Babies exposed to high amounts of air pollution in the womb show slower signs of development at 18 months than those exposed to lower levels.
The King's College London study is the first to investigate pollution exposure and development in Greater London, measuring language and motor skills of infants whose mothers were pregnant in the capital.
Pollution exposure during the first trimester of pregnancy (conception to week 12/13), emitted from sources such as traffic, was linked to worse language skills at 18 months.
The research is also the first to address whether being born prematurely changes the impact of pollution exposure during pregnancy. The study found that pollution had an even greater impact on babies born pre-term, with this group showing poorer motor as well as language skills.
The research follows the same group of babies found to have different brain structure sizes, attributed pollution exposure in the womb. Lead author Dr Alexandra Bonthrone, who completed the research at School of Biomedical Engineering & Imaging Sciences, at King's College London, said: "We wanted to establish if air pollution levels related to cognitive, language and motor abilities in this critical period of early childhood. The first 1,000 days from conception to age two shape long-term brain health and behaviour."
498 infants from Greater London were involved in the study, recruited from St Thomas' Hospital between 2015-2020. Of the cohort, 125 were born prematurely, and within that group 54 born less than 32 weeks – known as 'very and extremely preterm'. The infants underwent a standard clinical test of development, known as Bayley Scales, which measures cognitive, language and motor skills – with scores of 100 reflecting population average.
Those exposed to high pollution in the first trimester of pregnancy scored on average 5-7 points lower compared to babies exposed to low pollution on language tests. There was no association with pollution in second and third trimesters.
Premature babies exposed to the highest pollution levels in the womb across all of pregnancy, scored on average 11 points less than those exposed to low levels for motor skills.
Dr Bonthrone said: "While preterm babies are already more likely to have difficulties with motor development, we adjusted for different clinical factors, like time spent on support for breathing, to confirm the relationship between air pollution and motor development."
The King's team collaborated with the Environmental Research Group at Imperial College London to measure pollutants. The London Air Pollution Toolkit models pollution levels based on amount of traffic and average speeds travelled. This enabled researchers to estimate the amount of pollution mothers were exposed to during pregnancy based on their home postcode.
Dr Bonthrone said: "The levels in this study of toddlers born in greater London from 2015 to 2020 were within the annual limits set out by the Government in the 2010 legislation on air quality, although they are higher than the safe levels set out by WHO in 2021."
Pollutants analysed included particulate matter and nitrogen dioxide, which are emitted by traffic exhausts. Tiny particles of these molecules can be breathed in by humans. Breathing in air pollution is the second largest risk factor for death in children under 5 globally, second only to malnutrition, according to the State of Global Air Report.
Dr Bonthrone added: "At this stage, it is too early to say whether these babies will catch up with their peers – the only way will be to study them later in childhood. It could be that the development differences have effects into education and information processing, but we won't know for sure until we do future studies."
Senior-author Professor Serena Counsell, School of Biomedical Engineering & Imaging Sciences, King's College London, added: "This works adds to a growing body of evidence that maternal exposure to high levels of air traffic pollution may be associated with altered outcomes in their offspring. Reducing maternal exposure to air traffic pollution should be a public health priority."
Professor Frank Kelly, Environmental Research Group, Imperial College London added, "This study shows that even within what we currently call 'legal' levels of air pollution, we are seeing measurable impacts on the developing brain. That should fundamentally change how we think about 'acceptable' air quality for pregnant women and young children. Improving air quality is not just about cleaner skies; it is about giving every child the best possible start in life."
The latest study forms part of wider research following a cohort of children who underwent brain MRIs before or soon after birth, known as the Developing Human Connectome Project.
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