Between 2001 and 2011, eight coal and petroleum plants in California closed. The plants were scattered throughout the state: to the north, in Humbolt Bay, to the southern border, to Brawley. Like all power plants, they released particulate matter, sulfur dioxide (SO2), lead and other compounds, exposing people living nearby to polluted air.
When mills closed, levels of PM2.5 pollution (particles measuring less than 2.5 micrometers, about 30 times the size of a hair) in the vicinity decreased, as did nitrogen oxide. And in the year following their closure, fertility rates in surrounding areas increased.
Plant closures were a natural experience, providing researchers with a unique opportunity to measure the effects of eliminating pollution from the community. The resulting research has clearly demonstrated the emergence of a public health problem, namely the link between poor air quality and reduced fertility.
"There is growing evidence that there is a relationship between air pollution and fertility," says study author Joan Casey, a postdoctoral researcher at the University of California School of Public Health in Berkeley. "It's really convincing that something could happen."
Air pollution kills millions of people each year and is linked to health problems ranging from cardiovascular disease to Alzheimer's disease. The World Health Organization considers that bad air is the greatest risk to health and the environment in the world, and scientists are still trying to pin down the extent of the damage it can cause. Fertility is an area of intervention.
"One of the reasons people focus on reproductive outcomes is that we can see the effects of environmental exposures more immediately than something like cardiovascular disease, where we wait decades before we have impact, "says Audrey Gaskins, researcher at Harvard TH Chan School of Public Health. "These early reproductive criteria look like a canary in a coal mine. If we notice an effect on fertility and a loss of pregnancy, this indicates that the pollution is acting on the body in a detrimental way. Who knows what we are going to see again? "
Gaskins studies the effects of environmental exposures, such as air pollution, on women undergoing an in vitro fertilization procedure to become pregnant. Since the chronology of an IVF pregnancy is so closely monitored and regulated, she may attempt to determine the duration of an attempted pregnancy in which air pollution would have the greatest effect. "We know the day the fertilization takes place, the day the embryos are transferred, the day of implantation," she says. "We could never know it with a spontaneous pregnancy."
In a study published in June, Gaskins found that women living with IVF who lived closer to major highways – and therefore closer to traffic-related air pollution – were less likely to succeed. implantation of the embryo and live birth. A 2010 study had reached the same conclusion: women were less likely to succeed after IVF if they were more exposed to air pollutants, particularly nitrogen dioxide (produced by cars and trucks). .
These two studies are not outliers. Shruthi Mahalingaiah, assistant professor of epidemiology, obstetrics and gynecology at the Boston University School of Medicine, found a slight increase in infertility among women living nearby roads by analyzing data from the Nurses Health Study, a longitudinal survey of factors contributing to infertility. chronic disease in women. For a woman in Barcelona, each increase of about 3.5 micrograms per cubic meter of particles resulted in a 13% reduction in fertility, according to a 2014 study.
"We do not have a lot of data on embryos and IVF studies, population studies, studies of natural experiments like plant closure," says Mahalingaiah. "We are gathering the big story."
The story does not focus solely on women and the female reproductive system: sperm are probably also affected by poor air quality. In a study conducted in Taiwan, for example, and among men in Salt Lake City, exposure to air pollution reduced the ability of sperm to move. Each microgram per cubic meter of fine particles was associated with a drop in the normal shape and size of the sperm. correctly. Exposure to air pollution also increases the fragmentation of sperm DNA, which can lead to infertility and miscarriages.
Although public health research is increasingly interested in the relationship between air quality and fertility, the idea is not new to many reproductive professionals: In vitro fertilization laboratories and clinics have taken note of the adverse effects of pollutants at an early stage. They had a medical and probably financial incentive as the success of their business depends on high and consistent success rates of implantations and pregnancies. "The REI [reproductive endocrinology] the community internalized that a long time ago, "says Mahalingaiah. Fifteen years of research have shown that air control in laboratories where embryos are fertilized or implanted (with the help of filtration systems) improves the live birth rate. In addition, the European Union and Brazil have put in place air quality regulations for IVF laboratories, although the US does not. "All lab directors are improving their air quality measurements," she says.
Scientists are still working to identify the reason why air pollutants affect fertility, and there are several theories for the mechanisms involved. "Systemic and chronic inflammation or biological stress caused by pollutants is probably the most important factor." one of the key factors, "said Mahalingaiah. These particles may also contain minute particles of heavy metals that can interfere with hormonal systems important for reproduction; pollutants can alter or disrupt DNA and other genetic material in sperm and eggs.
The risks associated with these pollutants also persist after the beginning of pregnancy. Exposure has also been linked to both higher rates of premature delivery and an increased likelihood of low birth weight as pollutant concentrations increase. "Many people see reproductive outcomes as a continuum," says Gaskins. "Many women have trouble getting pregnant, carrying a baby, and so on.
Under the current presidential administration in the United States, actions on environmental issues such as air pollution have been downgraded and in many cases policies designed to improve air quality have been canceled . Notably, last month, the Environmental Protection Agency disbanded panels of experts to review and assess air quality standards. Work on air pollution and fertility is still in its infancy, and there are still issues to be resolved, but given the current regulatory environment, the work is even less likely to be integrated. political discussions, although general trends in fertility raise concerns. .
Whatever the case may be, Gaskins says it usually takes some time for policies to catch up on new areas of research. "As we begin to publish more and more good research demonstrating this link, more likely to be used in policies. Most of the research has been done in the last 5 years, "she says. "I think this will eventually become a topic of conversation, and it will be interesting to see when that happens."
Mahalingaiah, however, says that women's health and fertility should already be incorporated into the air quality discourses. She noted that air pollution disproportionately affects underfunded populations and communities of color, where people may have less access to fertility specialists and treatments if they have difficulty conceiving or carry out. And outside the United States, in places like India, where the air quality is considerably and constantly worse, the stakes are even more important.
"Air pollution and fertility should be an integral part of global health," she said. "We want to reduce emissions because they reduce fertility. I want the health of women and reproduction to be added to these conversations. "