Air pollution contributes to 4.2 million deaths each year, according to the World Health Organization, with 91 percent of the world population living in areas where it is greater than the limits set by the organization’s guidelines. Studies have linked inhalation of pollutants to health conditions such as asthma, which is the leading chronic illness in children in the United States. Early research into the health effects of air pollution in the 1960s focused on adults, and by the 1970s, the link between breathing in particulate matter and respiratory disease became well established.
Around the same time, researchers began to consider the pervasiveness of polluted air as affecting not just those who breathe it, but also fetuses developing in utero. In 1973, the first study of the effects of air pollution on birth outcomes in the Los Angeles area found a link between in utero exposure to air pollution and low birth weight. Since then, researchers have uncovered myriad health effects in children tied to the quality of the air their mothers breathed while pregnant.