Plants have long been known as the lungs of the earth. They play a major role in the exchange of moisture, carbon dioxide and oxygen with the atmosphere. However, no link has ever been found between trees and electrical effects in the atmosphere. Until now. The air around us contains both positive and negative ions in roughly equal number, with concentrations of a couple of hundred per cubic centimetre. These ions are created when air molecules are ionised by cosmic radiation from space and terrestrial radiation from the trace gas radon and its daughter products. Radon is a by-product of the radioactive decay of radium. This is present in minute quantities in rocks deep down in the ground and is continually exhaled by the ground. There is no reason to believe that the incidence of cosmic radiation in a forest would be any different to that outside. Similarly, we do not expect the radon exhalation from the ground to be different in the two types of locations. In fact, a search of the scientific literature confirmed this. These two processes are fairly constant in time and space. So you can imagine our surprise when our measurements at several locations around Brisbane, including the Brisbane Forest Park, Daisy Hill and Mt Coot-tha, showed that air ion concentrations in heavily wooded areas were twice as high as in open areas well away from trees. Once more, delving into the scientific literature, we found ample evidence in support of our observation, although nobody had noticed this difference until now. We know that because radium is found in rocks and radon is soluble in water, groundwater is particularly rich in radon. In some cases, there can be up to one hundred times more radon than found in surface water. Groundwater is brought up to the surface by trees and released into the atmosphere during transpiration. This is especially prevalent in trees with deep root systems, such as eucalypts. Using readily available data, we estimated that, in a eucalyptus forest, trees may account for up to 37% of the radon in the air when transpiration rates were highest during the middle of the day. So, why are these extra ions important and how does it affect us? Well, ions, in concentrations that are found in normal air are not harmful at all. In fact, studies have shown that negative ions may actually be beneficial to our health; which is why negative ion generators are commercially available for indoor use. Positive ions, on the other hand, have been associated with several health problems. Science is yet to provide an acceptable explanation for these human effects. Ionisation in the atmosphere – including from trees – produces ions of both signs. These ions quickly attach to particles in the air. In urban environments, the majority of these particles come from motor vehicle emissions and often carry dangerous toxins. There is an established link between airborne particles and human health. We do know that approximately one-half of the particles that we inhale during normal breathing are retained in our respiratory system and it has been shown that charged particles – those with ions attached – are more likely to be deposited in the lungs than uncharged particles. Therefore, looking at the pros and cons, the role of ions in human health effects is still very poorly understood. Negative ions may be helping us, positive ions harming us and both types of ions helping toxic particles attach to our lungs. But we do not believe that ions are dangerous. If there are no dangerous particles in the air to attach to the ions there is no risk of ill health: pollution is the risk, not ions. If you are concerned, please rest assured that it is perfectly safe to go for a walk in the woods! The above findings were recently published in the journal Environmental Science and Technology.