Air quality and mental health: evidence, challenges and future directions

Abstracts from:- BJPsych Open (2023) 9, e120, 1–12. doi: 10.1192/bjo.2023.507

Kamaldeep Bhui, Joanne B. Newbury, Rachel M. Latham, Marcella Ucci, Zaheer A. Nasir, Briony Turner, Catherine O’Leary, Helen L. Fisher, Emma Marczylo, Philippa Douglas, Stephen Stansfeld, Simon K. Jackson, Sean Tyrrel, Andrey Rzhetsky, Rob Kinnersley, Prashant Kumar, Caroline Duchaine and Frederic Coulon

Aim, scope and methodological approach

The purpose of this rapid narrative review is to gather expert opinions and summarise the existing body of knowledge on air quality and the long-term effects on mental health, highlight methodological challenges and knowledge gaps and identify future research directions. The perspective we take is broad, interdisciplinary and adopts a ‘life-course’ approach, considering psychiatric, cognitive and neurodevelopmental pathways and a wide spectrum of both indoor and outdoor air pollutants, including bioaerosols, heavy metal ions, inorganic particulate matter (PM) and gaseous pollutants.

Existing reviews have mostly focused on associations between air pollution and one type of mental health problem, using multiple study designs. For example, an excellent recent systematic review shows convincing evidence of associations between depression and PM2.5.Reference Borroni, Pesatori, Bollati, Buoli and Carugno1 This included five cohort studies and mostly cross-sectional and time-series studies from high- and low-income countries; the authors report significant heterogeneity and potential selection biases, but find convincing evidence of links between particulate matter and depression. There is much less research on psychoses, and specific conditions such as schizophrenia or personality disorders. One review argues cogently that exposure to xenobiotic heavy metals (such as lead and cadmium), particulate matter and nitrogen and sulphur oxides, organic solvents and other constituents of environmental pollution could be component causes of neurodevelopmental disorders such as schizophrenia.Reference Attademo, Bernardini, Garinella and Compton2

The health burden of air pollution

The World Health Organization (WHO) has ranked air pollution as one of the major environmental health risks, and the single biggest environmental threat to human health.7 Worldwide, it is estimated that 4.2 million and 3.8 million premature deaths were attributable to outdoor and indoor air pollution, respectively.7 There is more evidence of the adverse health effects of particulate matter.Reference Heal, Kumar and Harrison8 Particulate matter has diverse sources (natural/anthropogenic, indoor/outdoor), formation processes, composition (organic/inorganic) and sizes (ultrafine: PM0.1, particles that are <0.1?μm in diameter; fine: PM2.5, particles that are <2.5?μm in diameter; coarse: particles that are >PM2.5 and <PM10 in diameter).

The WHO guidelines implicate particulate matter with aerodynamic diameters of ≤2.5?μm (PM2.5) and ≤10?μm (PM10), ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide in poor air quality. The particle size can influence whether particulate matter can cross the blood–brain barrier and, along with duration of exposure, increase the risk of adverse health effects. Smaller particles are inhaled more deeply into the lung, leading to greater effects on health. The strongest evidence for adverse effects on health is for PM2.5, with an extensive body of evidence linking outdoor PM2.5 exposure to mortality, cardiovascular diseases, pulmonary diseases and cancer.Reference Heal, Kumar and Harrison8 ,Reference Kumar, Morawska, Birmili, Paasonen, Hu and Kulmala9 Therefore, modifying exposure to poor air quality in indoor and outdoor environments could reduce the population-level burden of poor health.

Mental health and air pollution

Alongside effects on cardiovascular and respiratory health, there is emerging evidence that exposure to air pollutants (both indoors and outdoors) may lead to neurocognitive disorders and affect mental health (directly and indirectly) through a range of potential causal pathways (see Figs 1 and 2 ).Reference Borroni, Pesatori, Bollati, Buoli and Carugno1 ,Reference Bakolis, Hammoud, Stewart, Beevers, Dajnak and MacCrimmon31 –Reference de Prado Bert, Mercader, Pujol, Sunyer and Mortamais35 Observational evidence has implicated outdoor air pollutants as risk factors for a variety of mental health problems, including depression, anxiety, personality disorders and schizophrenia.Reference Lim, Kim, Kim, Bae, Park and Hong36 –Reference Bai, Zhang, Zhang, Cheng, Duan and Gao40 In contrast, there is less research on the effects of indoor air quality and exposures to air pollutants on mental health. Yet, some aspects will be common; for example, inadequate housing is more common in urban spaces, where outdoor and indoor air quality is poorer.

Research gaps and challenges

In the early phase of the BioAirNet (https://bioairnet.co.uk/ ) research network, a sandpit event involving multidisciplinary experts and a range of stakeholders was held. This sought to identify key research questions, knowledge gaps and methodological challenges. In combination with the literature we identified, the following priority research questions and knowledge gaps warrant future attention:

1. (a) Could air pollutant exposure and inflammatory mechanisms explain higher rates of mental illnesses (psychoses and affective disorders) in urban areas; variations of incident mental illnesses by age, gender, sexuality, ethnicity and deprivation; and greater risk for chronic health conditions into adulthood, including psychoses, common mental illnesses and comorbid medical conditions?
2. (b) What future environmental designs and practices (outdoor, indoor, buildings and institutions) might prevent and reduce the risks of poor health, especially in specific at-risk populations?
3. (c) How might specific interventions be developed and tested for impact on the mechanisms?
4. (d) How do we evaluate policy interventions and major policy re-designs, such as the introduction of low emission zone restrictions, which are being adopted in many cities?Reference Mudway, Dundas, Wood, Marlin, Jamaludin and Bremner114
5. (e) Urban design evaluations also need methodologies that are feasible, adopting quasi-experimental designs, and collaboration between local government, building designer, epidemiologists/public health professionals, built environment architects and local residents.
6. (f) What constitutes an ‘anti-inflammatory’ environment that benefits young people in their worlds and adults at risk of or already experiencing mental illnesses and other health conditions?
7. (g) What role do social and behavioural factors have for creating or concentrating harmful exposomes in specific places and indoor environments, and mitigating these drivers of poor health?
8. (h) How do structural (socioeconomic, deprivation, poverty, geographical) and behavioural influences interact to promote or militate against a harmful exposome?
9. (i) How is child health and mental health affected, and what is the impact over the life course?
10. (j) How are specific high-risk groups affected: those with early psychoses, chronic depression and multimorbidity, including poor mental health?
11. (k) What are the implications for care environments for children and for those with mental illness?

In addition, specific approaches were identified to better quantify levels of exposure to indoor/outdoor pollution and links with impact on health in different scenarios (see Appendix 1); approaches to understand the mechanisms of harm to human health and well-being (see Appendix 2); and the need to specify more carefully which health conditions and causal models were being investigated (see Appendix 3).

These research gaps are broadly aligned with the six priorities proposed in a recent review of Environmental Science and Mental Health, including over 200 publications and six case studies.Reference Irvine, Roberts, Eastwood, Colley, Currie and Zhang115 In this report, five areas of opportunity were identified, which consider both the research approach and topics warranting further investigation: exploit large-scale data-sets, longitudinal approaches, integrative complex systems research, mixed-methods approach and community of practice.

Future directions and challenges

Air pollution and mental health are both major challenges that the world must grapple with, now and for years to come. This makes their intersection a doubly vital public health priority. This paper outlines evidence on the importance of indoor and outdoor air quality on mental health, research needs, challenges and future directions. There remain methodological challenges that must be overcome to provide insights into critical time points; place-based hot-spots for poor air quality; biological, psychological and social mechanisms; and strategies for prevention and mitigation. The clinical, public health and societal (well-being and economic) effects need to be modelled. Better quality primary research and longitudinal cohorts, especially for young people at critical points of maturation, are needed, alongside well-specified systematic reviews and network analyses. Specifically, areas of focus include evidence of links between pollution, specifically bioaerosols, and mental health, and better exposure measurement. An important linked but distinct subject that we have not addressed is climate change. The pathways between global warming, poor air quality, climate change and poor mental health may be mediated through natural disasters and social disruption, biodiversity loss and ecosystem destruction.Reference Sly62 Furthermore, there is little data on the contrasting effects of climate change on low- versus high-income countries. We know natural disasters and climate change will have more of an effect on low-income countries and poorer populations living in countries with less infrastructure, appropriate building design and protections around health and environmental policy.Reference Rao117 Furthermore, rising global temperatures are associated with more air pollution,Reference Anenberg, Haines, Wang, Nassikas and Kinney118 including stagnation and less ventilation, and greater production of particulate matter (wildfire smoke, airborne soil dust, ozone and PM2.5). These affect long-term medical conditions (heart, lung and kidney disease) via raised body temperature and inflammation. Heat also leads to more anxiety and depressive symptoms, and suicidal behaviours; those with pre-existing mental illnesses are more likely to die in hot spells than those without mental illnesses.Reference Cianconi, Betrò and Janiri119 Engagement of policy stakeholders from diverse sectors is necessary to translate emergent findings into actions. We anticipate this paper and related publications from a number of networks will help bridge knowledge gaps to stimulate a new wave of research, practice and policy actions. This will enable us to gain deeper understanding of the intricate and interconnected relationship among individuals, air pollution exposure and resulting mental health and well-being effects, amid the continuously changing air pollution sources and exposure patterns. Ultimately, the knowledge gained should be used to inform policies concerning air pollution interventions, urban and built environment design, land use planning and behaviour change.