Study links air pollution to mental illness

New research from the University of St Andrews has revealed that cumulative exposure to air pollution is linked to a heightened risk of admission to hospital for mental/behavioural as well as physical illness.

A team, led by Dr Mary Abed Al Ahad from the School of Geography and Sustainable Development, reviewed published data from across 16 years on the health effects of long term exposure to ambient air pollution. Previous research has tended to emphasise deaths rather than hospital admissions, and physical, rather than mental, ill health, suggest the researchers.

Dr Abed Al Ahad and her team have concluded that stricter environmental restrictions are needed to curb the impact on secondary care.

The research is published in the open access journal BMJ Open.

In a bid to plug this knowledge gap, the researchers drew on individual level data from the Scottish Longitudinal Study, which represents 5% of the Scottish population and includes demographic information from linked censuses.

In all, 202,237 people aged 17 and above were included in the analysis. Their health and hospital admissions for all causes; cardiovascular, respiratory, or infectious diseases; mental illness/behaviour disorders were tracked from Public Health Scotland data and linked to levels of 4 key pollutants for each of the years between 2002 and 2017 inclusive.

The 4 pollutants from road traffic and industry comprised: nitrogen dioxide (NO₂); sulphur dioxide (SO₂); particulate matter diameter of at least 10 μm (PM₁₀); and small particulate matter of 2.5 μm or less (PM_2.5) per 1 km² in each person’s residential postcode.

Fluctuations in pollutant levels were observed across the study period, with higher levels recorded in 2002–04. Over the entire period 2002–17 average levels of NO₂, SO₂, PM₁₀ and PM_2.5 were 12, 2, just over 11, and just over 7 μg/m³, respectively.

The average annual levels for NO₂, PM₁₀ and PM_2.5 were lower than the 2005 WHO guidelines but the levels of NO₂ and PM_2.5 were higher than the most recent 2021 WHO guidelines.

Average cumulative exposure to air pollution was strongly associated with higher rates of hospital  admissions.

Higher cumulative exposure to NO₂, PM₁₀, and PM_2.5 was associated with a higher incidence of hospital admissions for all causes, and for cardiovascular, respiratory, and infectious diseases before accounting for residential area.

When fully adjusted for cumulative exposure across time, the incidence rate for respiratory disease hospital admissions rose by just over 4% and just over 1%, respectively, for every 1 μg/m³ increase in PM_2.5 and NO₂ pollutants.

SO₂ was mainly associated with hospital admissions for respiratory disease while NO₂ was associated with a higher number of hospital admissions for mental illness/behavioural disorders.

Exposure to ambient air pollution was assessed yearly rather than monthly or daily, so masking seasonal variations, while residential postcode had to serve as a proxy for personal exposure to air pollution.

Nevertheless, the findings echo those of previously published research.

Dr Mary Abed Al Ahad said: “Policies and interventions on air pollution through stricter environmental regulations, long term planning, and the shifting towards renewable energy could eventually help ease the hospital care burden in Scotland in the long term.”

“Policies and interventions targeting air pollution emissions such as zero-emission zones or incentives for renewable energy in transportation and energy production sectors could help ease the hospital-care burden in the long-term both locally and globally.”