The researchers used cough sounds passively recorded by the mobile app via the phone’s microphone. Cough sounds were counted on individual devices and pooled by city to build an unprecedented dataset of this kind. The researchers accounted for external factors that could lead to an increase in coughing levels, such as weather conditions or influenza levels.
Analysis of the anonymized data also found that when pollution levels rose by 10 micrograms of particulate matter per cubic meter, there was a 2.4 percent rise in coughing levels. In Los Angeles, one of the cities included in the dataset, coughing levels spiked at the same time as the January 2025 wildfires, acting as a quasi-natural experiment that further supported the short-term association between daytime pollution and nighttime coughing.
The results, reported in the journal, Communication Health, suggest that air pollution is a leading environmental risk to human health and is linked to an estimated seven million premature deaths per year. Among airborne pollutants, tiny particles of less than 2.5 micrometers (PM2.5) are particularly dangerous, as they can penetrate deep into the lungs and bloodstream, potentially leading to inflammation throughout the body, heart disease or respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD).
“Everyone coughs from time to time, but coughing, whether it’s a one-off or is more persistent, is a sign of irritation or inflammation in the lungs,” said lead author Dr. Tong Xia from Tsinghua University. “A cough could be a sign of something that hasn’t become chronic yet, but might in future.” Sleep Cycle is a mobile app that uses sounds— such as breathing, snoring or coughing—to track sleep quality. These sounds are recorded locally and analyzed by an AI algorithm.
