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 younger than five years old, and can cause lifelong learning disabilities, visual and hearing problems, the World Health Organization (WHO) reports.
Researchers for the Stockholm Environment Institute (SEI), the London School of Hygiene and Tropical Medicine and the University of Colorado, 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, with sub-Saharan Africa, North Africa and South and East Asia most impacted by the issue.
Greenpeace has calculated the increased risk of death at varying levels of air pollution in 3,000 cities around the world – by combining risk analysis from the IHME's Global Burden of Disease project with annual average background levels of fine particulate matter (PM2.5s) from WHO.
Previous studies have looked at how effects of air pollution in utero might negatively impact babies' birth weight, 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 preterm births are associated with air pollution, we are quantifying the health impacts of fine particulate matter on babies before they are born,'' said Chris Malley, a researcher in SEI's York Centre, at the University of York and lead author on the study.
Each year, around one in every 10 babies worldwide are born prematurely, according to the WHO. Africa and South Asia (including India) bear a disproportionate burden of premature births, accounting for 60 per cent of all premature births globally. That region also dominated SEI's report of premature birth associated with air pollution.
SEI's new report provides 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.
Still, two experts who reviewed the study called estimates ''conservative''.
A number of factors have been identified in playing a part on premature birth, including poverty, infection, smoking and substance use, physical activity and maternal education. However, even with the report's limitations, it is still one of the first to argue that reducing air pollution could also be effective in reducing premature births across the world.
Because of a lack of research in regions such as south and east Asia and Africa, researchers used studies conducted in the US and Europe to estimate global outdoor air pollution exposure.
Indoor cooking fires could play a larger role than the study allows for. ''The exposure in Asia, and in part in say, China and India, to outdoor air pollution is significantly higher than outdoor air pollution in the US,'' said Malley. ''So that's the other part of the uncertainty, that at these higher levels of exposure we don't know whether this same relationship holds.''
The report focused on one kind of air pollution considered especially harmful to human health - fine particulate matter. This pollution is made up of tiny particles from a variety of emissions, such as diesel emissions and agricultural fires.
Other recent studies have pointed to the increasing levels of pollution-related deaths due to PM2.5 particles. (See: India pollution now worse than China, kills 1.1 mn a year). The particles, smaller than 2.5 micrometres in diameter, are considered harmful because they can lodge deep in the lungs, affecting the pulmonary and cardiovascular systems. Past studies have looked at these factors as ways that air pollution may impact preterm birth and low birth weight in babies.
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South Asia had the most preterm births associated with particulate matter pollution – as many as 1.6 million associated births. Notably, even though China has a relatively low rate of preterm births, researchers found that as many as 521,000 preterm births could be associated with air pollution because of high concentrations of particulate matter.
Researchers said the estimates support their conclusion that ''reduction of maternal [air pollution] exposure through emission reduction strategies should be considered alongside mitigation of other risk factors associated with preterm births.''
''This is one more reason why we need to be good stewards of the environment,'' said Jarris. ''The most vulnerable among us – unborn children – are affected, and really in a way that impacts families' lives for generations.''