By Angus Dalton
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Australia is blessed with some of the cleanest air in the world, especially compared to the smog-choked cities of India and China.
But a growing body of evidence is exposing worrying links between the relatively low levels of pollution clouding our cities and an array of surprising health effects.
Air pollution in Shanghai is astronomically worse than Sydney or Melbourne. But new studies show even relatively mild city smog can have surpising health consequences.Credit: iStock
One such study made international headlines last week. Melbourne researchers have discovered exposure to higher levels of air pollution is linked to developing a peanut allergy in childhood.
The study followed more than 5000 children who had a skin-prick allergy test at ages four, six and 10. Researchers estimated how much air pollution each child was exposed to based on their residential address, specifically the fuel exhaust pollutant nitrogen dioxide (NO2) and PM2.5, a catch-all term for particles of smoke, soil, dust or chemicals small enough to enter the bloodstream.
Air pollution irritates the lungs, fires up the immune system and causes systemic inflammation, the authors of the study write. Breathing in particulate matter also raises levels of immunoglobulin E – an antibody associated with allergy.
“Air pollutants have an irritant and inflammatory effect that may boost the immune system’s pro-allergic response, potentially triggering the development of food allergies,” says study author Dr Diego Lopez from the University of Melbourne.
“However, the underlying mechanisms of how air pollution increases the risk of a peanut allergy, and why eczema and egg allergy aren’t impacted in the same way, need to be explored further.”
Sydney respiratory researcher Professor Brian Oliver isn’t surprised by the results.
“In terms of air pollution and allergy, it doesn’t surprise me at all because it’s just basically saying that air pollution is causing the immune system to be dysregulated,” says Oliver, from the Woolcock Institute of Medical Research and the University of Technology Sydney.
Professor Brian Oliver says pregnant women should avoid frequent exposure to heavy traffic.Credit: Scott McNaughton
Oliver wasn’t involved in the peanut study but has been researching the effect of Sydney’s air pollution on brains – and he just reported some “paradigm-shifting” discoveries of his own.
Hazy memories
Oliver’s latest study began with a device that essentially acts as a souped-up vacuum cleaner. Researchers used it to suck up a sample of Sydney’s air about 300 metres from a busy road leading to the Anzac Bridge.
Then, they exposed pregnant mice to a dose of the polluted air scaled to emulate the amount a human living or working along that road would be breathing in.
A smoky day near the Anzac Bridge in Sydney.Credit: Kate Geraghty
When the pups were born, the effects were immediately obvious: the males were smaller than they should have been. Once the mice grew into adults, researchers checked their memory skills with a series of cognitive experiments that tested how well the mice remembered the orientation of coloured blocks.
The male mice who had been exposed to PM2.5 did badly. Oliver further analysed their brains to find out why.
“The male brains that had the PM2.5 exposure had less neurons,” he says. “Basically, the brains were not as brainy.”
The finding was particularly striking given that the levels of air pollution the mice’s mothers were exposed to – PM2.5 concentrations of 10 μg/m3 – were considered “good”, according to Australian air quality categories.
Mouse studies can’t be directly extrapolated onto human health, but Oliver believes it’s “highly likely” his findings would be similar in a human study. There’s limited research into how growing up in a polluted area may affect human brain structure, but a growing body of evidence is linking long-term air pollution exposure to smaller brains that are thinner in the cortex.
An Australian-led study last year based on data from 164,000 Chinese schoolchildren also found that for every 10 micrograms of PM2.5 per cubic metre children were exposed to, they were 1.65 times more likely to be diagnosed with ADHD.
“There’s a definite signal between air pollution and poor cognitive performance at school and air pollution and ADHD,” Oliver says.
Improving the health burden of air pollution systematically is a matter for urban planners and builders. Many buildings in Melbourne and Sydney were old, with outdated air-conditioners that simply sucked in air from outside with limited filtering, Oliver said.
Considering what we’re starting to understand about air pollution, pregnant women should choose to take a walk down quiet side streets when possible, rather than along busy main roads. Air purifiers to improve indoor air pollution also worked well for those who could afford them, he said.
Why did only male brains suffer?
A quirk in Oliver’s study was that only male mice had dimmed memories. A number of studies on children have flagged that the cognitive effects of NO2 and PM2.5 exposure seem to be worse in boys. How could this be?
Intriguingly, Oliver and his team hunted down two genes in female brains – Kdm5c and Kdm6a – that seemed to protect their cognition.
“With in vitro experiments, we could fiddle with those genes and either over-express them or take them out,” he says. “By doing that, we showed those two genes protected the neurons against air pollution-induced apoptosis.”
The exact mechanism of the genes isn’t known. But the discovery is an example of a fascinating and contentious biological idea called the “female protective effect”, which refers to the fact that some disorders impact women less.
The classic example is autism: three to four times as many men are diagnosed compared to women. (This is partly explained by doctors being worse at spotting the disorder in women, but several large studies have shown genetics and sex play a role.)
Oliver says the idea is the subject of ongoing research but makes sense evolutionarily: males are needed only for the easy part of reproduction, while females have to survive long enough to give birth.
“If a male is born a bit defective, as long as they can reproduce, they’ve done their job. But the female has to survive long enough to not only reproduce but also bring up the young, so the females are the more important side of the species.”
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