the 347 million cases of diabetes globally, according to the World Health Organisation.

The researchers collected blood samples from the 397 children who were included in two German birth cohort studies. Exposure to air pollutants at their birth addresses were estimated by analysing emissions from road traffic in the neighbourhood, population density and land use in the area.

The measurements of blood insulin levels and estimates of pollution were taken at different times, so the findings "should be regarded with caution", said Jon Ayres, a professor of environmental and respiratory medicine at the University of Birmingham in England.

A larger study should be conducted to confirm the possible link, Ayres said.

A follow-up study at age 15 would explore how these findings evolved during and after puberty and the effect of moving to a cleaner area, Heinrich and his colleagues said.

The research was funded by the German Federal Ministry of Education and Research and the European Community's Seventh Framework Programme.