More people are turning to IVF to have children than ever before. But new research, out last month, finds that the success of in vitro fertilization — a delicate process that works less than half the time — could be affected by air pollution.

IVF is an assisted reproductive technology that consists of extracting eggs from the patient and fertilizing them in a lab, where they are matured into embryos over a period of days. The embryos can then be stored frozen in liquid nitrogen or transferred back to the patient in the hopes that one of them implants and results in a healthy pregnancy.

Researchers at Emory University’s Rollins School of Public Health used air pollution data from the Environmental Protection Agency and matched that with addresses tied to the egg and sperm used in IVF cycles. They looked at air pollution levels while eggs were developing, as well as during the period of ovarian stimulation that gets them ready for extraction. Similarly they analyzed the 72 days when the sperm was developing before it was used for fertilization In each of these time windows. Researchers found a link between an increase in exposure to air pollution and lower egg survival and embryo quality. They also found a connection between higher concentrations of certain types of air pollutants on the day that eggs were thawed and lower survival of eggs, as well as lower fertilization rates.

Unique to the study, published in the peer-reviewed journal Environment International, was the use of donor eggs. That allowed researchers to parse out how air pollution was affecting the egg and the sperm separately, since the donor lives at another address and because of that, has different exposure to pollutants. “To be able to see that male partner contributions are also being affected by air pollution was a really powerful finding and one that we hope is unpacked in future research,” said Sarah LaPointe, the study’s first author and a postdoctoral research fellow at the Rollins School of Public Health.

The researchers used information from 500 egg donors and 915 couples where the male was supplying the sperm.

“This is a very clever study,” said Mary Willis, an assistant professor in the Department of Epidemiology at the Boston University School of Public Health, who is not affiliated with the research. “This is probably one of the only studies where you can get at really, really specific biological mechanisms and timing.” The reason, she said, is that “because it’s IVF, you know exactly when ovarian stimulation is happening. You know exactly when fertilization is happening. So you can get some really specific answers to biological questions that haven’t been answered in the literature.”

Organic carbon, part of what’s known as PM2.5, had one of the most consistent negative associations with egg survival and embryo quality. PM 2.5 refers to air particles that are so small they can enter the lungs and bloodstream, making them harmful to human health. This type of particle pollution mostly comes from vehicle and truck emissions, but can also be produced in wildfires.

One limitation of the study is that it only looked at the impact of individual pollutants, like organic carbon, but usually, pollution is composed of a variety of pollutants. LaPointe said they plan to do a multi-pollutant analysis in the future to better capture how air pollution may be affecting IVF.

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While air quality has consistently gotten better in the United States over the last few decades, almost 40 percent of people in the country live in areas with unhealthy levels of ozone or particle pollution, according to the American Lung Association’s annual State of the Air report. People of color are more likely to live in areas with unhealthy air. The study was published on the heels of research on IVF that came out in June, based on the analysis of 3,659 frozen embryo transfers in Australia between 2013 and 2021, which found that the chances of a live birth decreased with higher levels of air pollution.

Willis says that the expense and intensity of IVF procedures make it important to understand how to boost the odds of success for people undergoing fertility treatments. “It makes sense to look at these little things like air pollution,” she said.

Though these results cannot be used to draw conclusions about couples not using assisted reproductive technology, other research has found a link between air pollution and male and female infertility. A study out in September found an association between long-term exposure to PM 2.5 and low sperm count and motility.

LaPointe says her research makes a case for making more targeted recommendations for men — usually the focus is on the person trying to get pregnant. “They should also be involved in these conversations and recommendations to avoid these exposures during these periods,” she said.