The first study, published last week in the journal JAMA Psychiatry, studied a population of around 390,000 people in the UK over around eleven years.

The levels of pollution to which they were exposed were estimated based on their home address.

The researchers studied the levels of fine particles (PM2.5 and PM10), nitrogen dioxide (NO2) and nitric oxide (NO) -- pollution particularly caused by fossil fuel power plants and road traffic .

"Long-term exposure to multiple pollutants was associated with an increased risk of depression and anxiety," the researchers concluded.

The observed risk was non-linear, ie it was greatly increased beyond a relatively low level of concentration, then tended to stagnate thereafter.

“Knowing that the air quality standards of many countries still greatly exceed the latest World Health Organization (WHO) recommendations of 2021, stricter pollution standards or regulations should be put in place,” wrote the authors of this study.

A second, published Friday in the journal JAMA Network Open, focused on the effect of fine particulate matter (PM2.5), nitrogen dioxide (NO2) and ozone (O3) on people in over 64 years old.

The aim was to study the consequences of air pollution on the onset of depression late in life.

This work used a database of Medicare, the public health insurance reserved for the elderly in the United States, and studied a population of some 8.9 million people, including about 1.5 million cases of depression.

The results again show a strong association between pollution and depression, particularly by observing the levels of fine particles and nitrogen dioxide for disadvantaged populations.

This association could be explained by the observed link between high concentrations of pollutants and inflammation in the brain, according to the two studies.

Breathing polluted air over the long term leads to a greater risk of depression, according to two new studies © JUSTIN TALLIS / AFP/Archives

“There is a strong emerging link between inflammation and depression,” commented Oliver Robinson, professor of neuroscience and mental health at University College London, who was not involved in this research.

This work comes "in addition to the growing body of evidence showing that we should be concerned about the effects of pollution on mental health, in addition to the more obvious links" with respiratory disease, he added.

© 2023 AFP