Babies exposed to higher levels of air pollution in the early stages of pregnancy take longer to learn to speak than those exposed to lower levels in the womb, new research suggests.

A study by researchers from King’s College London found exposure to nitrogen dioxide and fine and ultra-fine particulate matter during the first trimester of pregnancy delayed speech development at 18 months.

For premature babies, the impact was worse: as well as delayed development of their ability to speak, they were also found to have impaired motor skills.

“This research should act as a wake-up call, because air pollution is not just an environmental issue, it’s a matter of justice and equality from the very start of life,” said Tyrone Scott, head of campaigns at War on Want.

“In cities like London, it is overwhelmingly working-class communities and communities from marginalised communities who are forced to live near busy roads and toxic air. That means the harm is not shared equally, it is concentrated on those already facing the greatest inequalities.

“When babies are being impacted before they are even born, we have to ask: whose lives are being put at risk, and whose are being protected? This is about systemic inequality, and it demands systemic change.”

The researchers behind the study believe it is the first to investigate pollution exposure and development in London by measuring the language and motor skills of infants whose mothers were pregnant in the capital. But the implications are global.

Across the world, almost the entire global population breathes air containing levels of pollutants that exceed World Health Organization guideline limits. The global health body says air pollution is now “the world’s largest single environmental health risk”.

With many polluting industries now outsourced from the global north, people in low-and middle-income countries in the global south suffer from the highest exposures. But even within wealthier countries the burden falls disproportionately on people from poorer and racialised communities.

Agnes Agyepong, chief executive of Global Child and Maternal Health, a black-led London-based campaign group, said: “We have to be honest that exposure to polluted air is not randomly distributed, but shaped by longstanding inequalities in housing, planning and power.

“This is not only an environmental issue. It is an equity issue, a maternal health issue and an early childhood development issue. If lawful pollution levels are still associated with measurable differences in outcomes, we need to ask whether current standards are truly protecting all children equally.”

Researchers from King’s College London studied 498 infants born in St Thomas’ Hospital, central London, between 2015 and 2020. Of those, 125 were born prematurely, 54 at less than 32 weeks – classifying them as “very and extremely preterm”.

Using their mothers’ home postcodes, the researchers estimated the amount of pollution – including nitrogen dioxide and PM10 and PM2.5 particulate matter – they were exposed to during each trimester of pregnancy. Then, once the infants reached 18 months old, the researchers gave them a standard clinical test to measure cognitive, language and motor skills.

Those infants exposed to high pollution in the first trimester scored on average five to seven points lower on language tests, compared with babies exposed to low pollution. Premature babies exposed to the highest pollution levels in the womb across all of pregnancy scored on average 11 points less for motor skills than those exposed to low levels.

“At this stage, it is too early to say whether these babies will catch up with their peers,” said lead author Dr Alexandra Bonthrone. “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.”

The study was “well-planned and executed” with findings that came “as no surprise” said Roy Harrison, professor of environmental health at the University of Birmingham, who was not involved in the research.

“Our own research has estimated that air pollution exposure is causing a collective loss of around 65 billion IQ points across the global population, providing further evidence of the massive benefits of air pollution abatement for public health,” he said.