Exposure to fine particulates from burning coal, vehicle exhausts and other pollution sources is linked to an increased risk of pre-term births, according to a study of more than 1 million Chinese births that included an Australian researcher.
The study, published Wednesday by the American Medical Association's JAMA Pediatrics journal, claims to be the first to examine the impact of particles of 1 micrometre (PM1) - a millionth of a metre - or smaller.
It found that an increase in PM1 of 10 micrograms per cubic metre over the entire pregnancy led to a 9 per cent increased risk of a pre-term birth. Where pollution was over 52 micrograms per cubic metre, the chance of a pre-term birth rose 36 per cent.
While governments around the world are starting to set guidelines or warn about PM2.5 and PM10 levels, the study suggests authorities should urgently review standards to include PM1 levels, and to broadcast those readings.
"PM1 is the major part of PM2.5 pollution," said Guo Yuming, an associate professor at Monash University's School of Public Health and Preventive Medicine in Australia, and one of the report's authors. "No study has looked at PM1 before."
Early births are the leading cause of death for neonates, infants and children of less than five years of age worldwide, the researchers said.
"Pre-term births is also a risk factor for [the whole] lifetime," Professor Guo said. "It's related to asthma, short life expectancy, and it's related to diabetes."
An association between exposure to PM1 particulates and pre-term births - with gestation of 37 weeks or fewer - could trigger the development of new health guidelines if established by further studies, Professor Guo said.
"If we want to improve air quality or reduce air pollution, I think field studies should look at PM1," he said. "I think the World Health Organisation should very interested in these results."
The impacts extend to countries, including Australia, which have tended to assume their air quality is relatively good, and looked mainly at readings of large particulates.
"People don't consider their health to be impacted by PM2.5 because they think their air quality is very good," Professor Guo said. "But if we look at PM1, their health may be at risk."
James Whelan, a researcher with Environmental Justice Australia, said the research had direct consequences for protecting community health in Australia, adding that other work showed children born within 50 kilometres of coal-fired power stations were more likely to have reduced birth weight, contributing to a range of adverse health impacts throughout life.
"Researchers have for decades advised that the relationship between adverse health impacts (including premature death) and particle concentrations is strongest for these particles smaller than one micron in diameter," Dr Whelan said, adding that state and territory governments do not routinely monitor concentrations of sub-micron particles.
"There is no safe concentration of fine particle pollution," he said. "Although concentrations in Australia are lower than in China where this study was undertaken, further reducing particle pollution has direct and immediate health benefits including full-term birth and increased birth weight."
"Coal-fired power stations are the leading source of fine particle pollution," he said. "Australian governments do not require the owners of our fleet of ageing power stations to install readily available pollution controls that would dramatically reduce emissions of these harmful ultra-fine particles."
The JAMA study covered more than1.3 million single live births between 20 and 45 weeks, 8 per cent of which were pre-term. The results also suggested those pregnant during autumn and of lower education levels were more exposed to pre-term births.