New research using data from a popular sleep-tracking app has uncovered a striking link between air pollution and nighttime coughing – even when pollution levels are within recommended safety limits.
Scientists from the University of Cambridge, Tsinghua University, and sleep technology company Sleep Cycle analysed coughing sounds from 32 cities across 12 countries over 500 days. Their findings show that higher daily air pollution levels were strongly associated with increased nighttime coughing, including in areas where pollution fell below the World Health Organization’s recommended daily exposure limit.
The study used cough sounds passively recorded by the Sleep Cycle mobile app via phone microphones. Coughs were counted on individual devices and pooled by city, creating an unprecedented dataset. Researchers accounted for factors like weather and influenza levels that could also increase coughing. They found that for every 10 μg/m³ rise in PM2.5, coughing levels increased by 2.4%.
In Los Angeles, coughing spikes coincided with the January 2025 wildfires, supporting the short-term link between daytime pollution and nighttime coughing.
Lead author Dr Tong Xia from Tsinghua University said: ‘Everyone coughs from time to time, but coughing, whether it’s a one-off or more persistent, is a sign of irritation or inflammation in the lungs. A cough could be a sign of something that hasn’t become chronic yet, but might in future.’
Sleep Cycle uses sounds such as breathing, snoring, and coughing to track sleep quality. These sounds are recorded locally and analysed by an AI algorithm, with no audio leaving the phone. The company’s technology has analysed over four billion sleep sessions.
Professor Cecilia Mascolo from Cambridge’s Department of Computer Science and Technology, who led the research said: ‘The dataset from Sleep Cycle is really valuable, because the app is widely used worldwide. Most public health studies only have data from one country, so having a dataset of this size gives it much more power.’
The anonymised dataset included information from tens of thousands of users over 18 months across cities in Australia, Brazil, Chile, China, France, Germany, Italy, Japan, South Africa, Sweden, the UK, and the US. Researchers linked cough data with city-level PM2.5 concentrations, meteorological variables and influenza surveillance data.
Jonathan Blades, Head of Policy at Asthma + Lung UK, said: ‘This robust observational study suggests that even at relatively low levels of fine particulate matter (PM2.5) exposure increases nighttime coughing – a sign that our airways are irritated. Air pollution is often invisible, but its effects can follow us into our homes and disturb us even while we sleep. The findings are another worrying reminder that lower pollution does not mean harmless pollution, and that there is no level of exposure to PM2.5 that can be considered completely safe.
‘The findings of this study add further weight to calls for a new Clean Air Act, committing the government to meet World Health Organization air quality guidelines and the urgent need to prevent future governments from weakening air quality standards. But it also reinforces evidence that there is no safe level of exposure to toxic air.’
The full research can be read here.
Photo: Polina ⠀