Breathing in microscopic air pollutants may raise the risk of developing dementia, according to a new analysis of existing research by public health scientists at Harvard.

It has long been known that inhaling these microscopic particles — also known as PM 2.5, particles less than 2.5 microns in width — can cause serious health issues. (In comparison, the width of a single human hair is 50 to 100 microns.)

But “their relationship to the brain and dementia is a relatively recent thing,” said lead study author Marc Weisskopf, Cecil K. and Philip Drinker Professor of environmental epidemiology and physiology at Harvard T.H. Chan School of Public Health. “Dementia is a massive problem worldwide. If we can reduce exposure to these particles, we can reduce the burden of dementia.”

More than 57 million people globally are living with dementia, a number expected to increase to an estimated 153 million by 2050.

The meta-analysis, published Wednesday in the British Medical Journal (BMJ), examined 16 observational studies and found consistent evidence of a connection between PM 2.5 and dementia, even when an individual’s annual exposure was less than 12 micrograms per cubic meter of air, which is the current Environmental Protection Agency standard, the study said. A grain of rice, in comparison, weighs about 30,000 micrograms, Weisskopf said.