TUESDAY, March 31, 2020 (HealthDay News) -- Smog drives up dementia risk, particularly for older men and women with heart disease, according to a new Swedish study.
For more than a decade, researchers tracked exposure to air pollution and dementia cases among nearly 3,000 Stockholm residents aged 60 and up.
Lead author Dr. Giulia Grande noted that exposure to dirty air has long been linked to an increased risk for lung and heart disease.
"More recently, several research groups have started to focus on the damages of air pollution on the brain -- for example, its impact on cognitive functions in older adults," she said.
The current research builds on that work. Participants were 74 years old on average and nearly two-thirds were women. All were free of dementia when the study began in 2001; they were tracked until 2013.
With strict air pollution rules in place, Stockholm has relatively good ambient air quality, said Grande, of the Aging Research Center at the Karolinska Institute and Stockholm University.
Her team pegged the city's average annual pollution levels at about 2.5 microns of particulate matter or less -- a level considered "low" by international standards.
Still, more than 12% of participants (364) developed dementia over the study period, the findings showed.
"And we found that people continuously exposed to higher levels of air pollution were at increased risk of dementia, as compared with those exposed to lower levels," Grande said. That link was especially strong among participants who had a history of heart failure, ischemic heart disease or stroke.
Almost half of the pollution-related cases of dementia were connected to stroke, Grande said.
The findings dovetail with other research that has linked cardiovascular disease to a more rapid rate of cognitive ("thinking") decline.
But why would air pollution increase dementia risk in the first place?
"Unfortunately, the biological mechanisms through which air pollution affects the brain are not completely understood," Grande said. "But several pathways are possible."
One possibility is that after inhaled pollutants penetrate the brain, they speed up accumulation of plaques that increase dementia risk, she said.
Poor air quality could also have an indirect effect, Grande added. Air pollution is an established risk to heart health and an "important trigger" for heart attacks and stroke. So, it could be the onset of heart disease that paves the way for development of dementia, she said.
That line of thinking made sense to Dr. Jesus Araujo, director of environmental cardiology at the David Geffen School of Medicine at UCLA. He said the kind of vascular damage brought on by heart disease may be an essential pre-requisite for dementia.
Araujo added that extensive evidence indicates that air pollution can trigger inflammation and hardening of the arteries (atherosclerosis), while also throwing the immune system out of whack (oxidative stress).
All of these factors "are important in the development of both cardiovascular disease and dementia," according to Araujo, who was not involved with the new study.
Grande said the findings are one more reason to strengthen existing air-quality laws.
"By 2050, 68% of the world population is expected to live in urban areas, being continuously exposed to air pollution," she pointed out. Her team projects that global dementia numbers will triple by 2050.
"Together with the worldwide aging of the population, this poses global challenges when it comes to preventive strategies for dementia," Grande said. "So establishing and characterizing the relationship between air pollution and dementia has enormous impact."
The report was published online March 30 in JAMA Neurology.
To learn more about the impact of air pollution on health, visit the World Health Organization.
SOURCES: Giulia Grande, M.D., Ph.D. candidate in geriatric epidemiology, Aging Research Center, Karolinska Institute and Stockholm University, Stockholm, Sweden; Jesus Araujo, Ph.D., M.D., assistant professor of medicine, director of environmental cardiology, David Geffen School of Medicine, and associate professor of environmental health sciences, Fielding School of Public Health, University of California, Los Angeles (UCLA); March 30, 2020, JAMA Neurology, online