Autism’s connection with air pollution explored in research

The view that air pollution contributes to autism is strongly held by a small but growing band of scientists who study pollution and health. They point to links between autism and air pollution uncovered in studies tracking thousands of children.

"We didn't know much about the effects of air pollution on the brain, say five-10 years ago, but now we are seeing its effects on the development of children's brains," says Dr Ebba Malmqvist, epidemiologist at Lund University in Sweden. "This can lead to cognitive dysfunction and studies show a higher risk of autism." The more air pollution, the more autism, it seems.

We should be worried about the particles coming out of traffic exhaust, because a lot of the studies show these fumes may be quite toxic

Malmqvist is completing a study of 48,000 women who were pregnant between 1999 and 2009 in southern Sweden, having followed the children until 2016. She wants to know what happens when a mother experiences high amounts of air pollution during pregnancy and what happens to a child who continues to breathe such air. Her study relied on mothers’ home addresses and estimated pollution levels to calculate exposure.

It is well accepted that air pollution contributes to illness around the world, especially lung conditions, heart disease and stroke in adults. What is newer is the realisation that the tiny particles cross from the lungs into our bloodstream, and travel into the brain. The particles also permeate and change the placenta; the critical tissue connecting mother and baby.

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Research in California this year (and a forthcoming study in Denmark) bolstered the pollution-autism link. We should be worried about the particles coming out of traffic exhaust, because a lot of the studies show these fumes may be quite toxic, says Prof Beate Ritz, an expert on environmental effects on health at the University of California who is involved in the Californian and Danish studies. The fumes could be toxic to young brains especially, he says.

There's evidence coming from lab studies too. When Prof Deborah Cory-Slechta at University of Rochester medical centre, New York, exposed young mice to polluted air, she detected changes in their brains that resemble what's seen in the brains of people with autism. "The levels we use are somewhat high, so the sort of thing you'd find on an Los Angeles expressway," she says. "We find a lot of changes in the mouse brain." Metals in particular accumulate to unusual levels.

“Genetics alone cannot explain [autism spectrum disorder],” says Cory-Slechta. “That is why people have looked at the environment.” She views air pollution as a contributor to autism and other developmental disorders. “It is like rain droplets in a bucket. Eventually there are enough hits and you trigger something.”

Not everyone agrees. Dr Kevin Mitchell, a geneticist and brain scientist at Trinity College Dublin, views genetics as the main contributor to autism and says the environment may account for less than 5 per cent of its development.Cory-Slechta puts the figures at 70 per cent environment, 30 per cent genetics.

Brain imaging

More studies are needed. A Spanish study is looking at air pollution, autism and brain imaging in European children. It is asking which time windows are especially critical for any effects on a child’s brain.

A previous study led by the Barcelona Institute of Global Health showed primary schoolchildren in highly polluted areas showed slower cognitive development. This group advised on how to protect schoolchildren's brains from air pollution in the scientific journal Environment International.

Their article recommended that schools be located away from trafficked roads, classrooms not face the busiest roads and air intake come from fresh air furthest away from traffic. Green areas and pedestrian space should be placed around schools, and parents and children should avoid the busiest roads when commuting to school.

A follow-on study, led by Prof Jordi Sunyer at the Barcelona institute, is looking to recruit more than 1,000 pregnant women to scan the brains of their growing foetuses. "This will answer whether traffic air pollutants in urban air are associated with structural changes in the brain during prenatal life," he explains.

Monitoring of air pollution tracks levels of nitrogen oxides (NOx) and particles measuring between 2.5 and 10 microns (a micron is a millionth of a metre) in diameter. The larger particles stick in our airways, exacerbating respiratory conditions such as asthma. Those smaller particles can travel throughout our body, potentially causing inflammation.

At a time when cases of autism are rising, air pollution does not seem to be visibly worsening. What is not monitored, however, are the ultrafine particles, less than 100 nanometres in diameter (a nanometre is one 1,000th of a micron). It is assumed that by regulating larger particles and NOx gases, you catch these nanoparticles.

Cory-Slechta is sceptical on this point. She suspects the smallest particles are causing harm. They should be monitored and regulated in future, she adds.