Short-term air pollution exposure is associated with lower severity and mixed features of manic episodes in hospitalized bipolar patients: a cross-sectional study in Milan, Italy

Bipolar Disorder (BD) alternates depressive, manic or hypomanic phases. A manic episode (ME) is the main psychopathological condition of BD and it often requires hospitalization. Air pollution is thought to play a role in onset and exacerbation of several psychiatric disorders. We aimed to verify the association between exposure to particulate matter ≤10mum (PM10) and ME severity, assessed through the Young Mania Rating Scale (YMRS). We evaluated clinical records regarding 414 hospital admissions of 186 patients residing in Milan (Italy), hospitalized for ME in the Psychiatry Unit of the Policlinico Hospital from 2007 to 2019. Patients were assigned mean daily PM10 and apparent temperature levels of the Milan municipality. As exposure windows, we considered single days preceding hospitalization (lag0 to 7) and their average estimates (lag0-1 to 0-7). We applied mixed effect models, adjusted for relevant confounders. Short-term PM10 exposure was associated with a reduction in YMRS, both when considering daily lags [beta: -0.43 (95% Confidence Interval: -0.83; -0.03) at lag0] and their average [-0.47 (-0.90; -0.04) at lag0-1]. YMRS was higher in psychotic patients (24.8) and lower in ME with mixed components (15.5) if compared to episodes characterized by neither mixed nor psychotic features (17.4, p<0.001). While PM10 did not influence the risk of psychotic symptoms at admission, it was associated with a higher risk of ME with mixed features, with Odds Ratios ranging from 2.43 (1.02; 5.76) at lag0 to 3.60 (1.22; 10.7) at lag0-2. Our findings show that increasing levels of PM10 move the ME towards the depressive pole of the BD spectrum and augment the probability of hospitalization for ME with mixed components. These results have important clinical implications, as mixed features worsen the course of ME and make the management of bipolar patients challenging.