Air pollution causes a “huge” reduction in intelligence, according to new research, indicating that the damage to society of toxic air is far deeper than the well-known impacts on physical health.

The research was conducted in China but is relevant across the world, with 95% of the global population breathing unsafe air. It found that high pollution levels led to significant drops in test scores in language and arithmetic, with the average impact equivalent to having lost a year of the person’s education.

“Polluted air can cause everyone to reduce their level of education by one year, which is huge,” said Xi Chen at Yale School of Public Health in the US, a member of the research team. “But we know the effect is worse for the elderly, especially those over 64, and for men, and for those with low education. If we calculate [the loss] for those, it may be a few years of education.”

Previous research has found that air pollution harms cognitive performance in students, but this is the first to examine people of all ages and the difference between men and women.

The damage in intelligence was worst for those over 64 years old, with serious consequences, said Chen: “We usually make the most critical financial decisions in old age.” Rebecca Daniels, from the UK public health charity Medact, said: “This report’s findings are extremely worrying.”

Air pollution causes seven million premature deaths a year but the harm to people’s mental abilities is less well known. A recent study found toxic air was linked to “extremely high mortality” in people with mental disorders and earlier work linked it to increased mental illness in children, while another analysis found those living near busy roads had an increased risk of dementia.

The new work, published in the journal Proceedings of the National Academy of Sciences, analysed language and arithmetic tests conducted as part of the China Family Panel Studies on 20,000 people across the nation between 2010 and 2014. The scientists compared the test results with records of nitrogen dioxide and sulphur dioxide pollution.

They found the longer people were exposed to dirty air, the bigger the damage to intelligence, with language ability more harmed than mathematical ability and men more harmed than women. The researchers said this may result from differences in how male and female brains work.

Derrick Ho, at the Hong Kong Polytechnic University, said the impact of air pollution on cognition was important and his group had similar preliminary findings in their work. “It is because high air pollution can potentially be associated with oxidative stress, neuroinflammation, and neurodegeneration of humans,” he said.

Chen said air pollution was most likely to be the cause of the loss of intelligence, rather than simply being a correlation. The study followed the same individuals as air pollution varied from one year to the next, meaning that many other possible causal factors such as genetic differences are automatically accounted for.

The scientists also accounted for the gradual decline in cognition seen as people age and ruled out people being more impatient or uncooperative during tests when pollution was high.

Air pollution was seen to have a short-term impact on intelligence as well and Chen said this could have important consequences, for example for students who have to take crucial entrance exams on polluted days.

“But there is no shortcut to solve this issue,” he said. “Governments really need to take concrete measures to reduce air pollution. That may benefit human capital, which is one of the most important driving forces of economic growth.” In China, air pollution is declining but remains three times above World Health Organisation (WHO) limits.

According to the WHO, 20 of the world’s most polluted cities are in developing countries. China, home to several of those cities, has been engaged in a “war against pollution” for the past five years.

Facebook Twitter Pinterest A migrant worker listens to the radio on his tricycle cart. Photograph: Andy Wong/AP

The results would apply around the world, Chen added. The damage to intelligence was likely to be incremental, he said, with a 1mg rise in pollution over three years equivalent to losing more than a month of education. Small pollution particles are known to be especially damaging. “That is the same wherever you live. As human beings we have more in common than is different.”

Aarash Saleh, a registrar in respiratory medicine in the UK and part of the Doctors Against Diesel campaign, said: “This study adds to the concerning bank of evidence showing that exposure to air pollution can worsen our cognitive function. Road traffic is the biggest contributor to air pollution in residential areas and the government needs to act urgently to remove heavily-polluting vehicles from our roads.”

Daniels said: “The UK’s air is illegally polluted and is harming people’s health every day. Current policies are not up to the scale of the challenge: government must commit to bringing air pollution below legal limits as soon as possible.”