For example, people with diabetes may have fruity or sweet breath. The smell is caused by ketones, chemicals produced when the body begins to burn fat instead of glucose for energy, a metabolic condition known as ketosis.

“The idea that exhaled breath might have diagnostic potential has been around for some time,” said Dr. Davis. “There are reports in both ancient Greek and ancient Chinese medical training texts relating to a doctor’s use of the smell to guide his clinical practice.”

Modern technologies can detect more subtle chemical changes, and machine learning algorithms can detect patterns in breath samples from people with certain diseases. In recent years, scientists have used these methods to identify unique “breath prints” for lung cancer, liver disease, tuberculosis, asthma, inflammatory bowel disease, and other conditions. (Dr. Davis and her colleagues even used VOC profiles to differentiate between cells that were infected with different strains of flu.)

Before Covid hit, Breathomix had developed an electronic nose to detect several other respiratory diseases. “We train our system: ‘Okay, this is what asthma smells like, this is what lung cancer smells like,” said Rianne de Vries, the company’s chief technology and scientific officer. “So it’s about building a large database and finding patterns in big data.”

Last year, the company – and many other researchers in the field – turned around and tried to identify a breath print for Covid-19. For example, during the virus’ first surge in spring 2020, researchers in the UK and Germany collected breath samples from 98 people who showed up in hospitals with respiratory symptoms. (Participants were asked to exhale into a disposable tube; researchers then used a syringe to collect a breath sample.)

31 of the patients were found to have Covid while the rest had a variety of diagnoses, including asthma, bacterial pneumonia, or heart failure, the researchers reported. The breath samples from people with Covid-19 showed higher levels of aldehydes, compounds produced when cells or tissues are damaged by inflammation, and ketones, which fits in with research suggesting the virus is damaging the pancreas and Can cause ketosis.

The Covid patients also had lower levels of methanol, which could be a sign that the virus had inflamed the gastrointestinal system or killed the methanol-producing bacteria that lived there. These changes in breathing together “give us a Covid-19 signal,” said Dr. Thomas, a co-author on the study.