Air pollution and urbanicity: common risk factors for dementia and schizophrenia?

Environmental pollution is a global problem and the subject of increasing worldwide public health concern.1WHOWHO global urban ambient air pollution database (update 2016). 2016http://www.who.int/phe/health_topics/outdoorair/databases/cities/en/Google Scholar In particular, air pollution is regarded as the largest single environmental risk to health. More than 80% of people living in urban areas that monitor air pollution are exposed to air quality levels that exceed the WHO limits, and all regions of the world are affected. Declines in urban air quality increase the risk of cerebrovascular accidents, coronary artery disease, lung carcinoma, and chronic and acute respiratory diseases (eg, asthma, obstructive lung disease, and acute lower respiratory infections). Associations between air pollution and central nervous system diseases have also been shown,2Block ML Elder A Auten RL et al.The outdoor air pollution and brain health workshop.Neurotoxicology. 2012; 33: 972-984Crossref PubMed Scopus (373) Google Scholar but we believe that relatively little attention has been given to the possible role of pollution as a risk factor for neurodevelopmental (eg, schizophrenia) and neurodegenerative (eg, dementia) diseases. Therefore, we read with great interest the articles by Chen and colleagues3Chen H Kwong JC Copes R et al.Living near major roads and the incidence of dementia, Parkinson's disease, and multiple sclerosis: a population-based cohort study.Lancet. 2017; 389: 718-726Summary Full Text Full Text PDF PubMed Scopus (498) Google Scholar in The Lancet and by Gao and colleagues in European Psychiatry.4Gao Q Xu Q Guo X Fan H Zhu H Particulate matter air pollution associated with hospital admissions for mental disorders: a time-series study in Beijing, China.Eur Psychiatry. 2017; (published online April 7.)http://doi.org/10.1016/j.eurpsy.2017.02.492Google Scholar The former was a large population-based cohort study conducted in Ontario, Canada, whose findings showed that urban residents, especially individuals living close to heavy traffic roads had an increased incidence of dementia. Notably, long-term exposure to fine particulate matter (≤2·5 μm in diameter; PM2·5) and nitrogen dioxide was positively associated with dementia. The latter study was a time-series analysis done in Beijing, China, investigating the acute effects of particulate matter on hospital admissions for mental disorders. The authors reported that elevated levels of PM10 (particulate matter ≤10 μm in diameter), PM2·5, and PMC (2·5–10 μm in diameter), were significantly associated with a small increase in hospital admissions for schizophrenia. Schizophrenia evidently has a multifactorial (genetic and environmental) aetiology, and is a complex, and clinically heterogeneous neurodevelopmental syndrome, associated with variable functional impairments in social, emotional, perceptual, and cognitive domains.5Owen MJ Sawa A Mortensen PB Schizophrenia.Lancet. 2016; 388: 86-97Summary Full Text Full Text PDF PubMed Scopus (957) Google Scholar A biological association between schizophrenia and dementia has been debated for a long time, since Emil Kraepelin named schizophrenia as dementia praecox (premature dementia). The question of whether cognitive impairment is a core feature of schizophrenia is still a matter of debate. Evidence of deficits in multiple cognitive domains such as memory, attention, and visuospatial orientation, is noted in individuals with schizophrenia.6Ribe AR Laursen TM Charles M et al.Long-term risk of dementia in persons with schizophrenia. A Danish population-based cohort study.JAMA Psychiatry. 2015; 72: 1095-1101Crossref PubMed Scopus (77) Google Scholar, 7Green MF Cognitive impairment and functional outcome in schizophrenia and bipolar disorder.J Clin Psychiatry. 2006; 67: 3-8PubMed Google Scholar Patients with schizophrenia show different degrees of cognitive impairment, and have poorer performance than healthy counterparts. Not infrequently, impairments in cognition can manifest years before the first psychotic episode, and usually cognitive dysfunctions are chronic.5Owen MJ Sawa A Mortensen PB Schizophrenia.Lancet. 2016; 388: 86-97Summary Full Text Full Text PDF PubMed Scopus (957) Google Scholar More recently, it has been proposed that schizophrenia could be a syndrome of accelerated ageing, suggesting that physiological abnormalities associated with schizophrenia could contribute to the increased mortality of individuals affected by this disorder.8Kirkpatrick B Messias E Harvey PD Fernandez-Egea E Bowie CR Is schizophrenia a syndrome of accelerated aging?.Schizophr Bull. 2008; 34: 1024-1032Crossref PubMed Scopus (212) Google Scholar It has been furthermore shown that patients with schizophrenia have a substantially increased relative risk of dementia that could not be explained by established dementia risk factors, such as cardiovascular disease and diabetes.6Ribe AR Laursen TM Charles M et al.Long-term risk of dementia in persons with schizophrenia. A Danish population-based cohort study.JAMA Psychiatry. 2015; 72: 1095-1101Crossref PubMed Scopus (77) Google Scholar A comprehensive hypothetical model may be explained by the presence of a progressive, neurodegenerative component in addition to a neurodevelopmental component in subjects with schizophrenia.5Owen MJ Sawa A Mortensen PB Schizophrenia.Lancet. 2016; 388: 86-97Summary Full Text Full Text PDF PubMed Scopus (957) Google Scholar The pathophysiological factors of the link between schizophrenia and dementia remain unclear. Urbanicity (being born or raised in cities) has been commonly described as one of the risk factors for schizophrenia and other non-affective psychosis, and pollution exposure (eg, exposure to xenobiotic heavy metals such as lead and cadmium, constituents of air pollution such as particulate matter, and nitrogen and sulphur oxides, organic solvents, and other constituents of environmental pollution) could potentially mediate the association between urbanicity and elevated risk, at least in part.9Attademo L Bernardini F Garinella R Compton MT Environmental pollution and risk of psychotic disorders: a review of the science to date.Schizophr Res. 2017; 181: 55-59Summary Full Text Full Text PDF PubMed Scopus (95) Google Scholar In our opinion, environmental pollution should be considered as a common risk factor for both dementia and schizophrenia. Studies are warranted to test this hypothesis, and if confirmed, the clinical implications could be of great importance for public and mental health. Future research, including large population-based studies, is likely to focus on investigation of the effects of specific components of environmental pollution on increased risk for mental, neurodevelopmental, and neurodegenerative diseases in human beings, with the aim to plan possible prevention strategies. Policy measures, regulatory interventions, and aggressive air quality management programmes are needed to reduce current air pollution levels and the associated burden of disease, so bringing substantial health benefits across a broad spectrum of disease.10Cohen AJ Brauer M Burnett R et al.Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015.Lancet. 2017; (published online April 10.)http://dx.doi.org/10.1016/S0140-6736(17)30505-6Google Scholar We declare no competing interests.

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