Study links air pollution to increased dementia risk

Fine particulate matter, also known as PM2.5, may increase dementia risk in older adults, according to a nearly 20-year study in the United States.

The analysis found fine particles from agriculture and fires are most strongly associated with increased risk of developing dementia. Fine particles from traffic and coal combustion for energy production are also risk factors.

The study estimated that nearly 188,000 new cases of dementia were attributable to total PM2.5 exposure in the US every year.

Published in the journal JAMA Internal Medicine on Monday, the study collected data from 27,857 participants who are nationally representative between Jan 1, 1998, and Dec 31, 2016. The participants were older than 50, without dementia at the beginning of the study.

A United Nations study last year found that almost the entire global population, or 99 percent, are breathing air that exceeds air quality limits set by the World Health Organization.

Particulate matter, especially PM2.5, is capable of penetrating deep into the lungs and entering the bloodstream, causing cardiovascular, cerebrovascular and respiratory illnesses, the WHO said.

Analysis of data showed that 4,105 participants, or 15 percent, developed dementia in the follow-up, and higher concentrations of total PM2.5 were associated with greater rates of incident dementia.

PM2.5 refers to fine, inhalable air pollutant particles with a diameter of 2.5 micrometers or smaller. In comparison, the average human hair is about 70 micrometers in diameter, the US Environmental Protection Agency said.

The monitored PM2.5 comes from various sources, including agriculture, traffic, energy production and other industry sectors.

According to the study, ammonia, a component widely used in pesticides, accounts for 30 percent of the formation of PM2.5 in the US. The common application of neurotoxic pesticides in agriculture could plausibly explain observed associations between PM2.5 from agriculture and dementia.

"At first, when agriculture and wildfires were the two that popped out, Boya and I were both surprised," Sara Dubowsky Adar, associate chair of epidemiology from the School of Public Health at the University of Michigan, who worked on the study with a team including research fellow Boya Zhang, told CNN.

"In hindsight, it really makes a lot of sense, mostly because... we're looking at impacts on the brain, and agriculture ... is using a lot of pesticides."

The study found that wildfires have become an increasing source of air pollution in the US. Wildfires release components that are likely to be highly toxic because they incinerate natural and synthetic materials in an uncontrolled manner.

More destructive

While individual wildfires may be short-lived, they have become more frequent and destructive in past decades, the study said. Long-range smoke from wildfires frequently impacts the same downwind locations, resulting in wildfire smoke becoming a more long-term presence.

Each year, many US cities experience more than 30 days affected by smoke. Overall, wildfires have been estimated to contribute up to 25 percent of total PM2.5 across the US, and up to 50 percent in some western regions where wildfires are more frequently observed.

Rural participants may breathe more PM2.5 emitted from agriculture and wildfires, the study said, and this may partially explain why there is persistently higher dementia rates in rural and suburban areas than in urban cities across age and sex groups.

The study found higher concentrations of PM2.5 in the Midwest and lower concentrations in the West. Fine particulate matter from wildfires and windblown dust was mainly concentrated in the West and Southwest.

Poor and minority people are more exposed to the risk, the study said. PM2.5 concentrations were higher for non-Hispanic black, those who were older, those with less formal education and less wealth, except for PM2.5 from wildfires and windblown dust.