Sooty air pollution in towns and cities increases the chances of women giving birth to small babies, new research has shown.
A study involving millions of births around the world found that higher pollution levels raised the risk of low birth-weight.
Although small, the effect is said to be statistically significant. At national population scales it could have an important impact on child health, said the researchers.
Babies are underweight at birth if they tip the scales at less than 2.5kgs, or 5lbs 8oz.
They face an increased risk of dying in infancy, as well as chronic poor health and impaired mental development.
The new study, the largest of its kind ever conducted, focused on tiny sooty carbon particles called PM10s and even smaller PM2.5s which are known to be linked to heart and lung problems and early death.
They originate from a number of sources, including diesel exhausts and the chimneys of coal-fired power stations and factories.
Professor Tanja Pless-Mulloli, who led the UK arm of the study at the Newcastle University, said: “As air pollution increases we can see that more babies are smaller at birth, which in turn puts them at risk of poor health later in life.
“These microscopic particles, five times smaller than the width of a human hair, are part of the air we breathe every day. What we have shown definitively is that these levels are already having an effect on pregnant mothers.”