A study published on Friday in JAMA Network Open found that long-term exposure to elevated levels of air pollution increases the risk of late-onset depression among the elderly.
The other study, published in JAMA Psychiatry, found that long-term exposure to even low levels of air pollutants was associated with increased incidence of depression and anxiety.
Air pollution has long been linked to cardiovascular and respiratory diseases.
The new studies add to a growing body of evidence that air pollution also affects mental health.
For the study of the effects of air pollution on elderly Americans, researchers from Harvard and Emory University examined the data of nearly nine million people on Medicare, the US government health insurance scheme for those aged over 64.
More than 1.52 million of them were diagnosed with depression during the study period of 2005 to 2016 according to Medicare claims.
"We observed statistically significant harmful associations between long-term exposure to elevated levels of air pollution and increased risk of late-life depression diagnosis," the researchers said.
"Socioeconomically disadvantaged individuals were observed to be at a much higher risk of late-life depression in this study," they said. "They are simultaneously exposed to both social stress and poor environmental conditions, including air pollution."
For the study, the researchers mapped pollution levels and compared them to the addresses of the Medicare patients.
The pollutants to which they were exposed were fine particulate matter such as dust or smoke, nitrogen dioxide, which stems mainly from traffic emissions, and ozone, which is emitted by cars, power plants and refineries.
The researchers said the elderly may be particularly susceptible to pollution-linked depression because of their pulmonary and neural vulnerability.
"Although depression is less prevalent among older adults as compared with the younger population, there can be serious consequences, such as cognitive impairment, comorbid physical illness and death," they said.
In the other study, researchers in Britain and China investigated the association of long-term exposure to multiple air pollutants and the incidence of depression and anxiety.
They studied a group of nearly 390,000 people, mostly in Britain, over a period of 11 years and found there was an increased risk for depression and anxiety even at pollution levels below UK air quality standards.
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