High pollution levels in towns and cities are linked to an increased chance of premature births.
The “toxic” chemicals emitted by urban traffic damages the health of unborn babies as well as their mothers, a study of 100,000 births has found.
The air was more polluted in winter than summer, while inland areas were affected more than the coast.
Dr Beate Ritz from the University of California said: “Air pollution is known to be associated with low birth weight and premature birth.
“Our results show that traffic-related polycyclic aromatic hydrocarbons (PAH) are of special concern as pollutants, and that PAH sources besides traffic contributed to premature birth.
“The increase in premature birth risk due to ammonium nitrate particles suggests secondary pollutants are also negatively impacting the health of unborn babies.
“To reduce the effects of these pollutants on public health, it is important that accurate modelling of local and regional spatial and temporal air pollution be incorporated into pollution policies.”
In the study, published in the journal Environmental Health, researchers looked at 100,000 births registered within five miles of air quality monitoring stations in Los Angeles County, from 22 months starting in June 2004.
They used information from health officials about the babies and their mothers together with air pollution levels to work out what effect the latter had on the former.
The study found that exposure to “critical pollutants” such as PAH, linked to car exhausts, led to a 30 per cent increased risk of giving birth prematurely.
Chemicals produced by diesel fumes were associated with a 10 per cent higher risk of premature birth, while ammonium nitrate fine particles were linked to a 21 per cent increase.
The pollutants were found in higher concentrations in summer than in winter, and more in inland areas than by the coast.