This 11-year cohort study analyses for the first time in France the relationships between PM air pollution and psychiatric outcomes, namely the number of emergency department visits for psychotic and mood (unipolar and bipolar) disorders. The results show a significant association between PM2.5 and PM10 peaks air pollution and the daily number of emergency visits for psychotic disorders. The number of daily emergency visits for unipolar depressive disorders were also higher during PM10 peak. Finally, no association was found for bipolar disorders.
Comparisons with previous studies
In a systematic review of studies focusing on associations between air pollutants and daily hospital admissions and/or emergency department admissions for psychiatric care, Bernardini et al. (Bernardini et al., 2020) identified 19 studies (including 9 that analyzed psychiatric disorders as a whole without specifying the diagnoses). Among the different analyzed air pollutants, the strongest and more consistent evidence of an association with psychiatric disorders concerned PM10: 13 of the 16 studies that analyzed the influence of PM10 and 8 of the 12 studies for PM2.5 found significant associations. Interestingly, these consistent results were found with different methods: (i) these studies analyzed the influence of PM with different lag periods (e.g., lag 0 and lag 0-1 days (Lee et al., 2019), lag 0 to 3 (S. H. Kim et al., 2019), etc.), and (ii) most of these studies analyzed the influence of 10 mg/m3 increase of PM, whereas we analyzed the number of visits according to the presence of a pollution peak (i.e., above predefined official French thresholds for defining a risk to human). Thus, our study using another design (lag 7 days, and binary peak variable according to air pollution thresholds) confirms the influence of air pollution on psychotic episode and exacerbation. Another strength of our analyzes is the fact that they were adjusted for daily temperature, daily relative humidity and daily sunshine (with for instance a negative association between the daily temperature and the number of admissions for unipolar depressive disorders, which is consistent with literature (Wortzel et al., 2019)). The fact that annual correlation analyzes suggest these associations show that PM air pollution could also have a cumulative effect. However, multivariable analyses are necessary concerning this issue (that were not possible in the present study, because of statistical powerful issue, as N = 11 in the present study).
Results concerning unipolar depressive disorders were similar to psychotic disorders, with a significant positive association between number of daily emergency visits and PM10 peak. Consistently, in their systematic review focusing on the psychiatric consequences of exposures of PM, Braithwaite et al. (Braithwaite et al., 2019) found that short-term increases in PM exposures appeared to be globally positively associated with emergency department visits for depression in seven studies. To our knowledge, only one study analyzed hospital admissions according to similar PM thresholds (25 µg/m3 for PM2.5 vs. 20 in the current study, and 50 µg/m3 for PM10 in both studies) and found significant positive associations with both schizophrenia and depression (fraction of hospital admission for depression attributable to exceeding PM2.5 threshold: 12.07 %, 95 % CI [0.10-21.81], PM10: 6.10 %, 95% CI [1.40%-10.32%]) (Qiu et al., 2019). Finally, in our analyses, the negative association between PM2.5 peak and the number of emergency visits for unipolar depressive disorders was inconsistent with the hypothesis of a negative effect of PM peak on depressive disorders. However, this negative association was not confirmed in the multivariable analyses, and could be related to confounding factors, for instance according to the weather, that is known to have a significant impact on the mood disorders (Pierre A. Geoffroy et al., 2017; Pierre Alexis Geoffroy, Bellivier, Scott, & Etain, 2014). Correlation analyses per quarter, that showed differences of the associations between PM2.5 peak and the number of emergency visits for unipolar depressive disorders according to the quarter, are consistent with this hypothesis.
Association between PM peak and bipolar disorders had never been studied and thus could not be compared to precedent studies, but the absence of any significant variations is still surprising, particularly regarding the fact that they share some (environmental or genetic) risk factors and pathophysiology with psychotic disorders (Bellivier et al., 2013; Demjaha, MacCabe, & Murray, 2012; Pignon, Geoffroy, Thomas, Roelandt, & Amad, 2019) and unipolar depressive disorders (Arango et al., 2021).
Several pathophysiological hypotheses have been made to explain the role of PM pollution in the etiology and the severity of psychiatric disorders. The central nervous system (CNS) may be affected by inflammatory and oxidative processes – particularly due to increased levels of inflammatory cytokines (Block & Calderón-Garcidueñas, 2009; Buoli et al., 2018), that have been considered as causal mechanisms of psychiatric disorders (Howes & McCutcheon, 2017; Leboyer et al., 2012, 2016; Müller, 2018). Increased stress cortisol production and hypothalamo-pituitary-adrenal axis (HPA) dysregulation – which are also involved in psychiatric disorders etiology (Cherian, Schatzberg, & Keller, 2019; Zorn et al., 2017) – may also play a role (Li et al., 2017; Miller, Gillette, Kircanski, LeMoult, & Gotlib, 2020). A body of evidence are in favor of white matter, cortical gray matter, basal ganglia and global blood vessels pollution-related damages (de Prado Bert, Mercader, Pujol, Sunyer, & Mortamais, 2018). Relatively, PM may also affect cognitive development (Zhang et al., 2018), or whole psychological well-being (Nakao et al., 2017; Orru, Orru, Maasikmets, Hendrikson, & Ainsaar, 2016). Of note, these alterations may reflect more chronic than acute (e.g., recent PM peak) exposure. Research remains to be done on the specific pathways and neurobiological impact of the PM short and long-term exposures, and its relationships with specific psychiatric disorders, such as mood and psychotic disorders, that have specific pathophysiology. This research could address the issue of the observed differences in the associations between air pollution and different psychiatric disorders.
Strengths and limitations
The major strengths of this study are the long duration of the data, the ecological design, and the adjustment on weather variables. Moreover, contrary to precedent similar studies (Duan et al., 2018; Gao et al., 2017), we did not use a large number of statistical analyses. However, several limitations should be acknowledged. First, individual factors related to mood or psychotic exacerbation, as stressful-life events, socioeconomic status, cannabis consumption, or inflammatory event and physical comorbidity, could not been analyzed. Second, the level of noise, or the level of green space frequentation, are in the same situation (Engemann et al., 2020; Gascon et al., 2018). Moreover, indoor pollution, daily exposure to other air pollutants (as nitrogen dioxide or ozone) automobile traffic, or tobacco use, that may have a large impact on the level of PM exposure, could also not been analyzed. Third, in the analyses, as the data was anonymous, we could not remove the duplicate.