Are we polluting children's brains?
There is a growing debate about the harmful effects of airborne neurotoxins – just how good is the science? By Norman Swan.
Let’s do a reality check. There is not a lot of evidence that we have a toxic brain disaster on our doorstep. For many decades, with each new generation we've seen a significant rise in the average IQ of the population. It’s not at all clear why, but our kids are truly smarter than us, and it isn’t just about better knowledge through wider education.
Second, there are few, if any, signs that we’re in the middle of an autism or Attention Deficit Hyperactivity Disorder (ADHD) epidemic. The only epidemic is in diagnosis, but that’s a topic for a future column. When it comes to dementia there is emerging evidence from Scandinavia that the incidence is topping out, suggesting that brain health is improving in lockstep with our general health.
Having said that, statistics can hide gaps and it doesn’t mean that we can stop worrying about potential risks from exposure to possibly neurotoxic pollutants. The most important neurotoxins are still tobacco smoke, alcohol, lead and mercury, all of which affect developing brains. In the early 1980s, Australian research known as the Port Pirie Cohort study showed significant developmental problems arising for children living close to a lead smelter. Children’s health also was a major driver for lead-free petrol. But what about pollution from traffic exhausts and coal-burning?
A fascinating study on babies born near a coal-fired power station in Tongliang County, China, was published this March in PLoS One by researchers at Columbia University. Some toxins found in coal emissions are polycyclic aromatic hydrocarbons (PAH), which tend to form damaging chemical bonds with DNA known as adducts.
Using cord blood from the newborns, the researchers measured levels of PAH adducts and levels of the protein "brain-derived neurotrophic factor" (BDNF), which serves as a marker of developing brains. The children’s development was assessed at the age of two. These measures were carried out on babies born in 2002 and 2005, before and after the power station was shut down. Cord blood of babies born after the shutdown showed lower levels of the adducts, higher levels of BDNF, and the children scored higher in developmental tests.
Traffic exhaust also contains PAH, but the most concerning pollutants are very fine particles that are thought to find their way into our bloodstreams via the lungs. The evidence that small-particulate air pollution from traffic exhaust is bad for adult brains is reasonably strong with links to stroke, possibly through inflammatory effects on arteries. And it’s this idea of inflammation, especially in developing brains, that has people worried about links to autism spectrum disorder (ASD).
What I’m about to describe isn’t fringe research; it comes from Harvard School of Public Health scientists. These scientists looked at the effects of particulates in California.
Children with ASD range from having severe behavioural, language and intellectual disabilities, through to high-functioning people whose chief problem lies in reading the emotions of others. The Harvard group’s findings relate to ASD in the offspring of mothers exposed to excessive air pollution during pregnancy.
Suspicions had been aroused by previous studies that found a link between high air pollution levels and rates of autism in the Bay Area of San Francisco. However these findings could have been confounded by the fact that mothers who live near freeways are also probably poorer, have lower levels of education and may also have increased rates of smoking and other adverse factors.
So the Harvard researchers came at it another way. They have decades of data on nurses from the famous “Nurses’ Health Study” – the first to highlight that hormone replacement therapy increased the risk of breast cancer. In 2005 the researchers determined which of the nurses had children with or without ASD. They gathered data on air pollution levels in the areas where the nurses lived during their pregnancies. They looked at the nine months of the women’s pregnancies compared to the nine months before conception and to the nine months after the child was born and found a significant association between ASD and exposure to particulates smaller than 2.5 microns. The strongest association was for women who were exposed in the last trimester of pregnancy when nerve connections are being made in the developing brain. They found that the higher the level of particulates, the greater the number of cases. While the background rate of ASD in the US is one child in 88, mothers exposed to the highest levels were up to 60% more likely to have a child with ASD.
Bottom line: ASD now joins the club of diseases, including stroke, heart disease and respiratory problems, that are associated with exposure to tiny particles in the air we breathe.
Is it 100% proven? Not yet. But it is nevertheless worrying and means that efforts to reduce traffic-related pollution should be maintained if not enhanced.