Three days ago at dawn, a woman in her 80s suffering from severe dehydration was rushed to the emergency room.

She had suffered a myocardial infarction, or heart attack, in which the coronary arteries supplying blood to the heart became blocked.

[Lee Jin-ho / Professor of Cardiology, Kyung Hee University Hospital: "Because the patient stayed in a hot environment for a long time and lacked energy, she wasn't able to drink enough water. She showed very severe dehydration, and because of the dehydration..."]

During heatwaves like these, our bodies sweat to lower body temperature, which leads to dehydration.

When bodily fluids are depleted, blood becomes thicker, leading to the formation of blood clots—commonly referred to as blood clots—which can block blood vessels and cause myocardial infarction or stroke.

[(Is this part that looks like this blocked?) Yes. The whitish areas in the middle of the dark parts are all visible blood clots. Blood should flow smoothly through the vessel, but it's blocked.]

This is why heart attacks are just as frequent during heatwaves as they are in winter, when blood vessels suddenly constrict in cold air.

According to statistics from the Health Insurance Review and Assessment Service, the cumulative number of acute myocardial infarction patients over the past five years has been over 13,500 higher in the summer than in the winter.

In 2018, which recorded 31 heatwave days, the burden of cardiovascular and cerebrovascular diseases surged more than fourfold compared to the years before and after, when there were about half as many heatwave days.

The elderly are particularly at risk, as they often do not feel thirsty even while sweating profusely in extreme heat.

[Ryu Jeong-bun / Dongdaemun-gu, Seoul: "Usually, I don't drink water. I don't drink much at all. About a cup a day?"]

[Seo Sang-ho / Dongdaemun-gu, Seoul: "I know I should drink a lot of water, but I just don't feel like drinking it."]

[Lee Jin-ho / Professor of Cardiology, Kyung Hee University Hospital: "If you remain exposed to hot summer days continuously, it can be seen as similar to running a marathon all day long. By the time you feel thirsty, it is highly likely that the water in your body has already been depleted..."]

Before feeling thirsty from the heat, it is advisable to drink water frequently—about a cup every hour or two rather than large amounts at once.

Reported by Han Sung-hee | Produced by Kim Da-yeon | Camera Filming by Yang Hyun-chul | Video Edited by Park Na-young | Design by Park Tae-young