A new study published in Environmental Health Perspectives suggests that pregnant women exposed to certain types of pollution are more prone to giving birth to an underweight baby.

Researchers observed three million births in nine countries that included the United States. As reported by Nature.com, they narrowed their studies to air pollutants that had particles produced from burning firewood and the combustion of fossil fuels from industry and transportation. Particles from dust and sea salt were also accounted for in the study.

The Cleveland Clinic reminds women “whatever they eat, smoke, or drink is passed on to their baby and can affect the baby’s development.”

Alcohol, tobacco and other drugs can contribute to low birth weights, but research centers in this study did not include information on whether the mother smoked during her pregnancy.

The World Health Organization is proposing that countries come up with more precise pollution standards.