Traffic in Copenhagen, Denmark Bill Bachmann/Alamy Stock Photo

A study published today adds to evidence that air pollution may be linked to mental health conditions. But it’s not clear yet how – and if – pollution may be affecting our brains.

What has this new study discovered?

Analysing data from 151 million people in the US and 1.4 million people in Denmark, researchers have found that there is a strong correlation between poor air quality and higher rates of bipolar disorder, schizophrenia, personality disorders and major depression. This suggests there is a link, but not necessarily that pollution is causing these conditions.

How strong is the link between pollution and these conditions?

When the team looked at health insurance claims in the US, they found that the strongest predictor of being diagnosed with bipolar disorder – after ethnicity – was air quality. Previous studies have unearthed a correlation in the UK between polluted areas and teenagers reporting psychotic experiences and local air pollution and psychiatric disorders in Swedish children.


How good is the evidence for these?

“We don’t really know very much overall. We’ve only got a handful of studies and most have methodological problems,” says Helen Fisher of King’s College London, who worked on the UK teenager study.

One problem is a lack of data on what an individual’s true exposure to air pollution has been, with some research looking at city-wide air quality measurements rather than specific addresses. That’s a big weakness given we know air pollution exposure can vary significantly from one street to an adjacent one.

In the new study, exposure in the US was mapped at a county level, some of which are thousands of square miles in area.

What else could explain the associations between dirty air and psychiatric conditions?

The study tried to take into account confounding factors where data was available, including income, ethnicity and population density. But an obvious factor that could be linked to both mental health and pollution could be traffic noise, which is known to increase stress and disrupt sleep.

In some other studies, Fisher says deprivation could be the key factor, given that poverty is associated with both psychiatric conditions and pollution.

So how seriously should we take the link between air pollution and mental health?

While the evidence is not very strong yet, Fisher says it shows there is an association between dirty air and mental disorders that warrants further research.

In what ways could pollution affect our brains?

Some of the smallest pollutants – particulate matter known as PM2.5s – can pass through the blood-brain barrier, potentially affecting the brain. Another possibility is that increased inflammation in the body, which air pollution is known to cause, may ignite the brain’s stress response. A third is pollution causing epigenetic changes, which affect the activity of DNA, perhaps leading to altered levels of brain chemicals. But Fisher stresses caution: “These are very tentative mechanisms”.

Why does it matter if a link is proven between air quality and our brains? Shouldn’t we care because of the physical effects anyway?

Stronger evidence of a link might not have a huge impact on policy because the case for action on air pollution – such as it shortening lives through lung and heart problems – is strong. But if dirty air was found to be causing mental conditions it would “open new avenues to the prevention and treatment of mental conditions,” writes John Ioannidis of Stanford University in a commentary article in PLOS Biology.

What’s being doing to find out more?

Fisher says research into air pollution and effects on mental health is about five to ten years behind the huge body of evidence on the physical health impacts. The two key things that need addressing are studies looking at exposure to dirty air over longer periods of time, and much better geographical resolution, helped by people carrying individual pollution monitors. Such research is underway.

Journal reference: PLOS Biology, DOI: 10.1371/journal.pbio.3000353