Researchers in 14 areas in 9 countries - including South Korea and Canada - to collect data on the concentrations of tiny particles in the atmosphere and exposure to dirty air of the women. Much micro dust arising from the smoke of motor vehicles and factories.

Then team analyzed data during pregnancy and childbirth by three million women in 14 regions. With the amount of three million cases, this is the largest-scale study of the relationship between infant weight and atmospheric pollution, LiveScience reported.

A group of women wearing a mask while exercising in the city of NorthBeijing, China to prevent dirty air. (Photo: Xinhua)

Analysis results show that, if the concentration of micro-particles increased by 10 micrograms per m3 of air, weight babies decreased 8.9 grams and a child's risk of low birth weight increased by 3%.

Infants weighing less than the standard (defined by the state of the people of each country) face the risk of cardiovascular disease, respiratory and behavioral disorders than other children .

"This study increases our confidence about the impact of dirty air for weight infants", Tracey Woodruff, reproductive health experts of the University of California in the U.S. and is a member of the group research, said.

However, the research team also acknowledges most likely infant's weight depends on a number of other important factors related to the mother - such as income, education level, smoking habits , the level of daily physical activity.