Smog in Beijing. Photographed by 大杨.

It is well known that polluted air can harm a developing fetus. The Olympic Games that took place in China in 2008 presented scientists with a perfect opportunity to evaluate this effect.

A campaign for cleaner air began several months before the opening ceremony in Beijing. As factories were closed and traffic was restricted, the concentrations of sulfur dioxide, carbon monoxide and nitrogen dioxide in the air fell by between 40 and 60 percent, and potentially harmful particles in the air reduced.

The researchers from the United States and China compared women who were in the eighth month of pregnancy during the Olympics with women who were just as far into their pregnancy a year before and a year after the Olympics.

The much cleaner air was found to have a significant correlation with children’s growth in the womb. Those children who were born during the Olympic year were on average 23 grams heavier than the other children.

The findings are published in the journal Environmental Health Perspectives.

Differences in Birth Weight Associated with the 2008 Beijing Olympics Air Pollution Reduction: Results from a Natural Experiment