In this July 27, 2018, file photo, the Dave Johnson coal-fired power plant is silhouetted against the morning sun in Glenrock, Wyoming. (AP)

Polluted air could be linked to an increased risk of dementia, and stricter air quality measures are called for to prevent conditions such as Alzheimer's, which have afflicted millions of Americans, according to a report by Bloomberg citing Harvard research.

Dementia was numerously associated with high levels of fine particles in the air, according to researchers at the Harvard Chan School of Public Health who analyzed 14 earlier studies. The association between particle levels and dementia remained even when they were below the US Environmental Protection Agency standard of 12 micrograms per cubic meter of air.

There is no known cure for dementia, which affects 57 million people globally, including the 6 million people who live with Alzheimer's disease in the US alone. According to Marc Weisskopf, a professor of environmental epidemiology and physiology at Harvard who contributed to the study that was published on Wednesday in the BMJ medical journal, even a decrease in annual levels of just 2 micrograms per cubic meter should result in lower dementia rates.

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"As far as we can tell, the lower you can go, the lower your risk is," he said in an interview, adding that while individuals have little control over their exposure to such pollutants, regulators have more to say.

The EPA proposed changing its annual fine particulate matter standards to between 9 to 10 micrograms per cubic meter of air in January, down from the current 12 micrograms. The requirements are less strict in other nations, such as the UK. According to Berkeley Earth scientists, smoking one cigarette per day is approximately equivalent to a level of 22 micrograms per cubic meter of air.

PM2.5, or fine particulate matter, is made up of particles that are about 30% the diameter of a human strand. According to the EPA, their tiny size enables them to enter the blood and settle deeply in the lungs. Exposure to PM2.5 has been associated with a number of illnesses, including lung cancer, type II diabetes, and cardiovascular disease, as well as premature death.

The average annual PM2.5 amount recommended by the World Health Organization should be less than 5 micrograms, but almost the entire world's population breathes air that is above those limits. Even though its estimated impact was smaller than for factors like smoking, Weisskopf said particulate matter is a concerning risk factor for dementia due to the broad scope of pollution exposure.

Other pollutants like nitrogen dioxide and nitrogen oxide may also increase the chance of developing dementia, but these links may not be as strong because there are fewer studies to support them. “Everybody has to breathe, so everybody is exposed to this,” Weisskopf said. “The population-level effect could actually be quite large because the number of people exposed is massive."