Pregnant women's exposure to air pollution and stress can raise the risk of autism-like social behaviour and differently wired brains in males, according to a study on mice.

Air pollution, such as exhaust emitted by diesel engines in trucks, is linked to increased rates of neurodevelopmental disorders, such as schizophrenia and autism.

While 99 per cent of people across the globe live in cities with unhealthy air, only one in every 44 children is diagnosed with autism (and four times more boys than girls). Why doesn't everyone develop autism then?

"Environmental toxins are worse for some people than for others and it's always the most vulnerable populations that are affected," said Staci Bilbo, psychology and neuroscience professor at Duke University.

In the case of autism and air pollution, Bilbo thinks the missing link is maternal stress stemming from poverty and housing insecurity.

Autism & Children: A Nutrition Guide
Autism & Children: A Nutrition GuideIANS

"It's not that wealthy people aren't stressed," Bilbo said. "But it's different when you have to worry about where you're going to live and whether you're safe in your home."

Even as the team couldn't experiment with the conditions in pregnant women for ethical reasons, they exposed pregnant mice to diesel exhaust particles, as a proxy for air pollution. Pregnant mice were also allotted less building materials than usual to construct their nests for their pups.

While their daughters grew up as expected, their sons misread social cues throughout life. As teenagers, males born by stress and smog exposed moms preferred hanging out with a yellow rubber duck rather than a nearby mouse (mice usually prefer the company of one of their own rather than a bath toy).

Air PollutionIANS

Further, the team found that stressed mothers who had inhaled diesel fumes gave rise to males who, as toddlers, had an overabundance of brain cell connections, called synapses, that need to get pared down as we grow up. The synapses leading to successful tasks, like picking up a glass, are maintained and strengthened, whereas the connections that lead to failed attempts get removed.

The researchers also found that their immune cells in the brain called microglia, which removes weak or dead synapses, had less of the protein that stimulates their appetite for synapses, which likely explains the observed overgrowth.

By adulthood, everything got flipped around. Males from smog- and stress-exposed mothers now had fewer synapses and were more gregarious than their unexposed peers.

This atypical tendency to be more outgoing rather than reserved mirrored the behaviour and brain activity of mice with autism-linked genes.

People with autism are often mistakenly assumed to be less social, instead, for people with autism, it's more of a misunderstanding of social cues and conventions rather than being inherently introverted, the team noted.