Air Pollution (Particulate Matter) Exposure and Associations with Depression, Anxiety, Bipolar, Psychosis and Suicide Risk: A Systematic Review and Meta-Analysis

Abstract

Background: Particulate air pollution's physical health effects are well known, but associations between particulate matter (PM) exposure and mental illness have not yet been established. However, there is increasing interest in emerging evidence supporting a possible etiological link.

Objectives: This systematic review aims to provide a comprehensive overview and synthesis of the epidemiological literature to date by investigating quantitative associations between PM and multiple adverse mental health outcomes (depression, anxiety, bipolar disorder, psychosis, or suicide).

Methods: We undertook a systematic review and meta-analysis. We searched Medline, PsycINFO, and EMBASE from January 1974 to September 2017 for English-language human observational studies reporting quantitative associations between exposure to PM $<1.0\mu m$ in aerodynamic diameter (ultrafine particles) and PM $<2.5$ and $<10\mu m$ in aerodynamic diameter (${\mathrm{PM}}_{2.5}$ and ${\mathrm{PM}}_{10}$, respectively) and the above psychiatric outcomes. We extracted data, appraised study quality using a published quality assessment tool, summarized methodological approaches, and conducted meta-analyses where appropriate.

Results: Of 1,826 citations identified, 22 met our overall inclusion criteria, and we included 9 in our primary meta-analyses. In our meta-analysis of associations between long-term ($>6\text{months}$) ${\mathrm{PM}}_{2.5}$ exposure and depression ($n=5$ studies), the pooled odds ratio was 1.102 per $10{\text{-}\mu g/m}^3$ ${\mathrm{PM}}_{2.5}$ increase (95% CI: 1.023, 1.189; $I^2=0.00\%$). Two of the included studies investigating associations between long-term ${\mathrm{PM}}_{2.5}$ exposure and anxiety also reported statistically significant positive associations, and we found a statistically significant association between short-term ${\mathrm{PM}}_{10}$ exposure and suicide in meta-analysis at a 0-2 d cumulative exposure lag.

Discussion: Our findings support the hypothesis of an association between long-term ${\mathrm{PM}}_{2.5}$ exposure and depression, as well as supporting hypotheses of possible associations between long-term ${\mathrm{PM}}_{2.5}$ exposure and anxiety and between short-term ${\mathrm{PM}}_{10}$ exposure and suicide. The limited literature and methodological challenges in this field, including heterogeneous outcome definitions, exposure assessment, and residual confounding, suggest further high-quality studies are warranted to investigate potentially causal associations between air pollution and poor mental health. https://doi.org/10.1289/EHP4595.

Figure 1.

Study selection diagram (studies published between 1 January 1974 and 20 September 2017).

Figure 2.

Summary of results of quality assessment of studies of particulate air pollution and adult mental health outcomes considered eligible for inclusion in this review, outcomes assessed, and overview of inclusion in meta-analyses. Quality was assessed using the Effective Public Health Practice Project (EPHPP) Quality Assessment Tool for Quantitative Studies (MERST 2010; Armijo-Olivo et al. 2012; Thomas et al. 2004), which was developed for evaluating public health research across heterogeneous study designs. We extended the list of study designs assigned a fair rating beyond those listed in the EPHPP Tool Dictionary. We rated all time-series analyses, case-crossover, and hierarchical cluster analyses as fair quality, provided case-crossover studies used bidirectional referent period selection. We rated all cross-sectional studies of associations with long-term PM exposure that used measured or modeled PM values from a period prior to (and not overlapping with) outcome assessment as fair to reflect the relative strength of these studies compared with typical cross-sectional studies with simultaneous exposure and outcome assessment; we rated those using an exposure period overlapping with or after outcome assessment as poor for study quality. We assigned overall ratings according to the EPHPP guidance (those with one poor rating were assigned an overall rating of fair; those with two or more, an overall rating of poor). See Table S3 for further information on the allocation of quality ratings for individual quality components. In the middle set of columns, the outcomes studied (indicated via tick marks) refer to any outcome related to the specified mental health outcome or diagnosis, such as physician diagnosis, meeting a specified threshold on a diagnostic scale, measures of symptom severity, hospital or ED attendance, and in the case of suicide, suicide attempts, ideation, or suicide death. Inclusion in primary meta-analyses (Figures 3, 5, and and funnel plots in Figure 4) is indicated via tick marks in the right-hand set of columns; the numbers and letters of the sensitivity meta-analyses indicated via an S correspond to those detailed in Table 4. Note: ED, emergency department; L-T, long term ($\ge 6\text{\hspace{0.17em} months}$) PM exposure (exposure assessment period $\ge 6\text{\hspace{0.17em} months}$); N/A, not applicable; PM, particulate matter; ${\mathrm{PM}}_{10}$, particulate matter of $<10\mathrm{\mu }\mathrm{m}$ in aerodynamic diameter; ${\mathrm{PM}}_{2.5}$, particulate matter of $<2.5\mathrm{\mu }\mathrm{m}$ in aerodynamic diameter; S-T, short term ($<6\text{\hspace{0.17em} months}$) PM exposure (exposure assessment period $<6\text{\hspace{0.17em} months}$).

Figure 3.

Forest plot of meta-analysis of associations between long-term ($\ge 6\text{- months}$) ${\mathrm{PM}}_{2.5}$ exposure and depression risk ($n=5$ studies). Results of meta-analysis are shown as pooled effect estimates of the OR of depression per $10{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ (95% CIs). The dashed vertical line indicates the overall effect estimate derived from DerSimonian-Laird random effects meta-analysis (DerSimonian and Laird 1986), and the blue diamond indicates the 95% CI of the overall (pooled) effect estimate. The horizontal lines indicate the 95% CI around each study’s central estimate for the adjusted OR (shown with a closed circle); arrowheads at the end of these lines indicate where the true location of the end of a line is not shown (for scale reasons) and the upper or lower 95% CI is farther from the central estimate, in the direction of the arrowhead. The percentage weights are weightings assigned to individual studies’ results in the DerSimonian-Laird random effects meta-analysis, and the sizes of the shaded squares around each effect estimate are scaled according to these relative weightings. The p-value of 0.972 shown at the bottom left is derived from a test of the null hypothesis of heterogeneity (Cochran’s Q). Covariates adjusted for are detailed in Table S2. Note: CI, confidence interval; OR, odds ratio; ${\mathrm{PM}}_{2.5}$, particulate matter of $<2.5\mathrm{\mu }\mathrm{m}$ in aerodynamic diameter.

Figure 4.

Funnel plots for all primary meta-analyses. Primary meta-analysis of depression with long-term (A) ${\mathrm{PM}}_{2.5}$ and (B) ${\mathrm{PM}}_{10}$ exposure. Meta-analysis of suicide risk with short-term ${\mathrm{PM}}_{10}$ exposure, (C) $\text{lag}=$ 0-1 d and (D) $\text{lag \hspace{0.17em}}=$ 0-2 d. A summary of the studies included in each funnel plot is shown in Figure 2; those indicated by tick marks in the right-hand column are included in primary meta-analyses. Funnel plots for sensitivity meta-analyses, the results of which are detailed in Table 4, are not included in this figure. The dark blue circles represent the central estimates for each included study or substudy’s results; the dashed diagonal lines represent pseudo 95% confidence intervals and the solid vertical lines represent the natural logarithm of the overall effect estimate. Note: L-T, long-term ($\ge 6\text{\hspace{0.17em} months}$) PM exposure (exposure assessment period $\ge 6\text{\hspace{0.17em} months}$); lnOR, natural logarithm of the odds ratio; lnRR, natural logarithm of the relative risk (both presented per $10{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ increase in ${\mathrm{PM}}_{10}$ or ${\mathrm{PM}}_{2.5}$ exposure); SE, standard error; S-T, short-term ($<6\text{\hspace{0.17em} months}$) PM exposure (exposure assessment period $<6\text{\hspace{0.17em} months}$).

Figure 5.

Forest plot of meta-analysis of associations between long-term ($\ge 6\text{- months}$) ${\mathrm{PM}}_{10}$ exposure and depression risk ($n=3$ studies). Results of meta-analysis are shown as pooled effect estimates of the OR of depression per $10{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ (95% CIs). The dashed vertical line indicates the overall effect estimate derived from DerSimonian-Laird random effects meta-analysis, and the blue diamond indicates the 95% CI of the overall (pooled) effect estimate. The horizontal lines indicate the 95% CI around each study’s central estimate for the adjusted OR (shown with a closed circle); the arrowhead at the end of the line for Zijlema et al. 2016, Substudy A (LifeLines) indicates that the true end of this line is not shown (for scale reasons) and the lower 95% CI is farther from the central estimate. The percentage weights are weightings assigned to individual studies’ results in the DerSimonian-Laird random effects meta-analysis, and the sizes of the shaded squares around each effect estimate are scaled according to these relative weightings. The p-value of 0.638 shown at the bottom left is derived from a test of the null hypothesis of heterogeneity (Cochran’s Q). Covariates adjusted for by individual studies are detailed in Table S2. Note: CI, confidence interval; OR, odds ratio; ${\mathrm{PM}}_{10}$, particulate matter of $<10\mathrm{\mu }\mathrm{m}$ in aerodynamic diameter.

Figure 6.

Forest plot of meta-analyses of associations between short-term ${\mathrm{PM}}_{10}$ exposure and risk of completed suicide (relative risk per ${10\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$), at cumulative lags 0-1 and 0-2 d. Results of meta-analysis are shown as pooled effect estimates for RR of depression per $10{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ ${\mathrm{PM}}_{10}$ (95% CIs). Lag 0-1 refers to the cumulatively lagged values (moving average) of concentrations across Day 0 (the day of the outcome event) and Day $-1$ (the previous day), whereas Lag 0-2 refers to the cumulative lagged values across Days 0, $-1$, and $-2$. The dashed lines indicate the overall effect estimates, separately for each cumulative lag, derived from DerSimonian-Laird random effects meta-analysis, and the diamond indicates the 95% CI of the overall (pooled) effect estimate. The horizontal lines indicate the 95% CI around each study’s central estimate for the adjusted RR at this exposure time lag (shown with a closed circle); the arrowhead at the right end of the line for Bakian et al. (2015) at lag 0-2 d indicates that the true location of the upper 95% CI for this study is farther from the central estimate. The percentage weights are weightings of the individual studies in the DerSimonian-Laird random effects meta-analysis, and the sizes of the shaded squares around each effect estimate are scaled according to these relative weightings. The p-values at the bottom left are from a test of the null hypothesis of heterogeneity (Cochran’s Q). The covariates that each study adjusted for are detailed in Table S2. Note: CI, confidence interval; ${\mathrm{PM}}_{10}$, particulate matter of $<10\mathrm{\mu }\mathrm{m}$ in aerodynamic diameter; RR, relative risk.