Air pollution may be harmful to babies even before they are born, a new study has found.

Researchers in London calculated mothers’ exposure to air pollution and traffic noise in various parts of the city from 2006 to 2010. Then they amassed data on birth weights of 540,365 babies born during those years to women who lived in those areas.

The average pollution exposure was 14 micrograms per cubic meter of PM 2.5, the tiny particles that easily enter the smallest airways in the lungs. The researchers found that for each 5 microgram per cubic meter increase in PM 2.5, the risk of low birth weight increased by 15 percent. Low birth weight is a predictor of an increased risk for diabetes, heart disease and hypertension in later life.

The study, in BMJ, found no effect of traffic noise on birth weight.

The Environmental Protection Agency standard for PM 2.5 is 12 micrograms per cubic meter averaged over three years, and the World Health Organization suggests 10 as a limit. But the lead author, Mireille B. Toledano, an epidemiologist at Imperial College London, said that there really is no safe level of air pollution.

“For every 10 percent reduction in PM 2.5,” she said, “we can prevent 90 babies being born with low birth weight in London. The current limits are not protecting pregnant women, and they’re not protecting unborn babies.”