Need2Know - Air Pollution and Mental Health
Air pollution is contamination of the indoor or outdoor environment by any chemical, physical or biological agent that modifies the natural characteristics of the atmosphere. Outdoor and indoor air pollution cause respiratory and other diseases and are important sources of morbidity and mortality. At the same time, the connection between air pollution and mental health is a growing area of research and advocacy. WHO data show that almost all of the global population breathe air that exceeds WHO guideline limits and contains high levels of pollutants, with low- and middle-income countries suffering from the highest exposures. Air quality is closely linked to the earth’s climate and ecosystems globally, and air pollution poses a major threat to health and climate.
Here is what you “need to know” from the current evidence base
Narrative summary of evidence
There is growing evidence highlighting how exposure to air pollution is associated with various negative mental health outcomes.
This includes:
A number of meta-analytic studies highlighting the relationship between air pollution and poor mental health outcomes.
For example, multiple meta-analyses have highlighted a small but increased risk of depressive symptoms among individuals more exposed to PM2.5 in the long-term (relative risks between 1.074-1.18 per 10μg/m3 average increase) and evidence of a positive association with PM10 in the short term.
One meta-analysis has highlighted an adverse relationship between PM2.5 exposure and cognitive decline.
Two meta-analyses of observational studies have reported increased risk for autism spectrum disorders with increasing PM2.5 and PM10 during pregnancy, the postnatal period, and early life, although results remain limited by the methodological heterogeneity of the literature.
Three meta-analyses have found that short-term exposure to NO2, PM2.5, and PM10 is associated with a small but significant increased incidence of suicide. A natural experiment in the US using drifting wildfire smoke from the US further indicated how air pollution exposure increased suicide rates, particularly among rural populations.
A number of longitudinal studies showing that air pollution exposure is associated with negative mental health outcomes. For example:
One prospective study of 1698 individuals from 1075 households in South East London found long-term exposure to PM2.5, NOx and NO2 to be associated with 18–39% increased odds of common mental disorders, 19–30% increased odds of poor physical symptoms and 33% of psychotic experiences only for PM10.
One longitudinal study found an association between residential air pollution exposure and mental health service use (an indicator of illness severity and relapse) in London and found residential air pollution exposure to be associated with increased mental health service use among people recently diagnosed with psychotic and mood disorders.
Various cohort studies have highlighted a negative association between PM2.5 exposure and incidence of all-cause dementia.
One large prospective cohort study using UK Biobank data (N = 389,185) found an association between long-term exposure to low levels of multiple pollutants and anxiety and depression.
One birth cohort in the UK of 9,065 people found that higher exposure to PM2.5 in pregnancy and childhood was associated with increased psychotic experiences and in pregnancy was associated with higher rates of depression.
Importantly, clean air policies have been shown to have positive impacts on mental health.
For example, one study using the China Health and Retirement Longitudinal study found that a 10-µg/m3 reduction of PM2.5 concentration was associated with a 4.14% (95% CI: 0.41-8.00%) decrease in depressive scores. The quasi-experimental nature of this study provides some of the most robust evidence for this association.
Another study, estimated that PM2.5 reductions under China’s Air Pollution Action Plan prevented 13,000–79,000 (95% confidence interval) suicides over 2013–2017, accounting for ∼10% of this period’s observed suicide rate decline.
The impact of air pollution on mental health is not equally distributed with people in more vulnerable communities being most likely to experience its negative impacts. One study from China using the China Panel Family Study for example found that the mental health of people from low socioeconomic impacts was more vulnerable to the negative impacts of air pollution.
It is not yet clear what the underlying mechanisms underpinning these associations are, with various potential mechanisms having been proposed including:
Neuroinflammatory and oxidative stress pathways
Implication of the hypothalamic-pituitary-adrenal axis
Mitochondrial dysfunction
Genetic mediation
Psychological mechanisms, e.g., distress associated with living in an environment perceived as unhealthy
Potential residual confounding due to factors such as socioeconomic status, residence, physical health comorbidities, and non-measured air pollutants
Additionally various methodological complexities make drawing causal inferences between exposure to air pollution and mental health problems challenging. These include:
Complexities of experimentally manipulating exposure to air pollutants and need to rely on observational data
Methodological limitations in measuring exposure and outcomes (e.g., using poorly validated measures, relying on self-report etc.)
Lack of measurement of potential confounders (e.g., socioeconomic status, other air pollutants etc.)
Lack of granularity of exposure to air pollutants (e.g., reliance on low resolution satellite-based estimates) leading to potentially inaccurate predictions of individual exposure (e.g., not taking into consideration possible adaptations such as use of air filtration devices, mask-wearing etc.)
Additionally, to date, most of the evidence has focused on outdoor air pollution, with less work exploring the relationship between indoor air pollution and mental health.
Furthermore, the vast majority of the research has been conducted in high-income countries.