The higher the level of particulates in the air, a new study shows, the greater the indications of psychological distress.

“This is really setting out a new trajectory around the health effects of air pollution,” says Anjum Hajat, an assistant professor of epidemiology in the School of Public Health at the University of Washington.

“The effects of air pollution on cardiovascular health and lung diseases like asthma are well established, but this area of brain health is a newer area of research,” Hajat says.

Air quality and quality of life

Where a person lives can make a big difference to health and quality of life. Scientists have identified “social determinants” of physical and mental well-being, such as availability of healthy foods at local grocers, access to nature, or neighborhood safety.

Previously, researchers have found association between air pollution and behaviour changes—spending less time outside, for instance, or leading a more sedentary lifestyle—that can be related to psychological distress or social isolation.

The new study looked for a direct connection between toxic air and mental health, relying on some 6,000 respondents from a larger, national, longitudinal study, the Panel Study of Income Dynamics. Researchers then merged an air pollution database with records corresponding to the neighborhoods of each of the 6,000 survey participants.

The team zeroed in on measurements of fine particulate matter, a substance produced by car engines, fireplaces, and wood stoves, and power plants fueled by coal or natural gas.

People can easily inhale fine particulate matter (particles less than 2.5 micrometers in diameter) and absorb it into the bloodstream. Fine particulate matter is considered of greater risk than larger particles. (To picture just how small fine particulate matter is, consider this: The average human hair is 70 micrometers in diameter.)

The current safety standard for fine particulates, according to the US Environmental Protection Agency, is 12 micrograms per cubic meter. Between 1999 and 2011, the time frame the researchers examined in the study, survey respondents lived in neighborhoods where fine particulates measured anywhere from 2.16 to 24.23 micrograms per cubic meter, with an average level of 11.34.

Researchers gauged participants’ feelings of sadness, nervousness, hopelessness, and the like with survey questions relevant to the study, scoring responses with a scale they made to assess psychological distress.

The researchers found that the risk of psychological distress increased alongside the amount of fine particulate matter in the air. For example, in areas with high levels of pollution (21 micrograms per cubic meter), psychological distress scores were 17% higher than in areas with low levels of pollution (5 micrograms per cubic meter).

Another finding: every increase in pollution of 5 micrograms per cubic meter had the same effect as a 1.5-year loss in education.

Breaking down the numbers

Researchers controlled for other physical, behavioural, and socioeconomic factors that can influence mental health, such as chronic health conditions, unemployment and excessive drinking.

But some patterns emerged that warrant more study, explains primary author Victoria Sass, a graduate student in the sociology department.

When researchers broke the data down by race and gender, black men and white women show the most significant correlation between air pollution and psychological distress: The level of distress among black men, for instance, in areas of high pollution, is 34 percent greater than that of white men, and 55 percent greater than that of Latino men. A noticeable trend among white women is the substantial increase in distress—39 percent—as pollution levels rise from low to high.

Precisely why air pollution impacts mental health, especially among specific populations, was beyond the scope of the study, Sass says. But that’s what makes further research important.

“Our society is segregated and stratified, which places an unnecessary burden on some groups,” Sass says. “Even moderate levels can be detrimental to health.”

Air pollution, however, is something humans can mitigate, Hajat says, and has been declining in the United States. It’s a health problem with a clear, actionable solution.

But it requires the political will to continue to regulate air quality, Sass adds.