Researchers from Harvard and Emory Universities have found a link between long-term exposure to air pollution and being diagnosed with depression after age 64.

“Depression in older adults is a concern and can be as important as dementia,” the authors write, noting that previous studies have shown the impact of air pollution on mental health.

The study was published Friday in JAMA Network Open. Researchers analyzed Medicare data from 2005 to 2016 for 8.9 million people age 64 and older, of whom 57% were female and 90% were white. During the study, more than 1.5 million people were diagnosed with depression.

The researchers looked at air pollution data for the ZIP code associated with where each person in the study lived during a 16-year period. The three air pollutants studied were: 

Fine particulate matter, which are tiny particles such as those that can make the air look hazy when pollution levels are high.

Ozone, also known as smog, which comes from sources such as tailpipes and smokestacks.

Nitrogen dioxide, which is among the group of gases that form when fossil fuels such as coal, oil, gas, or diesel are burned.

Prolonged exposure to each of the three pollutants was linked to an increased risk of a new diagnosis of depression. Researchers found that nitrogen dioxide exposure was particularly dangerous.

Depression later in life is often under diagnosed, according to the CDC, because “healthcare providers may mistake an older adult’s symptoms of depression as just a natural reaction to illness or the life changes that may occur as we age, and therefore not see the depression as something to be treated.”

“That’s one of the biggest reasons we wanted to conduct this analysis,” Harvard T.H. Chan School of Public Health researcher Xinye Qiu, PhD, told CNN. “Surprisingly, we saw a large number of late-onset depression diagnoses in this study.”

The authors write that they hope both environmental regulators and public health officials will take the impact of air pollution into account when considering the prevention of depression in older people.

“We hope this study can inspire researchers to further consider possible environmental risk factors (such as air pollution and living environment) for the prevention of geriatric depression, to understand the disease better moving forward, and to improve the delivery of mental health care services among older adults,” they write.

Air Pollution Linked to Depression in Older Adults: Study