Older adults exposed to air pollution long term – even at fairly low levels – have an increased risk of dementia, and cardiovascular disease (CVD) appears to both modify and mediate this association, according to the results of the Swedish National Study on Aging and Care in Kungsholmen (SNAC-K) study.

Giulia Grande, MD, a doctoral student at the Aging Research Center, Karolinska Institutet and Stockholm University, in Solna, Sweden.

Dr. Giulia Grande

Virtually all of the association between air pollution and dementia seemed to occur through the presence or the development of cardiovascular disease, which suggests a need to optimize treatment of concurrent cardiovascular disease and risk-factor control in older adults at higher risk for dementia and living in polluted urban areas, said lead author Giulia Grande, MD, a researcher at the Aging Research Center, Karolinska Institutet and Stockholm University, in Solna, Sweden.

In the longitudinal, population-based cohort study, investigators studied 2,927 randomly selected residents in a district of Stockholm who were aged 60 years or older (mean, 74.1 years), lived at home or in institutions, and were free of dementia at baseline (March 2001 through August 2004).

The investigators assessed the participants’ exposure to two major air pollutants – particulate matter ≤2.5 mcm and nitrogen oxide – yearly starting in 1990, from outdoor levels at their residential addresses. Both pollutants are generated by road traffic, among other sources.

Results reported in JAMA Neurology showed that, with a mean follow-up of 6.01 years, 12.4% of the older adults received a dementia diagnosis.

Dementia risk increased with the level of air pollutants at their residential address in the past, with strongest associations seen for exposure in the preceding 5 years: The hazard ratio (HR) for dementia was 1.54 for an interquartile range difference of 0.88 mcg/m3 in particulate matter ≤2.5 mcm and 1.14 for an interquartile range difference of 8.35 mcg/m3 in nitrogen oxide during that time period.

Of note, the study cohort lived in an area having “comparatively good ambient air quality” in which restrictions on air pollution have increased in recent decades, Dr. Grande and coinvestigators noted. “Interestingly, the higher limit reported herein is not only below the current European limit for fine particulate matter but also below the US standard. In other words, we were able to establish harmful effects at levels below current standards,” they wrote.

In analyses of effect modification, the elevation of risk related to particulate matter ≤2.5 mcm exposure and nitrogen oxide exposure was significantly greater among older adults who had heart failure (HRs, 1.93 and 1.43, respectively). Risk was marginally greater among those with ischemic heart disease (HRs, 1.67 and 1.36, respectively).

Analyses of potential mediators showed that preceding stroke accounted for the largest share of all dementia cases related to particulate matter ≤2.5 mcm exposure, at 49.4%.

The stronger association for exposure in the past 5 years is noteworthy for the big picture, they added. “From a policy point of view, this result is encouraging because it might imply that reducing air pollutant levels today could yield better outcomes already in the shorter term, reinforcing the need for appropriately set air quality standards,” they said.

Dr. Grande disclosed no relevant conflicts of interest. The study was funded by the Swedish National Study on Aging and Care in Kungsholmen (SNAC-K); the Swedish Ministry of Health and Social Affairs; the participating County Councils and Municipalities; the Swedish Research Council; funding for doctoral education from the Karolinska Institutet; and the Swedish Research Council for Health, Working Life and Welfare.

SOURCE: Grande G et al. JAMA Neurol. 2020. doi:10.1001/jamaneurol.2019.4914.