Infants whose mothers were exposed to higher levels of tiny air pollution particles during pregnancy are much more likely to develop asthma, according to research.

The study analysed the impact of ultra-fine particles (UFPs), which are not regulated by governments. These are thought to be even more toxic than the larger particles that are routinely monitored and have also been linked to asthma.

Sources of UFPs include vehicles and wood burners, and tens of thousands of particles can be found in each sugar cube-sized volume of city air. They are thought to pass through the expectant mother’s lungs and into her bloodstream, causing damaging inflammation. They are also likely to cross the placenta into the foetus’s circulation.

UFPs have been linked to other impacts including brain cancer and researchers said that demonstrating these health impacts should spur better measurement and action from policymakers to reduce dirty air.

“Our research is an important early step in building the evidence base that can lead to better monitoring of exposure to UFPs and ultimately to regulation,” said Prof Rosalind Wright, at the Icahn School of Medicine at Mount Sinai, New York, and who led the research. “Childhood asthma remains a global epidemic that is likely to grow with the anticipated rise in particulate air pollution exposures due to effects of climate change.”

Wright said foetuses were especially vulnerable to the oxidative stress that pollution particles cause in body tissues: “Foetal development is exquisitely sensitive to anything that throws the oxidation balance out of whack.”

Air pollution was already known to harm foetuses by increasing the risk of premature birth and low birth weight, and a study in 2019 reported that air pollution was as bad for pregnant women as smoking in raising the risk of miscarriage. Air pollution particles were also recently discovered on the foetal side of placentas.

The new research, published in the American Journal of Respiratory and Critical Care Medicine, followed almost 400 mothers and their children through pregnancy and afterwards in Boston, US.

The level of UFPs, which are those smaller than 0.1 micrometre, ranged from about 10,000 to 40,000 per cubic centimetre of air. The researchers found that children whose mothers had been exposed to levels of 30,000/cm3 during pregnancy were approximately four times more likely to develop asthma than those whose mothers had been exposed to levels of 15,000/cm3. This difference in pollution is roughly the change seen when going from a backstreet to a busy road.

Most of the diagnoses of asthma in the children occurred just after three years of age and overall, 18% of the infants developed asthma. The scientists took account of other factors, including the age of the mothers and obesity. They also took account of other air pollutants. “These UFPs have independent effects,” said Wright.

The researchers found that the period of gestation when foetuses were most sensitive to UFP exposure differed between the male and female foetuses, possibly suggesting the UFPs were interfering in the hormonal system.

A study in Toronto in 2019 also linked in-utero UFP exposure to asthma and studied a much larger group of 160,000 children. But the new study assessed the exposure to UFPs at a much finer spatial resolution – down to 20m rather than a city block – and had more complete data for the times of exposure. “The studies are complementary,” said Wright.

Prof Scott Weichenthal, at McGill University and part of the Toronto study team, said the new research was consistent with his team’s findings and added to rapidly growing evidence of adverse health impacts from UFPs, including heart and lung disease and cancer.

“Governments need to pay more attention to UFPs [and] people should try to reduce their exposures as much as possible, though this can be easier said than done if you happen to live close to [sources of pollution],” said Weichenthal.

As well as trying to avoid polluted places, Wright, a respiratory physician, said she tells her patients to consider taking antioxidants, as these have been shown to reduce inflammation caused by particle pollution.