Higher levels of air pollution were associated with an increased use of mental health services by patients with dementia, according to a retrospective cohort study of more than five thousand people.
Researchers from King's College London and Imperial College London said that reducing pollutant exposure might reduce the use of mental health services in this group, so freeing up resources in "already considerably stretched psychiatric services".
Air Pollution and Brain Health
Their study, published in BMJ Mental Health, built on previous research showing that ambient air pollution is a potentially modifiable population level risk factor for both the risk of developing dementia and the acceleration of cognitive decline.
A recent briefing document by InSPIRE — a UK policy and research consortium for mitigating the impact of air pollution on brain health — highlighted that exposure to high levels of air pollutants in early life increased the risk of later dementia and related disorders.
In the new study, the team noted that long-term exposure to particulate matter had been associated with an increased risk of dementia-related hospital admissions both generally and for Alzheimer's disease-specific emergency admissions. However, in the UK, dementia care is usually managed outside general hospital settings, and little is known about the role of air pollution in use of community mental health services.
The researchers used a clinical record interactive search system to identify all patients aged ≥65 years who had a first face-to-face contact with secondary mental healthcare providers in a large South London NHS Trust that resulted in a clinical diagnosis of dementia, including Alzheimer's disease, vascular dementia, dementia associated with other classified diseases, or unspecified dementia, at or following their contact with trust services.
From records collected between 1 January 2008 and 31 December 2012 they identified 5024 people living in four high-traffic South London boroughs served by the trust who met the study criteria. They then modelled air pollution levels for the four boroughs over 3-month periods in which the first face-to-face contacts took place.
Air pollution was expressed as mean concentrations of nitrogen dioxide (NO2 - representing a good proxy of diesel exhaust emissions, according to the researchers), particulate matter with an aerodynamic diameter <2.5 µm (PM2.5), and particulate matter with an aerodynamic diameter <10 µm (PM10).
Patients were followed for up to 9 years to assess their event counts, estimated as the number of distinct face-to-face attended appointments including outpatient visits and appointments with specialist teams. In addition, cognitive function was measured from recorded Mini Mental State Examination (MMSE) scores in the patients' records, and health and social functioning was measured using the Health of the Nation Outcomes Scale (HoNOS65+).
Mental Health Contacts Increased in Dose-Dependent Manner
In the first year, "increased exposure to all air pollutants was associated with an increase in the use of community mental health teams in a dose-response manner", the researchers reported. These associations were strongest when comparing the highest with the lowest air pollution quartiles, for example giving an adjusted incidence rate ratio of 1.27 (95% CI 1.11 to 1.45, p<0.001) for estimated NO2 levels, which were also linked with poor functional status — including the capacity for routine activities of daily living — but not cognitive function.
In addition, dose-response patterns between PM2.5 and number of events remained at 5 and 9 years, with associations strongest for patients with vascular dementia.
As it was an observational study, "no firm conclusions can be drawn about cause and effect", the researchers acknowledged. Nevertheless they speculated that cutting levels of nitrogen dioxide and particulate matter might reduce psychiatric demand in urban areas.
Cutting Pollution Could Improve Function in People With Dementia
They estimated that a fall in the annual PM2.5 exposure in London from the 2019 level of 11.6 µg/m3 to 5 µg/m3 could reduce the number of community mental health service contacts by people with dementia by 13% a year. Similarly, reducing annual levels of NO2 from 39 µg/m3 in 2019 to 10 µg/m3 could reduce annual mental health service contacts by these patients by 38%.
Public health interventions, such as ultra-low emission zones, "could potentially improve functioning and disease trajectories for people with dementia", they said. Plans to extend London's Ultra-Low Emission Zone (ULEZ) came a step closer last month after the High Court dismissed a legal challenge to its extension across almost all of the capital.
Commenting on the study , Dr Susan Mitchell, head of policy at Alzheimer's Research UK, said that the proposed changes could be "a decade too late" and that the Government couldn't "continue to turn a blind eye to air pollution".
While agreeing that there was a need for "urgent action" to bring pollution levels down, she said there was also "a pressing need" to discover more about how air pollution affects dementia risk.