While most olfactory scientists are students of the human brain, nose and smell-driven behaviours, Professor Jonathan Williams, an atmospheric chemist at the Max Planck Institute in Germany, studies air. The connection between atmospheric chemistry and the sense of smell may seem a little indirect and deserves explanation: when odour molecules are released into the air, let’s say from a fruit ripening on a vine, the initial concentration of the smell diminishes over time and fades away with distance from the source. This is not just because odour molecules become diluted into the surrounding air but also because sunlight breaks them down in a process known as photo-oxidation.

Human noses don’t detect all smells equally. The ones we are most sensitive to are compounds found in nature. Some of these can be detected at extremely low concentration while others we barely perceive at all. Williams and his colleague Akima Ringsdorf told the Royal Society that they now think they know why. They compared the contents of a smell sensitivity database, compiled over more than a decade by Japanese scientists, with an atmospheric chemistry database. This meant they could compare the smell thresholds with the lifespan of an array of organic molecules in the air. Their findings revealed a striking link between chemical families such as alcohols and esters associated with some foods, particularly fruits, and nitrogen and sulphur compounds associated with fire and bodily wastes.

Evolution and adaptation, they suggest, has led us to become most responsive to the chemicals that react most quickly with other elements in the air and then disintegrate and disappear just as quickly from our noses. These are specialised olfactory sensitivities that help us find – and quickly identify – danger or toxic substances.

Williams told colleagues at the Royal Society conference:

A good sense of smell is a big advantage when looking for food. If you want to find a nutritious ripe fruit in a dense, dark forest you need to be able to follow a smell gradient as it twists and turns in turbulent air streams. And the molecules with the shortest lifetimes have the steepest gradients. The results we have seem to support the general hypothesis that there is a link between atmospheric chemistry and the sense of smell, which nobody had thought of before.  

Williams came into the field of human smell by accident. He has spent more than two decades researching how gases emitted into the Earth’s atmosphere contribute to climate change and embarks on a twice-yearly odyssey involving large and small aircraft, cars and a final climb on foot to work in a 325-metre-high research tower built deep in the Amazon by his institute. Here, he and his research assistants and students use a suite of sensitive instruments to measure how specific gases emitted into the air by vegetation (volatile organic compounds) react in the air and contribute to the production of ozone and other particles.

In 2016, a member of his student group, driven by sheer curiosity, decided to blow into one of the highly sensitive instruments designed to capture the fresh forest air and, to the team’s surprise, the breath sample was found to contain a similar array of the substances released by Amazonian jungle plants, including lots of isoprene, one of its major constituents. This raised the question: could it be that we humans, all 7.5 billion of us, are contributing to climate change?

Williams returned to Germany determined to try to interrogate the possibility that human breath could be an ingredient in global warming and, as a classic football-fan scientist, decided he would test his idea by taking measurements of the air above a 31,000-seat football stadium. These would show that when something exciting happened in the match or anxiety and frustration were running high, levels of similar organic compounds in the air rose. Further studies conducted in a cinema during a range of different films seemed to confirm that the fluctuations were related to human emotion and that tension and even relief can literally be read in the air (something that anosmics would not be aware of). When pulses rise and people become tense in their seats as a suspenseful moment unfolds on the screen, the levels of carbon dioxide and isoprene rise, the latter seemingly stored in the muscle tissue as the audience tenses.

But what about other feelings? Could sadness, affection, arousal also be measured in the air? Williams and his team turned next to breaking responses down by movie type, turning to data specialists who normally analyse economic fluctuations. They found that the most obvious link emerged with suspenseful scenes followed by comedy and laughter, perhaps revealing an evolutionary advantage because “alert” or “stand-down” signals are so important for survival.

If these chemical changes can be detected in the air by instruments, is it possible that our noses are registering them too, and that, without consciously knowing it, we really can smell fear?

The expression and reading of fear in humans has fascinated psychologists and neuroscientists for decades, although research has focused primarily on examining axillary (armpit) sweat and facial expressions as participants are exposed to an array of stimuli, from aggression and violence during video games to the effects of watching winners and losers in martial arts contests. Previous studies have revealed that a person experiencing fear or terror emits sweat that is measurably different at molecular level to that of a person who is neutral or happy and that the production of these fear odours engages the same areas of the brain used to process frightened and fearful facial expressions. But while much work has been done to show that humans can decode this emotional intensity in each other’s faces, nobody had yet tried yet to see if this is also true of olfaction.

Jasper de Groot, an assistant professor at Radboud University in the Netherlands, was fascinated by the idea and started to investigate. He and his collaborators began testing their hypothesis on fear odours and intensity by collecting fear and neutral sweat from pads placed under the armpits of 36 participants as they watched half-hour movie clips using horror to elicit fear and gentle nature documentaries to induce calm. The scientists then measured their subjective responses along with physiological markers such as heart rate and ranked them into three groups: those who, when watching horror films, experienced low, medium and high amounts of fear. Those who exhibited most fear not only sweated more but also emitted a higher quantity of volatile odorant molecules.

The “receiver” group was female as women tend to have more sensitive noses and, according to the authors, are more likely to respond emotionally and less consciously to smell cues. An olfactometer, specially equipped to deliver air in a natural way as if through clothing or straight from the body, delivered the four categories of odours – neutral, low, medium, and high fear – and participants were asked to rate both the pleasantness and intensity. However, before the odour was delivered, they were shown images of two male actors who had been photographed showing fearful or disgusted faces but were Photoshopped to create more ambiguous, difficult-to-read facial expressions.

The female “reader” group was asked to choose if the face was showing fear or disgust. The results were a surprise. Instead of perceiving more fear in the ambiguous faces when exposed to sweat from the high-fear groups, the receivers could not distinguish between them, effectively reading fear in all the faces. When the effects of the different doses of fear sweat were examined at physiological and neural level, these also activated the parts of the brain linked to fight and flight or fear. So, even at minute doses, each level of fear sweat transmitted by a sender was perceived and processed very quickly by the receiver.

The scientists elegantly described their findings as a “temporal winner-takes-all principle”, one that suggests that we’re sensitively wired to smell fear at all levels because, let’s face it, it’s always better to be safe than sorry.

Excerpted with permission from On the Scent: Unlocking the Mysteries of Smell and How its Loss Can Change Your World, Paola Totaro and Robert Wainwright, Elliott and Thompson.