If anyone was born to study air pollution, it was Kari Nadeau. Growing up near smoggy Newark, New Jersey, she suffered as a child from terrible asthma and allergies, which she always suspected were related to the pollution surrounding her. Her mom was a public health school nurse, and her dad worked for the EPA. “I always had these questions lingering about how much the environment affects people with asthma or allergies,” she recalls. Was air pollution a culprit in her respiratory woes? Her instincts told her yes.

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And now her research does, too. An associate professor of pediatric immunology and allergy at Stanford University, Nadeau recently uncovered something surprising: When she looked at the blood collected from kids who came to her clinic from Fresno, California—kids who frequently had terrible asthma—she saw that they had different-looking immune systems than other California kids. Specifically, their immune systems’ “peacekeeping” functions didn’t work as well in keeping asthma-producing inflammation at bay. This observation piqued her curiosity: “I thought, what’s different in Fresno? So I went on the Internet and searched and saw that Fresno is the second-most polluted city in the country,” she says.

Nadeau is now collaborating with scientists in Fresno to study the link between immune system regulatory cells, asthma, and air pollution. To understand the differences among kids from Fresno and other areas in California, she collected blood from 71 asthmatic children who had spent their entire lives in Fresno, 30 healthy children from Fresno, 40 asthmatic kids from less-polluted Palo Alto, and 40 healthy kids from Palo Alto. She found that all the kids who grew up in Fresno had far higher levels of common pollutants in their blood than did the Palo Alto kids—and the higher their pollutant levels were, the more likely they were to have asthma.

Based on her research, Nadeau thinks that air pollutants stifle the activity of genes responsible for maintaining normal immune system function—and that by doing so, they increase the risk for asthma and allergies. Indeed, in a 2012 study, Nadeau and colleagues looked at pairs of identical (monozygotic) twins—one with and one without asthma. Because identical twins share the same genes, differences (such as the immune system function or occurrence of asthma) can be attributed to differences in their environment. The researchers found suppression of key immune system factors in each twin with asthma compared to his or her non-asthmatic sibling. A similar result was found for twins exposed to secondhand smoke (another significant air pollutant) compared to those not exposed. Twin studies such as these are invaluable in clearly showing a link between environmental exposure to air pollution and health problems.

Other studies also support the link between asthma symptoms and air pollution. More children in the South Bronx are hospitalized for asthma than anywhere else in New York State, and since many Bronx children live or attend schools adjacent to congested highways, Bronx Congressman José Serrano wondered if the two factors might be related. In 2002, he asked New York University environmental scientist George Thurston if he would be willing to conduct a study to find out. “We thought about it for a nanosecond, and then said, ‘sure,’” Thurston recalls. In a study reminiscent of Delfino’s, Thurston recruited 40 South Bronx fifth graders to tote wheeled backpacks containing personal air monitors for a month while rating their respiratory symptoms three times a day. “You rolled it, so it wasn’t really that heavy,” Derrick Reliford, one of the students in the study told the New York Times. “They were the rock stars of the class— everybody wanted to help them with the backpacks,” Thurston recalls.

The children came from four different schools, two of which were close to a highway and two of which were not. Thurston found that, sure enough, the children who went to schools or lived closer to highways were exposed to more air pollution—in particular, diesel fuel exhaust—and they also had more severe respiratory symptoms.

Low-income or minority areas often have some of the worst air. This raises questions of environmental justice—the concept that access to a clean, healthy environment is a basic human right. Sources of major pollution like power plants or waste incinerators are often placed in areas where residents have less ability to fight for their rights—less money, less education, little or no voice in local government. In some cases, even when socioeconomic status is accounted for, minority communities still face more exposure to pollution than average, an example of environmental racism. A 2002 study conducted in southern California by Brown University researcher Rachel Morello-Frosch found that a person’s risk for developing cancer from exposure to polluted air increased as income decreased. And, in general, cancer risk was higher for minorities (Asian, African American, Latino) than for the majority (Caucasians), no matter what the income level.

Children of low-income families are at particular risk: As in the Bronx, their homes and schools are near major roads or factories, and they often come and go to school during rush hour, when traffic is heaviest and smog forms.

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