Air pollution in childhood linked to schizophrenia

Summary: Children who grew up in areas with heavy air pollution have an increased risk of being diagnosed with schizophrenia.

Source: Aarhus University

Air pollution affects physical health, and research results now conclude that it also affects our psychological health. The study, which combines genetic data from iPSYCH with air pollution data from the Department of Environmental Science, shows that children who are exposed to a high level of air pollution while growing up, have an increased risk of developing schizophrenia.

“The study shows that the higher the level of air pollution, the higher the risk of schizophrenia. For each 10 μg/m3 (concentration of air pollution per cubic meter) increase in the daily average, the risk of schizophrenia increases by approximately twenty percent. Children who are exposed to an average daily level above 25 μg/m3 have an approx. sixty percent greater risk of developing schizophrenia compared to those who are exposed to less than 10 μg/m3,” explains Senior Researcher Henriette Thisted Horsdal, who is behind the study.

To put these figures into perspective, the lifetime risk of developing schizophrenia is approximately two percent, which equates to two out of a hundred people developing schizophrenia during their life. For people exposed to the lowest level of air pollution, the lifetime risk is just under two percent, while the lifetime risk for those exposed to the highest level of air pollution is approx. three percent.

Unknown cause

The results of the study have just been published in the scientific journal JAMA Network Open.

“The risk of developing schizophrenia is also higher if you have a higher genetic liability for the disease. Our data shows that these associations are independent of each other. The association between air pollution and schizophrenia cannot be explained by a higher genetic liability in people who grow up in areas with high levels of air pollution,” says Henriette Thisted Horsdal about the study, which is the first of its kind to combine air pollution and genetics in relation to the risk of developing schizophrenia.

This shows smoke stacks and a child's eyes
The association between air pollution and schizophrenia cannot be explained by a higher genetic liability in people who grow up in areas with high levels of air pollution. Image is in the public domain.

The study included 23,355 people in total, and of these, 3,531 developed schizophrenia. Though the results demonstrate an increased risk of schizophrenia when the level of air pollution during childhood increases, the researchers cannot comment on the cause. Instead, they emphasize that further studies are needed before they can identify the cause of this association.

Background for the results:

The study is a register-based study

Partners iPSYCH, BERTHA and Nordic Welfare

The scientific article: Association of Childhood Exposure to Nitrogen Dioxide and Polygenic Risk Score for Schizophrenia With the Risk of Developing Schizophrenia

Funding: Funded by The Lundbeck Foundation

About this neuroscience research article

Source:
Aarhus University
Media Contacts:
Henriette Thisted Horsdal – Aarhus University
Image Source:
The image is in the public domain.

Original Research: Open access
“Association of Childhood Exposure to Nitrogen Dioxide and Polygenic Risk Score for Schizophrenia With the Risk of Developing Schizophrenia”. Henriette Thisted Horsdal et al.
JAMA Network Open doi:10.1001/jamanetworkopen.2019.14401.

Abstract

Association of Childhood Exposure to Nitrogen Dioxide and Polygenic Risk Score for Schizophrenia With the Risk of Developing Schizophrenia

Importance
Schizophrenia is a highly heritable psychiatric disorder, and recent studies have suggested that exposure to nitrogen dioxide (NO2) during childhood is associated with an elevated risk of subsequently developing schizophrenia. However, it is not known whether the increased risk associated with NO2 exposure is owing to a greater genetic liability among those exposed to highest NO2 levels.

Objective
To examine the associations between childhood NO2 exposure and genetic liability for schizophrenia (as measured by a polygenic risk score), and risk of developing schizophrenia.

Design, Setting, and Participants
Population-based cohort study including individuals with schizophrenia (International Statistical Classification of Diseases and Related Health Problems, Tenth Revision code F20) and a randomly selected subcohort. Using national registry data, all individuals born in Denmark between May 1, 1981, and December 31, 2002, were followed up from their 10th birthday until the first occurrence of schizophrenia, emigration, death, or December 31, 2012, whichever came first. Statistical analyses were conducted between October 24, 2018, and June 17, 2019.

Exposures
Individual exposure to NO2 during childhood estimated as mean daily exposure to NO2 at residential addresses from birth to the 10th birthday. Polygenic risk scores were calculated as the weighted sum of risk alleles at selected single-nucleotide polymorphisms based on genetic material obtained from dried blood spot samples from the Danish Newborn Screening Biobank and on the Psychiatric Genomics Consortium genome-wide association study summary statistics file.

Main Outcomes and Measures
The main outcome was schizophrenia. Weighted Cox proportional hazards regression models were fitted to estimate adjusted hazard ratios (AHRs) for schizophrenia with 95% CIs according to the exposures.

Results
Of a total of 23 355 individuals, 11 976 (51.3%) were male and all had Danish-born parents. During the period of the study, 3531 were diagnosed with schizophrenia. Higher polygenic risk scores were correlated with higher childhood NO2 exposure (ρ = 0.0782; 95% CI, 0.065-0.091; P < .001). A 10-μg/m3 increase in childhood daily NO2 exposure (AHR, 1.23; 95% CI, 1.15-1.32) and a 1-SD increase in polygenic risk score (AHR, 1.29; 95% CI, 1.23-1.35) were independently associated with increased schizophrenia risk.


Conclusions and Relevance

These findings suggest that the apparent association between NO2 exposure and schizophrenia is only slightly confounded by a higher polygenic risk score for schizophrenia among individuals living in areas with greater NO2. The findings demonstrate the utility of including polygenic risk scores in epidemiologic studies.

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