Sandra Pérez Rodríguez
Firstly, I find this to be an interesting and timely study, as it broadens our understanding of the potential factors associated with suicidal behaviour. Suicidal behaviour is a multifactorial phenomenon and, as such, cannot be understood solely in terms of individual variables, but must also take into account social and contextual factors. In this regard, it is relevant to explore the potential role of environmental exposures such as pollution, in line with a growing body of literature on the influence of the environment on mental health.
Methodologically, the study is well-designed as a systematic review with a meta-analysis. One of its main limitations, acknowledged by the authors themselves, is the high heterogeneity of the included studies, which combine very different designs, use different methods to measure exposure to pollution, and do not always account in the same way for other factors that may influence the results—such as socio-economic status, which may be related to the environmental characteristics of the place where people live. Added to this is a wide geographical diversity, with a significant proportion of studies originating from Asia. All of this calls for caution when generalising the results to very different social, economic and political contexts.
Another important aspect is that, although the study appropriately distinguishes between suicidal ideation, suicide attempts and death by suicide, it subsequently analyses these outcomes together. These are related but not equivalent phenomena, and this decision limits the ability to determine whether the observed associations are similar for each type of suicidal behaviour.
Among the findings, although the study identifies associations with other pollutants, the most consistent evidence relates to air pollution and, in particular, to prolonged exposure to PM2.5 – extremely small particulate matter that can penetrate deep into the respiratory tract. This association remains when studies with a higher risk of bias are excluded, although it ceases to be significant when the different types of suicidal behaviour are analysed separately—an analysis for which, moreover, fewer studies are available. An association with nitrogen dioxide (NO₂), which is particularly linked to traffic and other combustion processes, is also initially observed, but this does not hold up when studies with a higher risk of bias are excluded.
Overall, the results provide evidence of an association between certain environmental exposures and suicidal behaviour, particularly in the case of prolonged exposure to PM2.5. However, the heterogeneity of the studies and the reduced consistency of some results in the more restrictive analyses call for caution. The findings are suggestive, but do not yet allow pollution to be considered an established risk factor or a causal relationship to be established. The study opens up an important line of inquiry that should be further explored to determine under what conditions, to what extent and for what types of suicidal behaviour these associations occur. If their relevance is confirmed, incorporating environmental factors into prevention could open up new avenues for action through public policy.