It is rush hour and you are crammed inside a train carriage with a stranger’s armpit pressing against your face. Are you feeling relaxed?

Studies have shown that repeated infringement of personal space in cities can trigger the brain’s threat system, which makes us feel stressed.

Other factors such as constant contact with strangers and traffic noise all contribute to city dwellers being most likely to suffer from chronic stress.

Mood and anxiety disorders, as well as schizophrenia, are up to 56% higher in urban environments when compared to rural locations.

To combat the ill-effects of city living on mental and physical health, some cities have built parks and urban green spaces.

In Japan, whose capital Tokyo is the world’s largest city, shinrin-yoku, or “forest bathing”, has been practiced since the early 1980s.

Forest bathing involves nothing more than spending time in a wooded environment, be it sitting or walking, but it has been found to lower blood pressure and reduce levels of the stress hormone cortisol.

What’s best for your brain?

While forest bathing has been shown to have positive health effects, scientists in Germany wanted to demonstrate the effect of forests and urban green spaces on the actual structure of city-dwellers’ brains.

They examined brain image sequences of 341 elderly residents of Berlin, focussing particularly on the amygdala – the part of the brain that controls our fear responses and is responsible for stress and anxiety disorders.

Previous research has shown increased levels of stress activity in the amygdala among city dwellers, so researchers at Berlin’s Max Planck Institute for Human Development wanted to see if there was any difference in the health of participants’ amygdala depending on which part of the city they lived in.

What they found was a strong correlation between residents living within 1km of the forests on the edge of the city and those with the healthiest amygdala.The health of the amygdala was gauged by its structural integrity, taking into account things like the volume of grey matter and level of interaction in that part of the brain.

Interestingly, there was no discernible benefit to living near a green urban space like a city park.

Image: Scientific Reports journal

The researchers, whose paper reporting their findings appears in the journal Scientific Reports, suggest this might be because forests are typically larger spaces.

Forests also usually lie at the outer edges of cities, as in Berlin, where there are fewer people, less air traffic and less air pollution.

Time to move to the woods?

People who live near forests have healthier amygdala, and this means being less prone to the stress and anxiety disorders controlled by this part of the brain.

But before we all move to the woods, the German scientists behind this research warn that correlation isn’t necessarily causation – it is possible that people with healthy amygdala choose to live near forests.

However, the researchers admit this is unlikely.

With the UN predicting that two thirds of the world’s population will be living in cities in 2050 – up from a little over half today – the researchers say more work urgently needs to be done.

They say that if we can identify the exact geographical features most likely to encourage a healthy brain, we might be able to start designing our cities in a way that improves people’s mental health.

In the meantime, taking a walk among some trees instead of squeezing inside a subway train might be a good start.