Nighttime coughing could be due to air pollution levels, research suggests

Has your partner ever woken you up at night with their coughing? Has your coughing ever woken them up? New research suggests it could be due to air pollution levels in your area, even if those levels are low.

Researchers from the University of Cambridge, Tsinghua University and the sleep technology company Sleep Cycle analyzed coughing sounds from 32 cities in 12 countries over 500 days, and found that higher daily air pollution levels were strongly linked to nighttime coughing levels, even where local pollution levels were below the World Health Organization's recommended daily exposure limit.

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 data, which was anonymized, also found that when pollution levels rose by 10 micrograms of particulate matter per metre cubed, there was a 2.4% 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 are reported in the journal Communications Health.

Air pollution is a leading environmental risk for human health, leading to an estimated seven million premature deaths per year. Among airborne pollutants, tiny particles of less than 2.5 micrometres (PM_2.5) are particularly dangerous, as they can penetrate deep into the lungs and into the bloodstream, potentially leading to inflammation throughout the body, heart disease or respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD).

Dr. Tong Xia, lead author, Tsinghua UniversityEveryone 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 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. The company's technology has analyzed more than four billion sleep sessions, making it a useful complement to traditional public health surveillance and epidemiological research.

"The dataset from Sleep Cycle is really valuable, because the app is widely used worldwide," said Professor Cecilia Mascolo from Cambridge's Department of Computer Science and Technology, who led the research. "Most public health studies only have data from one country, so having a dataset of this size gives it much more power."

"Every night, Sleep Cycle's technology turns millions of smartphones into precise sound analysis instruments," said co-author Dr Mikael Kågebäck from Sleep Cycle. "Coughs are detected and counted directly on each user's device, so no audio ever leaves the phone. The combination of scale and privacy lets researchers see the health effects of air pollution city by city, night by night, at a scale no sleep lab could match."

The dataset – which included no personal identifiers – contained information on cough sounds from tens of thousands of users over 18 months. The researchers used data from the 32 cities worldwide where usage rates of the app were highest. The cities were from 12 countries: Australia, Brazil, Chile, China, France, Germany, Italy, Japan, South Africa, Sweden, the United Kingdom and the United States.

The researchers linked the cough data with city-level PM_2.5 concentrations, meteorological variables and influenza surveillance data over a period of 18 months. Analysis of the data, even when accounting for the annual flu season when cough rates would normally be higher, showed a strong link between daytime pollution levels and nighttime cough frequency.

"I was surprised we could still see the signal at all," said Tong, who was formerly a PhD student in Mascolo's group. "I thought that adding in the confounding factor of flu season would make the link in the data disappear, but it didn't."

The link between pollution and coughing persisted in the data, even when pollution levels were below the WHO's recommended safe daily exposure limit.

"This suggests that there may be no such thing as a safe threshold for exposure to air pollution, although our research is purely observational," said Mascolo. "However, people may wish to consider how they might mitigate their own exposure to air pollution, by using masks or air filters, for example."

The research was supported by the European Research Council, the Tsinghua University Vanke School of Public Health Fund, and the Engineering and Physical Sciences Research Council (EPSRC), part of UK Research and Innovation (UKRI). Cecilia Mascolo is a Fellow of Jesus College, Cambridge.

Source:

University of Cambridge

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