SAN FRANCISCO (CBS SF) — A UC San Francisco study has revealed a link between elevated amyloid plaque levels among older Americans with cognitive impairment and the air pollution levels in their neighborhoods from cars, factories, power plants and wildfires.

In the study, which appears in JAMA Neurology on Monday, researchers found that among those tested the greater the air pollution in their neighborhood, the higher the likelihood of amyloid plaques – a hallmark of Alzheimer’s disease.

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The study adds to a growing body of evidence indicating that pollution joins established dementia risk factors like smoking and diabetes.

The UCSF researchers looked at the PET scans of more than 18,000 seniors whose average age was 75. The participants had dementia or mild cognitive impairment and lived in zip codes dotted throughout the nation.

The researchers found that those in the most polluted areas had a 10 percent increased probability of a PET scan showing amyloid plaques, compared to those in the least polluted areas.

When applied to the U.S. population, with an estimated 5.8 million people over 65 with Alzheimer’s disease, high exposure to microscopic airborne particles may be implicated in tens of thousands of cases.

“This study provides additional evidence to a growing and convergent literature, ranging from animal models to epidemiological studies, that suggests air pollution is a significant risk factor for Alzheimer’s disease and dementia,” said Dr. Gil Rabinovici of the UCSF Memory and Aging Center who helped author the study.

The more than 18,000 participants were recruited for the IDEAS study (Imaging Dementia – Evidence for Amyloid Scanning), which had enrolled Medicare beneficiaries whose mild cognitive impairment or dementia had been diagnosed following comprehensive evaluation.

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Researchers said that not all of the participants were later found to have positive PET scans – 40 percent showed no evidence of plaques on the scan, suggesting non-Alzheimer’s diagnoses like frontotemporal or vascular dementias, which are not associated with the telltale amyloid plaques.

Air pollution in the neighborhood of each participant was estimated with Environmental Protection Agency data that measured ground-level ozone and PM2.5, atmospheric particulate matter that has a diameter of less than 2.5 micrometers.

Researcher found that the probability of a positive PET scan rose progressively as concentrations of pollutants increased, and predicted a difference of 10 percent probability between the least and most polluted areas.

“Exposure in our daily lives to PM2.5, even at levels that would be considered normal, could contribute to induce a chronic inflammatory response,” said Leonardo Iaccarino, PhD, also of the UCSF Memory and Aging Center and a co-author of the study. “Over time, this could impact brain health in a number of ways, including contributing to an accumulation of amyloid plaques.”

Overall concentrations of PM2.5 would not be considered very high for it to have a significant association with amyloid plaques, amounting to annual averages in San Francisco during the study time, added Rabinovici.

Fires across the West emitted more than a million tons of the particles in 2012, 2015 and 2017, and almost as much in 2018 — the year a blaze in Paradise killed 85 people and burned 14,000 houses, generating a thick plume that blanketed portions of Northern California for weeks.

A confluence of events made the smoke especially bad this year. A rare dry lightning storm triggered the worst outbreak of wildfires in Bay Area history.

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The smoke got so bad that in the San Francisco Bay Area on September 9th, the plume from wildfires turned day into night, casting an eerie orange pall over a city where even before the pandemic facemasks had become common at times to protect against smoke.