L’existence d’un lien entre urbanicité et trouble psychotique est une donnée maintenant clairement établie avec l’existence d’une relation dose–effet : plus le taux d’urbanicité est élevé, plus le risque de psychose est important. Pour expliquer cette association, plusieurs hypothèses ont été proposées. La plupart des études ont évalué le rôle de facteurs psycho-sociaux comme le niveau socio-économique, le surpeuplement des logements, les inégalités sociales, le capital social pour expliquer ce lien. L’ensemble de ces facteurs ont pour dénominateur commun une exposition répétée au stress, responsable d’une neuro-toxicité susceptible de faire le lit des décompensations psychotiques. Plus récemment, des études suggèrent un rôle de la pollution de l’air. L’exposition aux polluants atmosphériques augmente la neuro-inflammation, le stress oxydatif, l’activation de la microglie, causant des dommages cérébraux et favorisant secondairement le développement des troubles psychotiques. Ces facteurs restent pour l’heure insuffisamment explorés et nécessitent des investigations plus poussées, ce d’autant que contrairement à beaucoup d’autres facteurs étiologiques (environnementaux ou génétiques), les stress psycho-sociaux et la pollution atmosphérique présentent l’avantage d’être accessibles à des interventions et des mesures de prévention susceptibles de réduire l’incidence de la maladie.

A growing body of evidence suggests that urbanicity contributes to the development of psychosis, with elevated risks being associated with growing up or living in environments with a higher level of urbanization. This article aims to provide a review of the scientific literature studying the link between urbanicity and psychosis in order to identify the concepts used so far to clarify this issue and the hypotheses that have emerged from them. Taken together, the findings reviewed in this paper confirm that urbanicity is associated with an increased risk of schizophrenia and other non-affective psychosis. However, the mechanisms underlying this phenomenon remain unclear. The impact of urbanicity may result from a wide range of factors from air pollution to stressful impact of social life leading to “urban stress”. Indeed, the main underlying putative causes explored that may underpin this association are the social environment and air pollution. The role of psychosocial stressors such as neighborhood fragmentation, low social cohesion and social capital, deprivation, social adversities have been singled out by several studies. These different factors could lead to several pathways, possibly unified by dopaminergic hyperactivity in mesocorticolimbic system. Urbanicity could also potentially be related, at least in part, to environmental pollution which has been largely neglected compared to some of the other non-genetic environmental risk factors. The findings from our review show that recent studies link environmental pollution and especially air pollution exposure, to an increased risk of schizophrenia. There are feasible biological mechanisms involving neuro-inflammation processes by which some of the specific pollutants could affect brain development in a way that could increase risk for schizophrenia. Further research – from the cellular to epidemiological levels – is clearly needed. To fully understand the causal role of the environmental risk factors, the integration of genomics with large-scale epidemiological studies is warranted. Moreover, new studies on urbanicity should therefore be more interdisciplinary, bridging knowledge from different disciplines (psychiatry, epidemiology, human geography, urbanism, etc.) in order to enrich research methods. The fact that more than 50% of the World population lives in cities, a proportion likely to increase in the future, makes the resolution of this question an urgent matter. If causation is proven, enhancements of policy intended to reduce human exposure to environmental stressors and pollution could reduce the burden of schizophrenia and possibly other mental illnesses.