More Evidence Links Air Pollution to Dementia

October 01, 2018

A new study has found a positive association between residential levels of air pollution and a diagnosis of dementia.

"We found that older patients across London (UK) who were living in areas with higher air pollution were more likely to be diagnosed with dementia in subsequent years," lead author Iain Carey, MD, St. George's, University of London, told Medscape Medical News.

He noted that most of the epidemiological evidence linking long-term concentrations of air pollution to adverse health effects has mainly focused on cardiovascular disease.

However, possible associations between traffic pollution and dementia have been shown in a few recent population studies, notably a large Canadian study published in The Lancet. The current findings are the first evidence from the United Kingdom.

Although acknowledging that causation cannot be established by these observational studies, Carey suggested, "these findings are another reason to minimize exposure to air pollution."

Senior study author Frank Kelly, MD, King's College London, suggested exposure could be reduced by avoiding travel during rush hour.

"Indoors, you can minimize pollution by not burning candles, open fires, have good ventilation/extraction when cooking and cleaning," he added. "Face masks do not usually work unless they are an extremely good fit to the face and have good filters in place — the most expensive."

The study was published online on September 18 in BMJ Open.

For the retrospective cohort study, researchers used data from the Clinical Practice Research Datalink (CPRD), a large, validated primary care database that has been collecting anonymous patient data from participating UK general practices since 1987. 

The current study involved 130,978 adults aged 50 to 79 years in 2005 with no recorded history of dementia or care home residence from 75 general practices in Greater London with clinical data available up to 2013.

During 2004, the average annual concentrations of nitrogen dioxide (NO2), particulate matter with a median aerodynamic diameter ≤ 2.5 µm (PM2.5), and ozone (O3) were estimated from dispersion models, and traffic intensity, distance from major road, and night-time noise levels were estimated at the postcode level.

All pollution measures were anonymously linked to clinical data using residential postcode. Hazard ratios (HRs) from Cox models were adjusted for age, sex, ethnicity, smoking, and body mass index, with further adjustments explored for area deprivation and comorbidity.  

Results showed that 2181 people (1.7%) received an incident diagnosis of dementia (39% mentioned Alzheimer's disease, 29% vascular dementia).

There was a positive exposure–response relationship between dementia and all measures of air pollution except O3, which was not readily explained by further adjustment.

Adults living in areas with the highest fifth of NO2 concentration (> 41.5 µg/m3) versus the lowest fifth (< 31.9 µg/m3) were at a higher risk of dementia (HR, 1.40; 95% CI, 1.12 - 1.74).

Increases in dementia risk were also observed with PM2.5, Associations were more consistent for Alzheimer's disease than vascular dementia.

"Since this is an observational study, it only tells us there may be a possible link between air pollution and dementia. There may be many factors involved, only some of which we were able to account for," Carey commented.

"We also have to consider our limitations, such as uncertainty around personal exposure, where we assume that traffic pollution levels at somebody's address can adequately represent their long-term exposure," he added. "There are also valid concerns about the under-diagnosing of dementia on electronic patient records."

Accounts for 7% of Dementia Cases?

Despite these limitations, the researchers attempted to quantify the risk.

"We calculated how much of the dementia was attributable to pollution (NO2), by assuming what would happen if all patients in the study had instead been estimated to have been exposed at the levels associated with the bottom 20%," Carey explained. "The difference between this theoretical scenario and what we observed gave us an attributable risk of 7%."

"Of course, this estimate only applies to our study and is not easily extrapolated elsewhere where exposure levels differ,” he cautioned. “However, in the large Canadian study by Chen and colleagues, they estimated a similar 6% for the combined effects of NO2 and PM2.5."

Asked to comment for Medscape Medical News, an author of that Canadian study, Ray Copes, MD, Public Health Ontario, Toronto, Canada, called these new findings, "a significant addition to the evidence implicating exposure to traffic-related air pollution as a risk factor for dementia." 

He said strengths of the study included that the authors were able to disentangle the effects of noise from air pollution and the use of individual level data on smoking status and body mass index, "things we were unable to do with our study."

The finding that only NO2 and PM2.5 remained significant in multi-pollutant models including noise and other potential confounders is not surprising "but most worthwhile to see confirmed," he said.

"Unfortunately, as is the case with other studies, specific data on the smallest particles included in PM2.5, the ultrafines, were not available," he noted. "It is possible that an even stronger link to dementia might be found with ultrafine particles, which are a component of fresh diesel exhaust."

In the BMJ Open article, Carey and colleagues conclude: "With the future global burden of dementia likely to be substantial, further epidemiological work is urgently needed to confirm and understand better recent findings linking air pollution to dementia."

"Our results suggest both regional and urban background pollutants may be as important as near-traffic pollutants. Future large-scale studies will need to rely on improved recording and linkage of dementia diagnoses across electronic systems, particularly Alzheimer's disease, where multiple sources can improve diagnostic accuracy," they write.

"Since exposure is lifelong, and most cases are diagnosed in later life, historical data are also ideally required to better estimate cumulative exposure over preceding decades," they conclude.

The study was supported by the UK Natural Environment Research Council, Medical Research Council, Economic and Social Research Council, Department for Environment, Food and Rural Affairs, and Department of Health. The research was also partly funded by the National Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Health Impact of Environmental Hazards at King's College London in partnership with Public Health England and Imperial College London. The authors have reported no financial disclosures.

BMJ Open. Published online September 18, 2018. Full text

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