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Soot from polluted air can reach the fetal side of the placenta

A new study hints at the ways in which air pollution may directly impact a fetus.

ByKatherine J. WuNOVA NextNOVA Next
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Researched have published the first peer-reviewed study showing that black carbon particles can reach the inside of the placenta. Image Credit: petrenkod, iStock

About 20 years ago, New Jersey and Pennsylvania began to replace highway toll booths with E-ZPass plazas. It was a major transformation: No longer required to stop and fish for change, motorists sailed down turnpikes instead of idling in long lines. The switch cleared the region of congestion and stop-and-go-traffic—as well as the plumes of pollutants they tend to produce.

By the end of 2001, both states’ transitions to open-road tolling were nearly complete. And in the months following, rates of premature births within a mile of revamped plazas began to drop.

Researchers have long suspected that the effects of air pollution can transcend generations, passing from mother to child as early as the first stages of pregnancy. But the details of this insidious inheritance have been tough to pin down, in part because scientists haven’t had a good way to demonstrate that pollutants can breach the placenta—the barrier around the fetus that exists throughout gestation.

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That might now be changing. Reporting today in the journal Nature Communications, a team of researchers has detected particles of black carbon—the sooty black material emitted when fossil fuels are burned—on the side of the placenta that faces the fetus.

The study, which measured particulates in placental samples taken after pre- or full-term birth, doesn’t prove that fetuses are actually internalizing the pollutants their mothers breathe. But it’s some of the strongest evidence yet that there’s direct exposure, says Janet Currie, a health economist at Princeton University who was not involved in the study. “This is the smoking gun,” she says.

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Black carbon is emitted by gas and diesel engines, coal-fired power plants, and other sources that burn fossil fuel. Image Credit: milehightraveler, iStock

If you’re reading this, you’ve breathed in black carbon, one of the many pollutants that fall under the category of particulate matter. During the combustion process, toxic chemicals tend to cling to bits of black carbon, which then drift into the atmosphere—and into human noses and mouths.

Where these particles go, adverse health outcomes seem to follow. Adults in regions with smoggy skies have more respiratory disease, heart problems, and cancer. Kids that grow up in these areas are at higher risk of asthma and perform worse on cognitive tests. Even babies born to mothers exposed to air pollution are more likely to be premature and weigh too little at birth.

These patterns have hinted that black carbon’s effects go far beyond the lungs, says study author Hannelore Bové, an environmental scientist at Hasselt University in Belgium. But most studies on air pollution are epidemiological, drawing on broad trends in health and disease on population-wide scales. To truly pin down cause and effect, she says, researchers need a way to track the particles themselves—and show that they’re able to access the rest of the body.

A few years ago, when Bové was a graduate student, no such technique existed. So she came up with one.

With the help of her colleagues, Bové discovered that black carbon glows when it’s hit with short, fast pulses of certain wavelengths of light. Against a backdrop of human tissue, the particles look like “white dots,” she says. “It’s really easy to distinguish.”

Recently, Bové and her colleagues used their technique to show that black carbon is capable of infiltrating the kidneys, passing out of the body in the form of urine. Curious to see where else the particles might end up, the team turned its attention to the placenta.

In their new paper, the researchers examined placentae from 28 women enrolled in the ongoing ENVIRONAGE birth cohort study, which recruits mothers to donate samples of tissue, including the placenta, after they give birth. Within this pool, 10 participants spent the entirety of their pregnancy living in residential areas in Belgium with relatively high black carbon levels, while another 10 women had experienced only minimal exposure. Unsurprisingly, Bové says, the placentae of the women in the first group lit up with black carbon, with some containing up to 35,000 particles per cubic millimeter of tissue. (Given that this study is the first of its kind, it’s hard to know what those numbers mean, Bové says, but more probably isn’t better.)

But black carbon was also detectable in the placentae of the women living in regions with less pollution, albeit in smaller amounts. In fact, the particles were present in the tissue of all 28 women in the study, including five who had given birth preterm—suggesting that pollutants can work their way into these tissues as early as the first trimester, when the fetus is most vulnerable.

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An image of placental tissue containing black carbon particles, shown in white and indicated by white arrowheads. The scale bar is 40 micrometers long. Image Credit: Saenen et al, Clinical Epigenetics, 2019

Given the global ubiquity of air pollution, it’s not surprising to see black carbon particles in every sample, says Audrey Gaskins, an epidemiologist specializing in fertility at Emory University who wasn’t involved in the study. “I would’ve been more shocked if they only found it in some,” she says.

Frederica Perera, director of the Columbia Center for Children's Environmental Health, praises the study for its rigor, but notes that it’s just a first step. The number of women sampled in the study was small, she says. And the illumination technique couldn’t reliably detect particles below 2.9 micrometers in diameter, despite the fact that ultrafine particles below 2.5 micrometers in diameter are some of the most concerning pollutants out there: the smaller their size, the more likely they are to escape the lungs.

The new study doesn’t address whether black carbon enters the fetus itself. But ENVIRONAGE’s protocol also includes sampling the umbilical cord, Bové says, and applying the same techniques to these tissues in the future could help solve that puzzle.

As ENVIRONAGE continues, Bové says, many of the children born to participating mothers will return for regular health checkups, allowing the researchers to tie specific environmental exposures to health outcomes. Some of the oldest kids in the cohort are now around seven years old, she says, and the team is still recruiting. She and her colleagues also hope to better understand how exactly the particles damage both maternal and fetal tissues—processes that could involve everything from uncontrolled inflammation to alterations in how cells express different sets of genes.

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Microscopy images of placental tissue from two different women exposed to relatively high levels of air pollution. Black carbon particles are black. Image Credit: Bové et al., Nature Communications, 2019

For now, simply seeing particles in the placenta “is very compelling,” Currie says. “There are some who will never be convinced by the statistics in an epidemiological study, but it’s pretty hard to explain how else these particles could have gotten there.”

Joan Casey, an environmental epidemiologist at Columbia University’s Mailman School of Public Health who was not involved in the study, agrees. “This is helping us connect the dots” between air pollution and poor health, she says. “If previous studies were linking A to Z, this starts to get at B, C, and D.”

In many countries, exposure to particulate matter is declining—but that’s not the case everywhere. Even in places where air pollution levels are relatively low, including the United States and Belgium (where the study took place), people are still feeling its ill effects, Gaskins says. What’s more, on scales both global and local, racial and ethnic minorities and people of lower socioeconomic status bear a disproportionate amount of the health burdens associated with these pollutants.

“This is more evidence that we need to continue working to clean up our air,” Casey says. After all, that may be the only way to make the health costs of pollution another toll that’s a thing of the past.

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