Does Hot Weather Raise Your Stroke Risk? What to Know

5 min read

June 18, 2024 – High cholesterol, high blood pressure, heart disease – most people know that these conditions can up one’s stroke risk. But a lesser-known risk factor can be a culprit as well: high outdoor and indoor temperatures.

A newly released study from German researchers found that 7% of patients treated at a hospital for a stroke had their symptoms on a warm night. The temperature threshold for defining warm air started as low as 14.6 C (58 F) during a May-to-October period each year for 15 years.

Our body temperature is kept steady through a process called circadian thermoregulation. This is tied to circadian rhythm, or the 24-hour clock in our brains that controls sleep-wake cycles. Researchers found that if this process is interrupted by excessive heat, a nighttime stroke could potentially be triggered two ways. 

Blood pressure variation is the first factor, said study co-author Cheng He, PhD, a postdoctoral fellow at Helmholtz Munich, a health and environment research center in Germany. “Blood pressure naturally decreases at night. If our circadian rhythm is disrupted, this decrease may not occur as expected, potentially raising the risk of stroke due to increased nighttime blood pressure.”

Second, through temperature. 

“The body cools down during sleep, a process controlled by the circadian clock. Disruption in this rhythm may impair the ability to lower body temperature, potentially leading to disturbed sleep and increased blood viscosity, both of which are risk factors for stroke,” he said.

 A new Chinese study also found that exposing people to high temperatures (a peak of 92 F) for just an hour increased their stroke risk up to 10 hours later, even after they had moved to places with lower temperatures. Interestingly, both blood and bacteria in the body could play an important role as to why. 

“When the temperature rises, your body sends more blood to your skin to cool it down and make you sweat. This can cause dehydration, which thickens the blood, making it more likely to form clots that can block blood flow to the brain and cause a stroke,” said co-study author Jing Zhao, MD, PhD, deputy director of neurology at Fudan University and chief of neurology at Minhang Hospital Fudan University in Shanghai, China. “Also, heat can make your gut lining more permeable, allowing bacteria to enter the bloodstream. These bacteria release toxins that cause inflammation throughout the body, which can also contribute to stroke by making artery plaques unstable.” 

Who Is Most Likely to Have a Heat-Related Stroke?

It's most likely in people with a condition that can make them more likely to get blood clots. In the Chinese study, the risk was particularly high in patients with atrial fibrillation, a condition where the heart beats irregularly, Zhao said. An irregular heartbeat can mean that blood can’t be pumped out of your heart well, causing clots to form. 

“Blood clots cause 90% of all strokes,” said Mingming Ning, MD, MMSC, director of the Cardio-Neurology Clinic at Massachusetts General Hospital in Boston and an associate professor of neurology at Harvard Medical School. 

Two other conditions can also raise your odds of getting a blood clot and make you more likely to have a heat-related stroke. The first is a hole in the heart, also known as a patent foramen ovale, or PFO.

“One in four people have a PFO,” Ning said. A PFO is a hole in the wall between the left and right atria of the heart. Normally, it closes after birth, and even if it doesn’t, it causes most people no symptoms at all. That said, “if you develop a clot in another part of your body, such as your leg, that clot can travel up to your heart,” Ning said. Pressure within your heart changes, and if you have a PFO, that pressure, combined with the open hole, allows blood to flow from the right to the left atrium. This flow could send that blood clot to your brain, causing a stroke.

The type of clot that does this is a deep vein thrombosis (DVT). DVT often happens when you are seated for long periods of time, as when you're driving long distances or flying and sitting in a cramped airplane seat, where you can’t stretch your legs. 

Ning said that 10% of of healthy travelers get a blood clot from flying or from a long car trip.

“It’s very important to get up and stretch your legs as often as possible on a plane, or to stop frequently and get out of your car,” she said. 

You’re more at risk of getting a DVT if you’re overweight, over the age of 60, have a blood clotting disorder, or take birth control pills or hormone replacement therapy medication. The symptoms of a DVT may include pain, redness, or swelling in your leg. Sometimes, there are no symptoms at all, so a good preventive measure is to wear compression stockings when traveling. Compression stockings stop swelling and direct blood flow back toward your heart, which helps prevent a DVT. 

How Can You Prevent a Stroke in Hot Weather? 

1. Stay hydrated.

“Water is the cheapest blood thinner you can buy,” said Ning. “When you don’t have hydration in your blood, your platelets will clump together.” Men should aim for around 15.5 cups (3.7 liters) a day, and women should try to drink around 11.5 cups (2.7 liters) a day. Drink even more in hot weather. 

2. Get a physical. 

Getting a physical can help your doctor to tell if you have a condition that may raise your stroke risk as temperatures rise. 

“Monitoring and managing underlying health conditions such as hypertension [high blood pressure], dyslipidemia [abnormal levels of fats in the blood], and atrial fibrillation is crucial,” said Zhao.

3. Get 7 to 9 hours of sleep a night.

“Your body doesn’t like extremes – rest allows your body time to repair itself,” Ning said. Keep your bedroom cool with air conditioning or fans. If you feel sweaty, move to a cooler room. 

“Monitor the indoor temperature and your physical condition in real time during summer nights,” said He.