There is also the physiological effect of heat itself on our breathing. “Heat can also disrupt breathing control, increase fluid retention, and promote dehydration, all of which can make the upper airway more collapsing and increase the likelihood of sleep apnea,” said Lucia Pinilla, another researcher at Flinders University who has studied the subject.

At the same time, sleep apnea is expected to be a growing problem for those living in cities, due to the constant impact of air pollution on breathing at night, something that is predicted to worsen. Last year, researchers in Hong Kong lead a study where they found that short-term and long-term exposure to PM2.5 particles—small particles in the air, less than 2.5 micrometers in diameter, produced by sources such as car exhausts, factory emissions, and forest fires and which can penetrate deeply into the lungs and bloodstream—make sleep apnea more likely.

Others have shown that the the same is true for nitrogen dioxidea reddish-brown gas released into the air from exhausts, power plants, and other industrial facilities, while exposed to pollution can also make symptoms worse for people with existing sleep apnea.

“Obstructive sleep apnea occurs when the upper airway becomes unstable and collapses during sleep,” said Martino Pengo, associate professor from the University of Milano-Bicocca who studies the subject. “Air pollutants increase this instability by irritating the tissues of the throat, making the airway narrower and more prone to collapse when muscle tone naturally falls at night. Nitrogen dioxide is a strong airway irritant and can promote local inflammation that disrupts sleep and interferes with breathing.”

Meanwhile, such research also points to possible ways to mitigate risk. Martha Billings, professor of medicine in the division of pulmonary, critical care, and sleep medicine at the University of Washington advises using indoor air filtration devices if online air quality databases show that your city ranks poorly. “I would recommend it especially if the air quality index is above 200 as it happens with forest fires or other stagnant air,” he said.

As well as increasing the temperature, losing weight can be a way to reduce your risk of sleep apnea. Research highlights how those who are overweight or obese are more at risk of experiencing more severe episodes of sleep apnea in hot weather, with their breathing stopping and starting more frequently during the night. Pinilla says it has to do with how the accumulation of body fat changes the regulation of the body’s internal temperature. “People with higher fat mass tend to retain more heat, and lose it less efficiently making it more difficult to maintain a comfortable core temperature at night,” he said. “On hot nights, this can lead to lighter and more fragmented sleep.”

Men, who are more prone to sleep apnea because they tend to accumulate fat more easily in the neck and have a longer and more collapsing airway, are especially at risk, as are those with preexisting mental health or sleep disorders.

The Flinders team hopes to eventually be able to develop personalized advice and a heat-warning system for those at greater risk of experiencing sleep apnea events during heatwaves, as well as simple solutions that anyone can implement. They hope to get funding to run experiments where people receive cooling mattress toppers or follow specific behavioral advice, such as making sure they are well hydrated when sleeping, to see if this can actively prevent apnea events in warmer weather.

“Ultimately, our goal is to develop practical, evidence-based recommendations that can be used during heat waves, especially for vulnerable groups and people at higher risk,” said Pinilla.