Scientists already knew that links could be drawn between air pollution and increased risk of certain disorders - ranging from obesity to diabetes and fertility - mostly because of stressful messages sent from the brain.
Researchers at the University of Maryland School of Medicine (UMSOM) have shown how air pollution reduces sperm count in mice, by causing inflammation in the brain.
"Our findings showed that the damage due to air pollution - at least to the sperm count - could be remedied by removing a single inflammation marker in the brains of mice, suggesting that we may be able to develop therapies that could prevent or reverse the damaging effects of air pollution on fertility," Zhekang Ying, lead study author and Assistant Professor of Medicine at UMSOM, said.
Published in September in the science review Environmental Health Perspectives, the latest study tried to plot the origins of a worldwide decline in the sperm count observed over the past decades.
When sperm and fine particles aren’t a good match
Around 92 per cent of the world population lives in areas where the level of fine particles in the air smaller than 2.5 micrometres in diameter “exceed the minimum safety standards set by the World Health Organisation,” the study noted.
Such pollution can come from cars' muffler pots, factory emissions, wildfires or woodburning stoves.
In previous studies, results have shown that when exposed to air pollution, mice did not always have inflammations on the testes, the male sex organs that produce sperm.
Such a discovery means that other mechanisms might be potentially responsible for reduced sperm counts.
The whole team of researchers, composed of Ying, Lianglin Qiu, Minjie Chen, Xiaoke Wang and Sufang Chen, tested healthy mice and mice bred without a marker of inflammation in the brain called Inhibitor Kappa B Kinase 2, or IKK2.
Exposing both healthy and IKK2 mutant mice to polluted air, they observed the effects and then tested their sperm counts.
The outcome was clear: the mice bred without the IKK2 inflammation marker in their neurons did not see any reduction in their sperm counts, unlike the healthy mice.
As a second step in the study, the researchers then removed IKK2 markers from specific neurons in order to determine more precisely how air pollution was leading to lower sperm counts.
They found that one specific kind of neuron typically associated with the sleep cycle and obesity was responsible for the reduced sperm count due to air pollution.
These neurons are typically found in the hypothalamus, the part of the brain which controls hunger, thirst, and sex drive.
The hypothalamus also works with the brain’s pituitary gland, which makes hormones that communicate directly with reproductive organs.
"Looking back, it makes perfect sense that the neurons in the hypothalamus are the culprits perpetuating this inflammation response that results in low sperm count, as we know that the hypothalamus is a major pathway link between the brain and the reproductive system," said Ying.
Sounding the alarm on air pollution
"These findings have wider implications than just fertility, as there are many conditions, such as high blood pressure, diabetes, and heart disease that can result from brain inflammation due to air pollution," Dr Charles Hong, Professor in Medicine and Director of Cardiology Research at UMSOM, said.
As the main topics at the UN’s COP 26 summit are to be linked to climate change and taking radical measures to reduce air pollution, such a study can serve as another wake-up call from scientists.
"Environmental pollution is a problem of equity in that some persons who are poor or of colour tend to face more severe health-related conditions due to greater exposure," Albert Reece, executive vice president for Medical Affairs at UM Baltimore, said in response to the study.
“It is important to explore the mechanisms by which pollution affects the body, so we can devise ways to prevent or treat these conditions to eliminate these health disparities".