Long-term exposure to air pollution may lead to depression in older people, according to US scientists. 

A research team led by experts at Harvard University has looked at the health records of nearly 9 million people in the USA, around 1.5 million of whom developed depression after the age of 64. 

They matched these health records with an estimate of people's long-term exposure to fine particulate matter (PM2.5), nitrogen dioxide (NO2), and ozone (O3), which they calculated by looking at where people had lived throughout their lives.

The scientists accounted for several other factors thought to be important in determining whether someone will develop depression. 

They found that for each five-unit increase in fine particulate matter exposure, depression risk increased by 0.9 per cent, for the same increase in nitrogen dioxide exposure it increased by 0.6 per cent, and for the same increase in ozone exposure, it increased by 2 per cent.

The full report is accessible here.