Childhood exposure to air pollution with PM2.5 and nitrogen dioxide (NO2) linked to self-harm in later life

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A study of over 1.4 million Danes has revealed a link between higher levels of exposure to two common pollutants during childhood and an increased risk of self-harm in later life.

The collaboration between academics at The University of Manchester and Aarhus University in Denmark is published today (16/09/2021) in a special issue on suicide prevention of the journal Preventive Medicine.

The team investigated if long–term residential exposure to particulate matter (PM2.5) with an aerodynamic diameter of less than 2.5µm and nitrogen dioxide (NO2) during childhood was linked to later self-harm risk.

The interlinked Danish national registers provided a uniquely powerful data source for researchers; the study including over 1.4 million people born in Denmark between 1979 and 2006.

The team were able to track each person’s place of residence from their birth to their tenth birthday, as well as the length of time that they resided at each address. They assed subsequent self-harm risk from tenth birthday onwards up to a maximum age of 37.

The information was linked to measurements of the outdoor levels of PM2.5 and NO2 in the local neighborhood over time.

Ambient outdoor air pollution consists of a complex mixture of gasses, particulate matters and inorganic and organic compounds. PM2.5 and NO2 are among the pollutants with the strongest adverse impact on physical health.

The gasses are emitted from a range of sources including car exhausts, power generation, shipping and domestic heating.

Lead author Dr. Pearl Mok, Research Fellow at The University of Manchester said: “This is the first population-based longitudinal study to reveal that long-term exposure to two common pollutants: PM2.5 and NO2, during childhood, is associated with subsequently elevated self-harm risks.

“Our findings add to the growing evidence-base indicating that higher levels of air pollution exposure are linked with poor mental health outcomes.

“Although air pollution is widespread, it is a modifiable risk factor and we therefore hope our study’s findings will inform policymakers who are devising strategies to combat this problem.”

The team controlled for some of the factors known to influence self-harm risk including history of mental illness in cohort members’ parents and socioeconomic position, information that is available from the Danish registers.

And they showed that exposure to 19 μg/m3 or above of PM2.5 on average per day from birth to 10th birthday was associated with a 48% higher risk of subsequent self-harm compared with a mean daily exposure of less than 13 μg/m3.

And for every 5 μg/m3 increase in exposure to PM2.5 on average per day during childhood, they found the risk of subsequent self-harm rose by 42%.

Similarly, exposure to 25 μg/m3 or above of NO2 on average per day from birth to 10th birthday was associated with a 50% higher risk of subsequent self-harm compared with a mean daily exposure of less than 10 μg/m3.

And for every 10 μg/m3 increase in exposure to NO2 on average per day during childhood, the risk of subsequent self-harm rose by 21%.

Professor Roger Webb from The University of Manchester, a co-author on the study said: “A growing body of evidence in recent years has indicated that exposure to air pollution is also associated with adverse mental health outcomes.

“Children living in neighborhoods with higher levels of air pollution have been reported to have increased risks of developing a range of psychiatric disorders including schizophrenia, depression, and attention-deficit hyperactivity disorder (ADHD).

“Though the mechanisms linking air pollution and development and exacerbation of mental disorders or its associated distress have not yet been explained, it has been well-established that it can cause inflammation and oxidative stress.

“This study is the first to provide valuable evidence on the association between childhood exposure to air pollution and subsequently elevated self-harm risk.

“However, further research is needed to investigate to what degree our findings can be generalized to other countries, especially lower- and middle-income countries where levels of ambient air pollution are far higher than they are in Denmark.”

“Exposure to ambient air pollution during childhood and subsequent risk of self-harm: a national cohort study” is published in Preventive Medicine.


Mental health problems are diagnosed in 10–20% of children and adolescents worldwide (Polanczyk et al., 2015). Incidence and typical age of onset varies by diagnosis, but all can have negative impacts on numerous facets of life including daily functioning, social interactions, and educational achievement (e.g. McLeod and Kaiser, 2004, Parker et al., 2015, Van Ameringen et al., 2003).

Research has demonstrated that the majority of adult psychopathologies begin in childhood and adolescence (Kim-Cohen et al., 2003), with the poorest prognosis for those whose problems begin early and persist (Copeland et al., 2015, Kessler et al., 2007). The identification of early risk factors may inform the development of interventions to prevent the emergence of mental health problems in the first two decades of life.

Numerous factors involved in the etiology of child and adolescent mental health have been identified and well-validated, including family psychiatric history (e.g. Rasic et al., 2013, Silberg et al., 2012) and exposure to victimization in childhood (e.g. Arseneault et al., 2011, Jaffee et al., 2004, Norman et al., 2012).

However, the role of the wider environment is less clear. Previous reports have demonstrated that cities have an elevated prevalence of adult psychiatric diagnoses (Peen et al., 2010, Sundquist et al., 2004, Vassos et al., 2016) and child and adolescent mental health problems (Newbury et al., 2018, Newbury et al., 2016, Rudolph et al., 2014), with a number of environmental factors suggested to account for the differences between urban and rural environments, including poor air quality.

Air pollution is a worldwide environmental health issue (Health Effects Institute 2010, World Health Organisation 2013), but is a particular concern in urban environments, especially large cities such as London (UK) where air quality is substantially lower. Despite some improvement in recent years due to increased regulation of vehicle emissions, air pollution levels in London are consistently higher than the limits set by the European Union (EU) and WHO guidelines (Beevers et al., 2016).

Furthermore, research suggests that the risk of adverse outcomes is significantly elevated even at levels below legal limits (Beelen et al., 2013), suggesting that there may be no “safe” level for air pollution exposure (WHO, 2013). Substantial variability in air pollution concentrations exists in large cities, and comprehensive data for London is available via the KCLurban air pollution exposure model at a resolution of 20 m × 20 m and based on measured air pollution from several sources (Kelly et al., 2011). Thus, London is an ideal city in which to explore associations between air pollution exposure and the development of mental health problems.

Research has demonstrated that exposure to high levels of air pollution is a strong risk factor for poor cardiovascular and respiratory outcomes (Kelly and Fussell, 2015, Rückerl et al., 2011), partly resulting from inflammation and oxidative stress in exposed organ systems (Kelly, 2003, Laumbach et al., 2014). Inflammatory processes have been suggested to play a crucial role in the etiology of a wide range of psychiatric diagnoses (Coccaro et al., 2014, Miller and Raison, 2016, Najjar et al., 2013), suggesting a link between air pollution exposure and mental health problems is plausible (Guxens and Sunyer Deu, 2012).

Experimental research in animals (Levesque et al., 2011) and post-mortem observations in humans (Calderón-Garcidueñas et al., 2008) have demonstrated that air pollutants, particularly fine and ultrafine particles, are capable of reaching the brain, potentially by crossing the blood-brain-barrier or translocation along the olfactory nerve. Once in the brain, pollutant particles can modulate vasoregulatory pathways and trigger neuroinflammation (Block and Calderón-Garcidueñas, 2009). Indeed, exposure to inflammatory stimuli may have an even greater impact in childhood and adolescence owing to ongoing brain development (Danese and Baldwin, 2017).

A growing number of reports have demonstrated associations between air pollution and psychopathology. A large study in adults found an association between exposure to PM2.5 (particulate matter less than 2.5 µm in diameter) and anxiety symptom scores (Power et al., 2015), while associations have also been reported for depression (Lim et al., 2012, Szyszkowicz et al., 2009), and suicidality (Bakian et al., 2015, Kim et al., 2010).

However, one of the largest studies to date that explored associations between air pollution exposure and depression in adult general population samples from 4 different European countries reported inconsistent results between samples (Zijlema et al., 2016).

Children may be especially susceptible to neurologic injury from air pollution because their brains are still developing (Rice and Barone, 2000), and they are likely to have less established natural barriers in the lungs to protect against inhaled particles (Brockmeyer and D’Angiulli, 2016). In addition, children have a higher breathing rate to body size ratio, and tend to spend more time outdoors than adults, further increasing their risk of exposure to air pollution (Brockmeyer and D’Angiulli, 2016).

A handful of existing studies have reported associations between air pollution exposure and higher rates of attention-deficit hyperactivity disorder (ADHD; Min and Min, 2017, Newman et al., 2013, Siddique et al., 2010), autism (Becerra et al., 2013), anxious/depressive symptoms (Perera et al., 2012), and behavioral problems (Forns et al., 2016; Yorifuji et al., 2017) in children.

A recent population-based study reported increased rates of dispensed psychiatric medication amongst children and adolescents living in areas with higher air pollution concentrations (Oudin et al., 2016). Whilst indirect measures such as medication use can powerfully maximize sample size in population studies, it is a crude measure which includes a wide range of dispensed medications, for a large spectrum of mental health problems of varying severities, and is not consistent across consecutive years.

These factors may limit the conclusions that can be drawn from such studies, as mental health problems may be under-detected, causing a bias in the results, and the association with specific mental health outcomes cannot be inferred. In order to extend these findings, it is crucial to use more comprehensive and direct measures of psychopathology as well as higher resolution estimates of pollution exposure.

Though some previous studies have used high-quality personal monitoring of air pollutant exposures (e.g. Perera et al., 2013), most existing studies have tended to rely on relatively low resolution, regional-level pollution exposure data. Given the potential for variability in short-range pollutant levels within urban centres, such as London, these measures may not be sufficient to accurately capture personal levels of exposure.

Additionally, longitudinal samples are required to better tease out the temporal relationship between pollution exposure and development of mental health problems, and assess the potential longer-term effects of early and/or cumulative exposure to pollutants. Indeed, results in adult samples suggest that longer durations of exposure to pollutants are associated with more adverse outcomes (e.g. Brunekreef et al., 2009).

The aim of this exploratory study was to investigate potential associations between estimated exposure to ambient air pollution in late childhood and mental health problems assessed concurrently in childhood and prospectively in late adolescence. To date, associations between exposure to air pollution and a range of mental health outcomes have not been explored in a longitudinal UK-based sample of children and adolescents.

We utilized previously modelled high-resolution annualized average concentration estimates based on air pollution measurements from several sources for a major urban center (London, UK), focusing on two air pollutants (PM2.5 and NO2). We undertook a novel integration with a well-established UK population-based birth cohort study, which comprised previously collected comprehensive assessments of major mental health problems at ages 12 and 18.

We then explored whether these air pollutants were associated with concurrent and longitudinal mental health problems, and checked the robustness of associations for other important risk factors, including socioeconomic status, family psychiatric history, childhood victimization, and smoking, as well as controlling for age-12 symptoms in age-18 analyses. We hypothesized that estimated exposure to higher levels of PM2.5 and NO2 would be associated with elevated rates of mental health problems concurrently at age 12 and longitudinally in late adolescence (age 18), and that these associations would hold after adjustment for key confounders.

reference link : https://www.sciencedirect.com/science/article/pii/S016517811830800X?via%3Dihub


More information: Pearl L.H. Mok et al, Exposure to ambient air pollution during childhood and subsequent risk of self-harm: A national cohort study, Preventive Medicine (2021). DOI: 10.1016/j.ypmed.2021.106502

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