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.

Health a risk: A woman and child wear masks during a hazy day in Beijing. Credit:AP

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.