An online survey of more than 4,000 adults shows that people who live in places with a high degree of air pollution are more likely to have kids than those living in areas with lower levels of pollution.

    The results were published Tuesday in the Journal of Environmental Health.

    The poll, which polled more than 400 people in 21 U.S. states, found that those living with a highly polluted air are twice as likely to become parents as those living within 10 miles of a relatively cleaner air source.

    People who live near polluted areas are also twice as unlikely to become mothers.

    The study is part of a national survey conducted by the Environmental Protection Agency and the University of Chicago.

    The findings come as states are debating policies aimed at fighting climate change and tackling air pollution.

    Pollution is the second-leading cause of death in the United States, behind heart disease, with more than 16,000 people dying from it annually, according to the Centers for Disease Control and Prevention.

    A majority of Americans now live in areas that are less than 10 miles (16 kilometers) from an EPA-designated source of clean air, according a 2017 report by the nonprofit Environmental Defense Fund.

    “Our results show that air pollution is the single most important cause of premature birth and death for U.N. children,” study author Mark Z. Jacobson, a research scientist at the University at Buffalo in New York, said in a statement.

    “The findings suggest that the U.P.A. could be doing a lot more to protect children from the risks of air pollutants, particularly when they are close to sources of clean, healthy air.”

    Jacobson said that the research indicates that the public health response to air pollution should focus on preventing exposure to it.

    “What is happening in the U