Forest and seaside walks may benefit us by exposing us to low doses of natural toxins that our ancient ancestors once breathed in.

“We need the tonic of wildness… we can never have enough of nature,” wrote Henry David Thoreau in Walden: Or, life in the woods – his 1854 account of living among nature. It turns out that Thoreau was right about the “tonic” of the natural environment. Evidence is mounting that, as many of us assume, being by the sea or in the countryside is good for our health. The question is why. Some of the restorative effects of this “blue-green space” could be psychological. This stems from the biophilila hypothesis, put forward by the biologist E. O. Wilson in the 1980s, that we have a hardwired need to connect with nature. For instance, a study at the time found that just looking out of a window on to a natural scene helped patients recover more quickly after surgery than those whose view was of a brick wall. More recently, a UK study showed that people who live in coastal locations take significantly more exercise than those inland. And a Japanese study on physiological effects ofshinrin-yoku – literally “forest bathing” – found that participants had lower blood pressure and lower levels of the stress hormone cortisol when they took country walks than when they walked city streets. These are not the only ways in which the countryside could impact our health. Graham Rook of University College London argues that blue-green space improves the regulation of our immune system, because it exposes us to bacteria and parasites with which we co-evolved. I have a new theory to add to the mix: the country and sea air is good for us because it is slightly poisonous. Take a stroll by the ocean or in the woods, and you can smell the difference compared to a town or city. It’s more than just a lack of pollution. What you are sensing is an airborne plethora of particles and chemicals produced by plants, fungi and bacteria. Wave action produces microscopic droplets of seawater, bringing with them compounds from marine cyanobacteria, algae and seaweed. The presence of these biomolecules in the atmosphere represents a radical departure from the urban environment, which is more likely to contain synthetic pollutants and mildew. Of particular interest to me are compounds called phytochemicals, derived from plants, algae and cyanobacteria. Some of these are polyphenols – antioxidants, which often act as natural defences against pathogens and predators. Others are volatile terpenoids, which are responsible for the scents of plants. What is so special about these chemicals? When I came across the concept of biophilia 10 years ago, I wondered if chemical factors in the environment might be responsible for the beneficial effects of nature. I now think that breathing in and ingesting these natural chemicals may have a positive effect on our physiology. They may even be able to block the progression of many diseases such as diabetes and cancer, as well as certain neurodegenerative processes (Environmental Research, vol 140, p 65). During the course of human evolution our ancestors were exposed to a legion of phytochemicals, through inhalation and their diet. Though many of these are toxic at high concentrations, this long-standing exposure made us tolerant to low doses. In fact, exposure to low levels of these toxins can be of benefit due the mild stress they induce, which triggers repair mechanisms and enhances tolerance to bigger doses – an effect known as hormesis. Abundant evidence exists that dietary phytochemicals have this beneficial hormetic effect, particularly the antioxidants found in many vegetables, fruits and medicinal plants. If eating food containing these substances is good for us, such as in the Mediterranean-style diet, then ingesting them via an airborne route may also be beneficial. How exactly could these chemicals boost health? By inhibiting an important system, the mTOR pathway, that controls many aspects of cell function and growth. When this signalling system is overactive, it can lead to diabetes, inflammation, certain cancers and neurodegeneration. Inhibiting the mTOR pathway activates a number of protective systems such as tumour suppression and autophagy – the destruction of damaged cellular components, allowing new ones to form. Some phytochemicals also appear to have biological effects similar to those of severe calorie restriction, which inhibits mTOR and has been shown to have health benefits for humans and increase lifespan in many animal species. And mTOR may not be the only cell-signalling pathway targeted by these airborne molecules. Phytochemicals in food can influence other systems, such as the MAPK/ERK pathway, which plays a key part in cancer progression, and AMPK, a protein that monitors the energy level of cells. This “biogenics” hypothesis remains untested at the moment. It is uncertain whether the levels of bioactive chemicals in natural aerosols are sufficient to influence cell signalling, although my colleagues at the University of Exeter and I plan to study the effects of low concentrations of phytochemicals in mice. However, we already know that certain airborne phytochemicals can interact with cell signalling pathways to cause hypersensitivity in the immune system – giving rise to hay fever, for example – and that minute amounts of airborne algal toxins can induce asthma. Planners are putting increasing emphasis on creating urban parks as ever more people move to the city from the country. But maybe there are other ways to bring the country to the city. If it turns out that airborne natural molecules do indeed boost our health, then this leads to the question of whether we could “bottle” the benefits of country and seaside air. Natural phytochemicals could be added to aerosols for use in the home and in public spaces such as shopping malls. They could even be added to our diet. The modern built environment represents a significant change of airborne milieu for humans. City air is not only more polluted but lacks the diversity of biologically derived molecules that humans have been exposed to throughout our evolutionary history. In light of this, any health benefits we experience in the natural environment should perhaps be regarded not as a boost to well-being but as a return to our baseline state. It seems that when we venture out to “take the air”, we get a great deal more besides.

New Scientist, 28 June 2015 ; ;