Air pollution could be a contributing factor in millions of premature births around the world each year, a new report has found.
Nearly 15 million babies are born annually before reaching 37 weeks' gestation.
Premature birth is the leading cause of death among children below five years old and can cause lifelong learning disabilities, visual and hearing problems, said the World Health Organisation (WHO).
Stockholm Environment Institute (SEI) researchers have concluded that as many as 3.4 million premature births across 183 countries could be associated with fine particulate matter, a common air pollutant. The places most affected are sub-Saharan Africa, north Africa and South-east Asia.
"Pre-term birth and associated conditions are the biggest killers of children worldwide," said Dr Paul Jarris, chief medical officer at the March of Dimes, a US-based non-profit organisation focusing on maternal and baby health.
IMPACT ON BABIES
We have known for a long time that air pollution contributes to asthma and heart disease in adults. What I think people fail to recognise is that so many of these risk factors impact babies before they are even born.
DR PAUL JARRIS, chief medical officer at the March of Dimes, on the impact of air pollution on premature births.
"We have known for a long time that air pollution contributes to asthma and heart disease in adults," said Dr Jarris. "What I think people fail to recognise is that so many of these risk factors impact babies before they are even born."
Previous studies have looked at how in-utero air pollution might negatively impact the birth weight of babies, or the likelihood that they will be born early.
SEI's study, which examined data from 2010, attempted to calculate how those factors might influence the global rate of premature births.
"By showing in our study that 18 per cent of pre-term births are associated with air pollution, we are quantifying the health impacts of fine particulate matter on babies before they are born," said Dr Chris Malley, a lead author of the study.
Each year, around one in every 10 babies worldwide are born prematurely, according to the WHO.
Africa and South Asia account for 60 per cent of all premature births globally. Those regions also dominated SEI's report of premature birth associated with air pollution.
SEI's new report gives an estimate of potential birth impacts associated with air pollution, but the authors acknowledged that the study had considerable caveats because of a lack of research in some of the most affected areas.
However, the report is still one of the first to argue that reducing air pollution could be effective in reducing premature births.
The report focused on one kind of air pollution considered especially harmful - fine particulate matter. This is made up of tiny particles from emissions, such as diesel emissions and agricultural fires. The particles are considered harmful as they can lodge deep in the lungs.
Notably, even though China has a relatively low rate of pre-term births, researchers found that as many as 521,000 pre-term births could be associated with air pollution because of high concentrations of particulate matter.
Researchers said the estimates support their conclusion that the "reduction of maternal (air pollution) exposure through emission- reduction strategies should be considered alongside mitigation of other risk factors associated with pre-term births".
Dr Jarris said: "The most vulnerable among us - unborn children - are affected and in a way that impacts the lives of families for generations."