A large international study has found that pregnant women who are exposed to air pollution have a significantly increased risk of having a baby that is underweight.

Particulate air pollution that the study is referring to is emitted by coal burning power plants, vehicles and other industrial sources.

The study looked at data from North America, South America, Europe, Asia, and Australia covering more than 3 million births.

"What's significant is that these are air-pollution levels to which practically everyone in the world is commonly exposed," study co-principal investigator Tracey Woodruff, a professor of obstetrics and gynecology and reproductive sciences at the University of California, San Francisco, said in a university press release. "These microscopic particles, which are smaller than the width of a human hair, are in the air that we all breathe."

Although the research showed a statistical correlation between pollutant concentration and newborn birth weight, it did not indicate that the particulate matter was a direct cause.

Dr. Woodruff also noted that those countries that do have tighter emissions requirements so have lower levels of the airborne pollutants.

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

Study co-author Dr. Mark Nieuwenhuijsen noted the recent exceedingly high levels of particulate air pollution in Beijing. "From the perspective of world health, levels like this are obviously completely unsustainable."

As western nations are trying to lower their reliance on fossil fuels and many developing countries are increasing theirs, this poses a serious public health issue.

The research study has been published on February 6 in the journal Environmental Health Perspectives.