"Pregnant women should probably take protective measures when exposed to air pollution, such as wearing masks, or avoiding walking on busy roads with heavy traffic," Lan Jin of Yale University, US, told environmentalresearchweb. "City authorities are already making efforts to reduce air pollution. However, they can probably design some educational programmes to help pregnant women to understand the risk of air-pollution exposure, and try to reduce traffic in residential areas."

Lanzhou, in China's northwest, was the nation's most polluted city, according to a 2011 World Health Organization report. Sandstorms, factories and traffic all contribute to the air pollution. The city has 3.6 million residences over an area of 5058 square miles, and lies in a trough surrounded by mountains and hills that trap pollutants at ground level.

To carry out the study, Jin and colleagues investigated 8969 births of live, non-twin babies in one Lanzhou hospital during 2010–2012, to women who lived within 50 km of an air-monitoring station. There were 8.4 congenital heart defects per 1000 of these births. The team assessed the women's exposure to air pollution on a daily basis during their pregnancy by weighting their distance from four air-monitoring sites while they were at home and at work. Then the researchers used linear regression to look at the correlation between air-pollution exposure and incidence of congenital heart defects, allowing for other factors such as maternal age, education, income, body-mass index, disease, folic-acid intake, therapeutic drug use, smoking, season of conception, fuel used for cooking and temperature.

"Congenital heart defects are the leading cause of infant mortality and morbidity, and have the highest prevalence in Asia," said Jin. "However, to my knowledge, no studies have been done to understand the impacts of air pollution on congenital heart defects in Asia."

The researchers found significant positive associations of the heart-condition patent ductus arteriosus with PM10 exposure during the first trimester, second trimester and the entire pregnancy. The condition also showed associations with NO2 during the second trimester and the entire pregnancy. Congenital malformations of cardiac septa showed positive associations with PM10 exposures in the second trimester and the entire pregnancy, and SO2 exposures in the entire pregnancy.

It's not yet clear how the pollutants could cause birth defects. "Several hypotheses have been proposed for the effects of air pollutants on foetal development, including oxidative stress, pulmonary and placental inflammations, increased blood coagulation and viscosity, and oxidation of lipids and proteins by NO2," said Jin.

This was the first study to allow for different levels of exposure to air pollution at women's homes and workplaces, and the second to take account of women moving house during their pregnancy, rather than using only their address on the date of the birth. "Worldwide, studies on this issue are limited and inconclusive," said Jin. "Taking advantage of detailed maternal information in this cohort, we also provided insights on the impacts of residential mobility and work locations on exposure misclassification in the Chinese population."

The team believes that Lanzhou could be representative of other big cities in China, which also have major air-pollution sources such as coal combustion and traffic. Now, the researchers will focus on understanding the impact of traffic pollution on adverse birth outcomes in Lanzhou.

"I was hoping that this proposed study will provide more scientific support on the effective control of traffic pollution and the prevention of future adverse birth outcomes," said Jin. "I am conducting a series of air-pollution monitoring campaigns in Lanzhou to build a land-use regression model, which can capture the spatial variability of traffic pollution in the city more accurately than the four regulatory monitors used in this paper."

The team reported the findings in Environmental Research Letters (ERL).

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