"Toxic air pollution particles are found in pregnant women's PLACENTAS for the first time," reports the Mail Online.
The placenta connects the mother to her baby. It allows nutrients and oxygen to pass from the mother to the baby, and removes harmful waste products to protect the baby.
Researchers in Belgium scanned the placentas of 28 women who had recently given birth or had miscarriages. 23 women had full-term births and were the main focus of the study. The researchers found traces of black carbon particles in all 28 placentas, with higher concentrations in women who lived in areas with worse air pollution.
Black carbon particles (more commonly known as soot) are created when fuel (such as wood or diesel) does not burn fully. Previous studies have linked exposure to black carbon in pregnancy to premature birth and growth restriction in the womb.
The researchers said the particles were found both on the mother's side of the placenta and the baby's side, suggesting that black carbon could reach the babies. However, we do not know whether particles transfer from the placenta to the baby.
While there is little women can do to avoid living in areas of high air pollution, the study suggests another reason to press for better environmental standards to reduce air pollution.
The researchers who carried out the study were from East-Limburg Hospital (where the women gave birth), Hasselt University and KV Leuven, all in Belgium. The equipment used was funded by Interuniversity Attraction Poles Program. The study was published in the peer-reviewed journal Nature Communications on an open access basis so it is free to read online
The study was widely reported in the UK media. Most of the stories were reasonably accurate although some overplayed what we know about the links between air pollution and poor outcomes in pregnancy.
The researchers used placentas collected from women taking part in a related cohort study.
Like all observational studies, this is good at picking up links between factors (in this study, between how much pollution women were exposed to and how much carbon was in their placenta) but it cannot prove that one caused the other.
Researchers collected the placentas of 28 women who gave birth at a hospital in Belgium, who had all agreed to take part in the study. The placentas were collected within 10 minutes of delivery and researchers took biopsies from the side where the blood vessels go towards the baby (foetal side) and the side where the blood vessels come from the mother (maternal side).
The biopsies were analysed with pulsed white light lasers, which made the carbon particles look white in images. The scans were then analysed to calculate the number of carbon particles per millimetre cubed of placenta.
All the women in the study were non-smokers, to avoid contamination with tobacco-based carbon. 5 samples came from women who had miscarriages or gave birth prematurely, so that the researchers could see how early in pregnancy black carbon particles appear in the placenta.
From the overall sample, researchers picked 10 women living in areas of high air pollution during their pregnancy, and 10 living in low air pollution areas.
The researchers said they found black carbon particles in all 28 samples studied, on both the maternal and foetal side of the placentas. That included all the samples from the 5 pre-term placentas, which ranged from 12 weeks of pregnancy to 31 weeks.
The amounts of black carbon particles were linked to how much air pollution women had been exposed to during pregnancy:
The placenta biopsies of the other 3 women looked for differences between the maternal and foetal sides of the placenta. They found some differences, but they did not go into detail about the findings so the implications of this is unclear.
The researchers said: "Our study provides compelling evidence for the presence of BC [black carbon] particles originating from ambient air pollution in human placenta and suggests the direct fetal exposure to those particles during the most susceptible period of life."
They added: "The evidence of particle translocation to the placenta might be a plausible explanation for the observed detrimental effects of ambient particulate air pollution on fetal development."
Headlines about soot particles in the placenta are obviously alarming, but there are a few things to bear in mind.
First, we do not know whether the black carbon particles actually reach the unborn baby. And we do not know what effect they have, if they do. The study did not report on actual harm to babies as a result of black carbon. While previous studies have suggested links with pre-term birth and restricted growth, we do not know for sure that's down to exposure to air pollution in pregnancy.
The study was also based on a small number of samples. Only 28 women contributed samples to the study, and only 20 of them were analysed according to pollution levels where the women lived. That's a small sample to draw conclusions from.
Black carbon particles, however, are known to harm lung health and are likely to have other effects on health, too.