"Growing up in air-polluted areas linked to mental health issues," reports The Guardian.
Researchers in Denmark and the 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.
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.
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.
The coverage in the UK media was reasonably balanced and accurate. The Guardian, The Independent, The Daily Telegraph and The Times all included expert comment making it clear that the study did not prove a link between pollution and mental health.
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.
Researchers first carried out separate studies in the US and in Denmark.
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.
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.
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:
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.
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."
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.
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.