Exposure to air pollution may be tied to the risk of developing depression later in life, a large new study finds.
Scientists are finding more and more evidence that people who live in polluted areas have a higher risk of depression than those who live with cleaner air. But this study published Friday in JAMA Network Open is one of the first to examine the associations between long-term exposure and the risk of depression diagnosed after age 64.
Depression itself is a serious health condition. When it develops in an older adult, it can also contribute to problems with the ability to think clearly, studies show, as well as physical problems and even death.
Previous research has found that a new diagnosis of depression is less common among older adults than in younger populations.
“That’s one of the biggest reason we wanted to conduct this analysis,” said Dr. Xinye Qiu, co-author of the new study, published Friday in JAMA Network Open. Qiu is a postdoctoral research fellow in the Department of Environmental Health at Harvard T.H. Chan School of Public Health. “Surprisingly, we saw a large number of late-onset depression diagnoses in this study.”
The researchers looked at information on more than 8.9 million people who got their health insurance through Medicare and found that more than 1.52 million were diagnosed with depression later in life during the study period of 2005 to 2016. But the number is probably an undercount; studies show that late-in-life depression is often underdiagnosed.
To determine the study participants’ pollution exposure, Qiu and her co-authors looked at where each of the people diagnosed with depression lived and created models to determine the exposure to pollution at each ZIP code, averaged across a year.
The researchers looked at the study participants’ exposure to three kinds of air pollution: fine particulate matter, also known as PM2.5 or particle pollution; nitrogen dioxide; and ozone.
Particle pollution is the mix of solid and liquid droplets floating in the air. It can come in the form of dirt, dust, soot or smoke. Coal- and natural gas-fired power plants create it, as do cars, agriculture, unpaved roads, construction sites and wildfires.
PM2.5 is so tiny – 1/20th of a width of a human hair – that it can travel past your body’s usual defenses.
Instead of being carried out when you exhale, it can get stuck in your lungs or go into your bloodstream. The particles cause irritation and inflammation and may lead to respiratory problems. Exposure can cause cancer, stroke or heart attack; it could also aggravate asthma, and it has long been associated with a higher risk of depression and anxiety.
Nitrogen dioxide pollution is most commonly associated with traffic-related combustion byproducts. Nitrogen oxides are also released from traffic, as well as through the burning of oil, coal and natural gas. Exposure can increase inflammation of the airways, cause coughing or wheezing and reduce lung function.
Ozone pollution is the main ingredient in smog. It comes from cars, power plants and refineries. This particular pollution is best known for exacerbating asthma symptoms, and long-term exposure studies show a higher risk of death from respiratory diseases in people with higher exposures. The American Lung Association calls it one of the “least well-controlled pollutants in the United States,” and it’s one of the most dangerous.
In the new study, the scientists found that people who lived in areas with higher pollution levels long-term had an increased risk of a depression diagnosis. All three of the pollutants studied were associated with a higher risk of late-onset depression, even at lower pollution levels.
“So there’s no real threshold, so it means future societies will want to eliminate this pollution or reduce it as much as possible because it carries a real risk,” Qiu said.
There were greater associations between depression and exposure to particle pollution and nitrogen dioxide among socioeconomically disadvantaged groups. That may in part be because they are simultaneously exposed to social stress and these poor environmental conditions, the study says.
Older adults who had underlying problems with their heart or breathing were also more sensitive to developing late-in-life depression when exposed to nitrogen dioxide pollution, the study found.
The study has some limitations. The majority of the participants were White, and more research would be needed to see whether there would be a difference among diverse populations.
This is also a population-level study, so there is no way to pinpoint exactly why people exposed to these kinds of air pollutions would have a higher risk of depression.
Other studies have found that exposure to air pollution may affect the central nervous system, causing inflammation and damaging the body’s cells.
Some air pollution, studies show, can also cause the body to release harmful substances that can hurt the blood-brain barrier, the network of blood vessels and tissues made up of closely spaced cells that protect the brain, and that may lead to depression and anxiety.
Because aging can impair the immune response, older adults may be particularly vulnerable to the negative effects of air pollution. More research will be needed to fully understand these connections, as the neural basis for depression is not completely understood.
Another possibility may be that people who live in polluted areas develop physical problems that are associated with worsening psychiatric health, the study said.
“Late-life depression should be a geriatric issue that the public and researchers need to be paying more attention to, like on a similar level with Alzheimer’s and other neurological conditions,” Qiu said.
She is particularly concerned about the effects that climate change will have on this phenomenon. Ozone pollution will increase as the world gets warmer, and the study found that ozone pollution had a stronger association to late-onset depression than particle and nitrogen dioxide pollution.
“Because of this concerning effect we are seeing with ozone, it makes more sense for the government to put some regulation on pollution and also climate mitigation, because rising temperatures and ozone pollution are definitely linked to each other,” Qiu said.