New research indicates that air pollution may be contributing to poor sleep, particularly through increased nighttime coughing. A study involving Cambridge University researchers, in collaboration with experts from China and the tech firm Sleep Cycle, assessed coughing sounds from 32 cities across 12 countries over 500 days.
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The analysis demonstrated a strong correlation between higher daily air pollution levels and increased instances of nighttime coughing, even in areas where pollution was beneath the World Health Organization (WHO) recommended daily limits. Cough sounds were recorded passively by the Sleep Cycle app, which uses smartphone microphones. This data was compiled by city to form an extensive dataset.
Researchers took into account other variables that could affect coughing, including weather conditions and flu activity. Their findings, published in the journal Communications Health, revealed that for every 10 micrograms per cubic meter increase in particulate matter, coughing levels increased by 2.4%. In Los Angeles, for example, coughing spiked during the January 2025 wildfires, reinforcing the link between daytime pollution and nighttime symptoms.
Air pollution is associated with approximately 7 million premature deaths annually. Research has previously identified fine particulate matter (PM2.5) as particularly harmful, capable of entering the lungs and bloodstream, leading to inflammation and respiratory issues. Study lead author Tong Xia from Tsinghua University stated that while occasional coughing is common, persistent coughing may indicate underlying lung irritation.
The Sleep Cycle app monitors sleep quality by analyzing sounds like breathing and coughing, processed through AI algorithms. This technology has reviewed over 4 billion sleep sessions, enhancing traditional public health studies with a large global dataset. Cecilia Mascolo, a study leader from Cambridge's computer science department, emphasized the dataset's value, noting that it provides insights that many studies limited to one country cannot offer.
The dataset comprised cough-related sounds from tens of thousands of users, reflecting usage in cities across the U.K., U.S., Australia, Brazil, Chile, China, France, Germany, Italy, Japan, South Africa, and Sweden. Researchers used this data to connect cough occurrences with city-level PM2.5 levels and other relevant variables over 18 months. Their analysis, even after considering flu season spikes in cough rates, revealed a persistent link between pollution levels and coughing frequency.
Tong expressed surprise that the relationship remained evident despite controlling for flu seasons, suggesting there might be no "safe threshold" for pollution exposure. Mascolo added that individuals might want to consider steps to reduce their exposure to air pollution, such as using masks or air filters.