Air pollution has long been known to cause respiratory diseases and cancer, but researchers recently found another risk of exposure — dementia.

In a study published in Neurology, Western University researchers compared the rate of air pollution for those with and without dementia, finding that those who did not develop the condition had lower average daily exposure.

Researchers compared data from multiple studies, which involved over 91 million people over the age of 40 globally, and found that 5.5 million people (6 percent) developed dementia.

The cause is exposure to fine particulate matter, a mixture of solid particles and liquid droplets found in the air, often containing dust, dirt, soot, or smoke. Experts are still unsure why particulate matter affects the neurological condition.

Janet Martin, professor at Western’s Schulich School of Medicine and Dentistry, told The Weather Network that "we don't know exactly why there is this relationship between fine particulate matter and dementia."

"There are a lot of unknowns as to what really is the main mechanism or related mechanisms that lead to the development of dementia," she said.

For every microgram of fine particulate matter per cubic meter, the risk of dementia increases by 3 percent. The particles are able to enter the bloodstream through the respiratory system when inhaled.

"Because they are so small, they can actually enter the bloodstream and evade some of your immune system," Martin explained. "We think that is the reason why they are so risky ... because they do have a direct entry into the bloodstream. [This] may then have ultimate impacts in terms of the risk of dementia."

Though it's not just fossil fuels that contribute to pollution, as emissions from chemical processes and even wildfires or crop burning can negatively impact the air.

Exposure has steadily decreased in recent years, as Martin said that some nations have also begun to place limits on emissions.

"Different countries around the world have named different thresholds based on what's feasible to achieve," she said. "In Canada, our standards have been on the lower side to be safer than what some countries have named as their maximum exposure."

Martin noted that many people, primarily minority and low income comminutes, do not live in areas that have "full control over their own exposures," and that more regulations are needed globally to curtail fine particulate matter.

"It's a very serious reduction in the quality of and length of life. Dementia [has an] increasing incidence. It's sort of like that thief in the night that is coming upon our population as we age," said Martin. "It's one of the most important things that we learn to address and prevent, so that we don't have this huge burden of disease as our population ages."