Air pollution is linked to mental health problems

Researchers in Denmark and the United States (US)  found that people in Denmark who grew up in more polluted areas up to the age of 10 were more likely to develop depression, bipolar disorder, schizophrenia or personality disorder.


While US counties with worse air pollution had higher rates of bipolar disorder and depression.

The World Health Organisation estimates that air pollution kills 7 million people each year – equivalent to 13 deaths every minute, and more than the combined total of war, murder, tuberculosis, HIV, AIDs and malaria.

However, showing a link does not mean that air pollution was the direct cause of mental health conditions in these studies.

Other factors could have been involved.

For example, people living in more polluted areas (which tend to be in urban environments) may have lower incomes, have had more traumatic life experiences, different drug use habits and less access to green space.

And all these factors may increase the chances of mental health problems. The study did not take all of these factors into account.

There is growing interest in the impact of air pollution on our health. This study is thought provoking, but should only be seen as a way of exploring ideas at this stage.

It does not prove that pollution causes poor mental health. More research is needed to look into whether this link still exists after taking into account more risk factors for mental health problems.

Where did the story come from?

The study was carried out by researchers from the University of Chicago and University of California Los Angeles in the US, and from the Aarhus University in Denmark and Karolinksa Institut in Sweden.

It was funded by the Nordfosk project, which co-ordinates joint research funding in Nordic countries, DARPA (the US Defense Advanced Research Projects Agency) and US National Institutes for Health. It was published in the peer-reviewed) journal PLOS Biology on an open access basis so it is free to read online.

What kind of research was this?

The researchers carried out 2 observational studies, using large databases from the US and from Denmark, which included information about environmental conditions (including air pollution) and treatment for mental health conditions. They wanted to see whether air pollution in the environment was linked to a higher chance of mental illness.

While observational studies can show interesting links between risk factors (such as pollution) and medical conditions, they cannot show that one directly causes the other. This is particularly the case in this type of study, as the researchers had to make assumptions about people's environmental exposures based on the geographical location of their residential addresses.

What did the research involve?

Researchers first carried out separate studies in the US and in Denmark.

United States

Researchers used US Environmental Protection Agency data to look at pollution at county level. There are 3,142 counties in the US. They recorded air pollution, water pollution, land quality and quality of the built environment, which included amount of traffic, for each county. They used data from 2000 to 2005 and divided counties into 7 groups – from most to least polluted.

The researchers used a large database of insurance claims to identify people in each county who had the diagnoses of interest.

The IBM Health MarketScan commercial claims and encounters database, which records health insurance claims for more than 151 million people, was used to estimate the proportion of people in each county who had bipolar disorder, schizophrenia, personality disorder, major depression, epilepsy or Parkinson's disease. They used data from 2003 to 2013.

The researchers also used information about the weather to get an idea of how much time people might spend outside, and about the ethnic background of people living in each county, their average income, population density, proportion of poor people and urban population.

Researchers analysed whether each of these factors was associated with a county having a higher or lower proportion of residents with each medical condition.

The figures were adjusted to take account of people's age and gender, and of the county's population density, ethnic diversity, average income, the quality of air, water, land, built environment and weather; and percentages of poor and insured population in the most polluted counties.


Researchers used data from Denmark's national treatment and pollution registers. Using daily recorded data on pollution (to 1km squared levels) they assessed air pollution using atmospheric concentration of 14 compounds linked to air pollution.

They then looked at people born in Denmark during 1979 to 2002, who were still living in Denmark at the age of 10. This group of 1.4 million people had data available from birth to 2016.

The researchers estimated the average amount of air pollution each person experienced from birth up to age 10. The researchers divided the population into 7, from those who experienced the lowest to highest air pollution levels.

They then looked to see whether people exposed to more pollution by age 10 were more likely to have developed a mental health problem. Information was available on depression, bipolar disorder, personality disorder and schizophrenia.

Joint figures

Finally, the researchers carried out some statistical analyses to try to reconcile the 2 sets of data, including restricting the US figures to the air pollution measures taken in Denmark, and adding some basic socioeconomic figures into the Danish calculation to see if this affected the results.

How good is the evidence?

"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

What were the basic results?

United States

Counties with the highest level of air pollution had 27% more people with bipolar disorder than counties with the lowest levels (95% credible interval (CrI) 15% to 40%). Counties with the highest levels of air pollution had a very small (6%) increase in the levels of major depression (95% CrI 0% to 12.4%)

Air pollution was not linked to rates of schizophrenia or personality disorder.

Other factors, such as ethnicity, density of population, land pollution and urban living were also linked to the rates of some mental health conditions.


Rates of all 4 mental health conditions studied were higher among those from areas with more pollution. However, the figures in the paper (reported below) do not seem to have been adjusted to take account of social and economic factors that might affect risk of mental health diagnoses. The paper reports that compared to those who lived in the least polluted areas:

  • schizophrenia was 148% more likely for people who’d lived in the most polluted areas up to age 10 (95% confidence interval (CI) 119% to 180%)
  • bipolar disorder was 24.3% more likely (95% CI 4.5% to 47.9%)
  • personality disorder was 162% more likely (95% CI 142% to 183%)
  • depression was 50.5% more likely (95% CI 42.8% to 58.7%)

The figures adjusted for social and economic factors are reported only as graphs and seem to show that the increase in risk for bipolar disorder is not statistically significant.

How did the researchers interpret the results?

The researchers said: "We observed a strong positive association between exposure to environmental pollution and an increase of prevalence in psychiatric disorders in affected patients."

They caution: "These strong associations do not necessarily mean causation [proven cause and effect]; further research will be needed."

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

The study tried to consider confounding factors where figures were available, including income, ethnicity and population density. But an obvious factor that could be linked to both mental health and pollution is traffic noise. This is known to increase stress and disrupt sleep, which are both linked to mental ill health.

In what ways could pollution affect our brains?

Some of the smallest pollution particles can pass through the blood-brain barrier, potentially affecting the brain. Air pollution is known to cause inflammation in the body, which may ignite the brain's stress response. Or perhaps pollution can cause epigenetic changes that affect the levels of signalling chemicals in the brain. But these are only tentative ideas.


This study is interesting for researchers who want to find out more about the possible causes of mental health conditions, and for those wanting to understand the health effects of air pollution.

However, the study is only exploratory, and the analyses do not tell us much yet. We certainly do not know whether polluted air can directly cause depression, bipolar disorder or other conditions.

The main limitation of the study is that it relies on crude data about where people live and the air pollution in that area. It's not certain exactly what pollution levels each person was exposed to.

It also did not take into account the possible effects of many other factors that could raise the chances of mental health problems, such as a family history of mental health problems, having traumatic life experiences, or use of drugs such as cannabis.

While the researchers did try to account for some socioeconomic factors, the results are not presented in a way that makes this clear. The US figures are based on an insurance database, so do not include people without health insurance. That means poorer people with mental health conditions may not have been included.

Also, for the US data, it was not possible to be certain that air pollution was measured before individuals developed their mental health condition.

The researchers speculate that pollution could cause mental health problems via inflammation and damage to the brain. But so far this idea is based on experimental laboratory animal tests, and we do not know whether it translates to humans in the real world.

Previous studies have linked poor air quality to a range of conditions including asthma, heart disease, and various types of cancer. Air pollution is strongly linked to risk of other diseases, especially respiratory disease.

So, there's no doubt that reducing air pollution is an important goal. However, we do not yet know if it's a major factor in mental health.

Why does it matter if air quality affects our brains?

Shouldn't we care because of the known physical effects it has anyway?

Stronger evidence of a link to mental health might not have a huge impact on policy because the case for action on air pollution - such as it is shortening lives through lung and heart problems - is strong.

But if dirty air was found to cause mental illness, it would "open new avenues to the prevention and treatment of mental conditions", John Ioannidis at Stanford University in California wrote in a commentary in PLoS Biology.

Air pollution could be damaging your memory

The findings showed that memory scores were significantly worse for participants living in areas with high levels of NO2 and PM10.

New research has found that exposure to air pollution is significantly affecting our memory, causing a loss in memory which could be equivalent to up to ten years of ageing.

Carried out by researchers from the University of Warwick, the new study looked at a nationally-representative sample of 34,000 individuals across 318 geographical areas in England.

The researchers collected information on air quality for each district, including levels of both nitrogen dioxide (NO2) and particulate matter (PM10), which are particles with a diameter of 10 micrometres or smaller. Both are produced by burning fossil fuels from car and other vehicle exhausts, power plants and industrial emissions.

The participants were asked to remember 10 words in a standardised word-recall test and were given a score from zero to ten based on their answers.

The researchers also took into account participants’ age, health, level of education, ethnicity, and family and employment status, which are factors that can impact memory.

The findings showed that memory scores were significantly worse for participants living in parts of England with high levels of NO2 and PM10.

In fact, the researchers estimated that the difference in memory between England’s cleanest areas, found on the west coastline in districts such as Devon and West Somerset, and most-polluted areas, places like Kensington and Islington in London, is equivalent to the loss of memory from 10 extra years of ageing.

“When it comes to remembering a string of words, a 50-year old in polluted Chelsea performs like a 60-year old in Plymouth. We are still not exactly sure how nitrogen dioxide and air particulates act to do this,” commented co-author Professor Andrew Oswald.

The researchers say that although caution is always needed when interpreting a causal relationship, they describe the results as “concerning”, and add that they are consistent with those produced by animal studies, although this is one of the first studies to confirm the results in humans.

“There is a little prior evidence of a negative association between levels of traffic pollution and memory using data on elderly individuals and in children,” said co-author Professor Nattavudh Powdthavee, “but almost all research in human studies on this topic has been based on elementary correlations and not on nationally representative samples of individuals in a country. We have tried to solve these two problems in our study.”

Exposure to pollution linked to 'silent miscarriages'


Pregnant women who have been exposed to high levels of pollution face an increased risk of "silent miscarriage" in the first trimester, according to a new study. A "silent miscarriage" happens when a fetus hasn't formed or has died, but the placenta and embryonic tissue remain.

Researchers analyzed data from more than 250,000 pregnant women in Beijing between 2009 and 2017.The study published Monday in the journal Nature Sustainability.

Earlier research has shown a connection between pollution and health problems for pregnant women, such as hypertension and pre-eclampsia. Pollution is also linked to low birth weight, but there has been little research to connect pollution to miscarriage.

This study found that the women who lived in neighborhoods in Beijing with higher levels of concentrated pollution including particulate matter pollution, sulfur dioxide, ozone and carbon monoxide saw a greater risk of miscarriage.

Particulate matter pollution seemed to put the women at the greatest risk among miscarriages linked to pollution. The authors thought that was because these tiny particles can cross the maternal-fetal blood barrier and disrupt fetal growth and development.

This study seems to agree with earlier work. A study in that United States published in February found that women in Utah who were exposed even short-term to pollution faced an increase risk of miscarriage.

Other research has found that pollution can breach a mother's placenta and potentially reach fetuses in the womb, raising the possibility of miscarriage or, if the woman is able to carry the baby to term, future health problems for the child.

A 2017 study of women in London found that exposure to pollution from traffic led to giving birth to low birth weight babies. Babies born with a low birth weight are at a much greater risk of dying than healthy weight babies and face a much greater risk of chronic disease later in life, such as cardiovascular problems.

When these tiny pollution particles enter the fetal bloodstream, they may interact with the tissue leading to irreversible damage to the dividing cells. Maternal exposure may also damage the placenta.

Women who were older than 39 at age of conception and women who worked as farmers or as blue collar workers seemed to be most at risk.

Earlier studies have shown that even when air pollution is at levels below air quality guidelines and regulatory limits, it can still pose a real hazard to public health. Exposure to high levels of pollution have lead to an increasing number of deaths around the world.

A study in July found that long-term exposure to air pollution, especially ground-level ozone, is like smoking about a pack of cigarettes a day for many years and can cause problems such as emphysema. Another study found that it can cause COPD and age lungs faster. Air pollution also increases the risk of heart disease, stroke and lung cancer.


 "Growing up in air-polluted areas linked to mental health issues," reports The Guardian.

Air pollution linked to mental health issues, research suggests 

The growing link between pollution and mental health By Dr Layla McCay, Director, Centre for Urban Design and Mental Health

Briefing Mental health: Adam Vaughan; New Scientist 31 August 2019 number 3245 pg 14


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