Unborn babies exposed to toxic smoke from the Hazelwood coal mine fire in 2014 were more likely to develop respiratory infections and wheezes as toddlers compared with babies who inhaled the smoke themselves, a study has found.

Acrid smoke from the coal mine hovered over the regional community of Morwell and the broader Latrobe Valley for six weeks in February and March 2014.

A mine fire inquiry later found the pollution likely led to deaths in the region.

In 2015, researchers formed the Latrobe Early Life Follow-up study to track the health of Latrobe Valley children for the first 1,000 days of their lives.

Seventy-nine children exposed to smoke in utero and 81 children who were aged zero to two years participated in the study.

Another 129 children conceived after the fire, who were not exposed to the smoke, formed a comparison sample.

Their parents provided monthly online diaries for 29 months after the fire in which they reported their children's symptoms, medical appointments and medication use.

Nine researchers from four universities around Australia analysed the data.

They found babies exposed to the coal mine smoke in utero were more likely to have runny noses, coughs, wheezing, colds and flus as well as upper respiratory tract infections two to four years after the fire.

"And because there was a single exposure route for most participating children."

For an unborn baby to be exposed, finer particles of pollution needed to cross the placenta directly to the foetus.

"Because we had children who were exposed after they were born, we found effects in those children," said Graeme Zosky of the Menzies Institute for Medical Research at the University of Tasmania.

Professor Zosky said that broadly speaking, there was approximately a 50 per cent increase in the risk of wheeze and a 35 per cent increase in the risk of having a doctor-diagnosed upper respiratory tract infection, cold or flu compared to the control group for every 10ug/m³ (10 micrograms per cubic metre) increase in daily average particulate matter.

"The highest average exposures for some children in the study was 50ug/m³," he said.

Unborn babies vulnerable to pollution

The Hazelwood coal mine fire was one of Victoria's worst pollution events.

In Morwell's south, within 1 kilometre of the fire, the 24-hour average concentration of fine particulate matter exceeded the national air quality standards on 23 days.

Between June 2016 and October 2018, two to four years after the fire, researchers sent participating parents monthly text messages with a link to an online survey to collect information about symptoms during the preceding month.

Parents reported a range of their toddler's symptoms including runny noses, coughs, wheezing, fevers, ear infections, eczema, bronchiolitis, chest infections and rashes as well as contact with doctors, maternal health nurses and pharmacists.

The use of oral antibiotics, asthma inhalers and topical steroid containing creams was also documented.

From the 4,672 monthly parent diaries analysed, a cough or runny nose were the most frequently reported symptoms, and 11 per cent of the diaries reported upper respiratory tract infections.

Professor Zosky said the finding that pre-natal exposure was more damaging than during early childhood was unexpected, as inhalation was presumed to be the primary route of exposure to air pollutants.

"Pre-natal environmental exposures are thought to be particularly important for long-term health because germ and fetal cells are more susceptible to disruption than mature cells," he said.

"In conclusion, the study highlighted the vulnerability of the very young, including unborn babies … and the importance of protecting them during landscape fire smoke events and other causes of air pollution.

"Protecting pregnant women and young children from episodic severe smoke events should be central to public health responses to poor air quality.

The study was published today in the Medical Journal of Australia.