Mothers who are exposed to certain types of air pollution are much more likely to have babies with a low birth weight, the largest study of its kind has shown.

It included data on over three million births from countries around the world. According to the findings, areas with the highest levels of pollution also had the highest rate of low birth weight babies.

A low birth weight was considered less than 5.5lbs. Babies with a low birth rate are at an increased risk of death and serious health problems later in life.

The US scientists found that particulate air pollution was to blame. Particulate is a general term used to describe any tiny solid or liquid particples. Particulate air pollution includes the pollution emitted by motor vehicles (in exhaust fumes) and some power plants.

"What's significant is that these are air pollution levels to which practically everyone in the world is commonly exposed. These microscopic particles, which are smaller than the width of a human hair, are in the air that we all breathe," explained lead investigator, Prof Tracey Woodruff, of the University of California in San Francisco.

Currently in the US, regulations state that the annual average concentration in the air should be no more than 12 µg/m3 (micrograms per cubic meter) of particles measuring less than 2.5 microns. In the European Union, the limit is 25 µg/m3, however this is currently being debated by regulatory agencies to decide whether it should be lowered.

Meanwhile, particulate air pollution in Beijing in China was recently found to be 700 µg/m3, a figure which Prof Woodruff described as ‘completely unsustainable'.

"In the United States, we have shown over the last several decades that the benefits to health and wellbeing from reducing air pollution are far greater than the costs. This is a lesson that all nations can learn from," she added.

Details of these findings are published in the journal, Environmental Health Perspectives.