A new study reveals that high levels of air pollution may increase nighttime coughing, even when pollution levels are below the daily exposure limits recommended by the World Health Organization.
Researchers from Cambridge University and Tsinghua University, in collaboration with the sleep technology company Sleep Cycle, analyzed cough sounds in 32 cities across 12 countries over an 18-month period. They found that higher levels of fine particulate matter (PM2.5) during the day were associated with increased coughing rates at night.
The analysis showed that an increase in fine particulate matter concentration of 10 micrograms per cubic meter was associated with a 2.4% increase in coughing rates, even after taking other factors into account, such as weather and flu season.
The researchers relied on data automatically collected from the Sleep Cycle app, which uses phone microphones to detect sounds associated with sleep, including coughing, snoring, and breathing. Cough sounds are analyzed on the device itself, without sending the recordings outside the phone, and the data is then aggregated at the city level without identifying any users.
The link between pollution and coughing was clearly evident in Los Angeles, where coughing rates spiked in conjunction with the wildfires that broke out in January 2025. Researchers considered this a "natural experiment" that supported the existence of a short-term relationship between daytime air pollution and nighttime coughing.
Fine particulate matter (PM2.5) is among the most dangerous air pollutants because of its ability to penetrate deep into the lungs and reach the bloodstream. Exposure to it is linked to inflammation throughout the body and an increased risk of heart and respiratory diseases, such as asthma and chronic obstructive pulmonary disease (COPD).
Dr. Tong Xia, the lead researcher from Tsinghua University, said that coughing may be a sign of irritation or inflammation in the lungs, even before the condition develops into a chronic disease.
Professor Cecilia Mascolo, who led the research from Cambridge University, said that the continued emergence of the link between pollution and coughing even at low levels suggests that there may not be a clear "safe limit" for exposure to air pollution, but stressed that the study is observational and does not prove a direct causal relationship.
The study results were published in the journal Communications Health.
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