Trees are Good for Human Health in Unexpected Way

Australian researchers set out to discover the kinds of microbes people are likely to encounter during walks in urban parks and made an unexpected discovery.

When you visit an urban park, you'll no doubt see numerous trees, some of which may have lived for hundreds of years, providing food and refuge for generations of fungi and insects. Maybe you'll spot some wildflowers attended by pollinating insects, and you may find yourself enjoying the chorus of songbirds fervently competing for mates.

But there’s also another world of wildlife floating all around you. This is the biodiversity that we can’t see with the naked eye - the secret life of the air we breathe. The air is full of microscopic life forms.

Humans are bombarded by all these tiny organisms on a daily basis. Studies have shown that up to a million microbial cells can be found in a single cubic meter of air, and people can inhale a whopping 100 million bacteria each day.

But where does all this invisible life come from? And what does our exposure to it mean for our health? Together with colleagues, we set out to discover the kinds of microbes people are likely to encounter during walks in urban parks.

A recent study in Nature: Scientific Reports shows that many of the life forms floating in the air actually originate in the soil beneath our feet. This makes a lot of sense. Soil is arguably the most biodiverse habitat on Earth, and a single gram of it can contain more microbes than there are humans on the planet.

Microbes are incredibly light, so they become airborne really easily and are carried far and wide on the wind. The study showed that distinct layers of bacteria form in the air, with different species and quantities of microbes occurring at different heights. At the average head-height of a standing adult, there were fewer but also different kinds of bacteria compared with those in the air lower down at the head height of a child or sitting adult.

This means that we may be exposed to different kinds of microbes - some good for us, some bad - depending on our height and posture. Exposure to lots of different types of microbial life, particularly in childhood, is generally considered to be a good thing, because it allows our immune systems to build up a strong army of cells that protect us from pathogens. The greater number of microbial species we detected closer to the ground could be vital in ensuring children develop robust immunity later in life.

But it also matters which environments we spend time in. After collecting 135 samples, the researchers found that the air in the wooded areas of an urban park near Adelaide in Australia contained more bacterial species but fewer potential human pathogens than nearby sports fields.

Trees appear to filter the microbial communities in a given airspace, reducing the risk of exposure to microbes that cause disease. Because trees also seem to increase microbial diversity in the air, allowing more of them to grow in urban areas could provide an important health benefit by enhancing our immune systems.

This wouldn’t only benefit human health. Although we can’t see the microbes and the other members of the microscopic world around us, they’re fundamental to the proper functioning of ecosystems, plant health and communication (yes, trees can communicate with each other), and even climate regulation.

We still know relatively little about the unseen life in the air we breathe, but this new, preliminary study reveals a few of its secrets.