Nitrogen dioxide pollution, churned out by diesel engines like those in lorries, vans and buses, could disrupt how well children burn fat, scientists say.
A study found 10 year olds who lived in polluted areas when they were babies are, on average, 2.2lbs (1kg), heavier than those who grew up around cleaner air.
The first year of a baby's life is a 'critical window', experts say, and parents should think carefully about where they raise their children and how it affects their health.
The research comes after the World Health Organization revealed 90 per cent of the world's children are breathing unsafe air.
Researchers from the University of Southern California followed 2,318 children in the Golden State to track how pollution affected their bodyweight later in life.
They found those who spent their first year of life in a polluted area gain weight significantly faster as they grow older.
In the study, they wrote: 'Our study suggests that early life may represent a critical window of exposure where increased [air pollution] may result in increased risk for higher childhood [weight] trajectories, which in turn may lead to childhood obesity.'
Although the study did not determine the reasons for this, it could be because pollution causes inflammation which affects the brain, The Guardian reported.
Lead researcher PhD student Jennifer Kim told the newspaper: 'The most common thought is inflammation of body systems like the lungs which may spill over into the entire body – the brain which regulates appetite and changes in fat metabolism.'
The research focused on the effect of nitrogen dioxide (NO2), which is at illegal in levels in most UK cities and urban areas around the world.
People are becoming more concerned about the effects of pollution as scientific studies stack up the damage it's doing to people's physical and mental health.
Other recent research has shown air polluted by vehicles raises the risk of a low birth weight, birth defects and cot death.
And a study published in October claimed as many as 33million visits to A&E by people with asthma around the world are due to air pollution.
'We would urge parents to be mindful where their young children spend their time, especially considering if those areas are near major roads, Ms Kim said.
'The first year of life is a period of rapid development of various systems in the body [and] may prime the body’s future development.'
The research was published in the journal Environmental Health.
Professor Jonathan Grigg, of Queen Mary University, was not involved in the research but told The Guardian: 'This study showing an association between increased body mass in children and exposure to air pollution from roads is important since it is compatible with previous studies showing an association between type 2 diabetes and air pollution in adults.
'However, more research is needed to explain how toxins inhaled into the lungs affect fat cells throughout the body.'
The Daily Mail
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