Scientists have identified the “trigger molecule” that makes pleasant aromas smell like burning rubbish or sewage in people whose sense of smell is disrupted by Covid. The loss of smell is a defining symptom of Covid-19, with about 18% of adults in the UK estimated to have been affected. Some people also experience disturbances in their sense of smell – a condition known as parosmia – but the biological basis for this has remained a mystery. Now scientists have identified a highly potent odour molecule that appears to be a trigger for the sense of disgust experienced by many of those with parosmia. The molecule, called 2-furanmethanethiol, found in coffee, was described by those with a normal sense of smell as being coffee- or popcorn-like, but those with parosmia typically described its scent as disgusting, repulsive or dirty. Dr Jane Parker, the director of the Flavour Centre at the University of Reading and co-author of the research, said: “This is solid evidence that it’s not ‘all in the head’, and that the sense of disgust can be related to the compounds in the distorted foods. The central nervous system is certainly involved as well in interpreting the signals that it receives from the nose.” According to one recent international survey, about 10% of those with Covid-related smell loss experienced parosmia in the immediate aftermath of the disease, and this rose to 47% when the respondents were interviewed again six or seven months later. Some of the most common triggers for parosmia include coffee, chocolate, meat, onion and toothpaste. The latest study investigated whether there were particular compounds within these substances that were to blame. By trapping the aroma of coffee, the team were able to test individual coffee compounds on volunteers who had parosmia and compare their reaction with those who didn’t. From the hundred or so aroma compounds present in coffee, people with parosmia could point to those responsible for the sense of disgust. Among the 29 volunteers, scientists found 15 commonly identified compounds that triggered parosmia, with the prime culprit being a chemical called 2-furanmethanethiol, which 20 of the volunteers said had a horrible smell. The nose has more than 400 different types of olfactory receptor in the nose, each sensitive to different aromas. The chemical 2-furanmethanethiol has an exceptionally low threshold for being detected and so is possibly one of the first chemicals to come back on a person’s radar after losing their sense of smell. Parker said that the brain appeared to be mis-categorising the smell, but that more work was needed to understand this element of the condition. The paper noted that those with parosmia could also experience the most objectionable smells differently, for instance describing the smell of faeces as “less unpleasant or biscuity”. Simon Gane, one of the researchers, from the Royal National Ear, Nose and Throat and Eastman Dental hospital, said: “We still have a long way to go in understanding this condition, but this research is the first to zoom in on the mechanism in the nose. We now know this has to be something to do with the nerves and their receptors because that’s how these molecules are detected.”