A recently published study has added evidence to the concern that air pollution might be linked with mental health conditions. However, it is still not clear if or how pollution might affect the human brain.

What has been discovered by the new study?

Researchers analyzed data from 1.4 million individuals in Denmark and 151 million individuals in the US and found a strong correlation between higher rates of major depression, personality disorders, schizophrenia, and bipolar disorder with poor air quality. This suggests that there is a link, but does not necessarily prove, that these conditions are caused by pollution.

How strong is this link between those mental health conditions and pollution?

When the researchers examined US health insurance claims they discovered that the strong predictor of a bipolar disorder diagnosis was air quality – after ethnicity. Earlier studies uncovered a correlation in the UK between teenagers who reported psychotic experiences and polluted areas and in Swedish children between psychiatric disorders and local air pollution.

How good is this evidence?

Due to methodological problems and limited studies, the evidence is still being studied by scientists.

One problem is the lack of data related to what a person’s true exposure has been to air pollution. Some researchers have looked at city-wide measurements of air quality instead of for specific addresses. That is a major weakness given that air pollution exposure is known to vary greatly from one street to the next.

A new study mapped US exposure at the county level, with some of them having areas of thousands of square miles.

What else might explain these associations between psychiatric conditions and dirty air?

The study attempted to take confounding factors into account where there was data available, including population density, ethnicity, and income. However, an obvious factor that might be linked to both pollution and mental health might be traffic noise, since it is known to disrupt sleep and increase stress.

In some of the other studies, the key factor might be deprivation, since poverty is associated both with pollution and psychiatric conditions.

So just how seriously do we need to take this apparent link between mental health and air pollution?

Although there is no strong evidence yet, scientists do agree that an association between mental disorders and dirty air does warrant further research.

How can our brains be affected by pollution?

Some of the tiniest pollutants – particulate matter referred to as PM2.5s – is able to pass through our blood-brain barrier, which can potentially affect the brain. Increased inflammation within the body is another possibility. It is known that air pollution does cause this and it can ignite the stress response of the brain. A third possibility is epigenetic changes being caused by pollution, which can affect DNA activity that can lead to altered brain chemical levels. However, many experts are very cautious about the results still since tentative mechanisms are used by the studies.

So why is it important to prove a link between our brains and air quality?

Shouldn’t we care anyway due to the physical effects that poor air quality causes?

Stronger evidence fo this link might not have a major impact on policy since the case for acting on air pollution – like it shortening our lives via heart and lung problems – is already strong. However, if it was found that dirty air was causing mental conditions as well it would open up new avenues for preventing and treating mental conditions, according to Stanford University’s John Ioannidis, in a commentary article published by PLOS Biology.

What is currently being done to learn more?

Research on the effects that air pollution has on mental health is lagging behind the large body of evidence on its impacts on physical health by five to ten years. There are two key things that need to be addressed that studies are looking at which are dirty air exposure over longer time periods, and improved geographical resolution, which is helped by the people who carry individual pollution monitors. This research is currently underway.