Explainer | The difference between heatstroke and sunstroke, their symptoms, and how to stay safe in hot weather

Wear a hat, drink plenty of fluids, seek shade and avoid outdoor exercise. These are the golden rules on hot days to avoid sunstroke and heatstroke – but what’s the difference between the two? In both cases, high temperatures harm the body. Put simply, “sunstroke affects only the head – that is, the brain. The rest of the body is fine”, says Dr Frank Erbguth, president of the German Brain Foundation. Heatstroke causes the body’s entire thermoregulation system to break down and can be fatal. This doesn’t mean that sunstroke isn’t serious. As Erbguth says, there’s often a seamless transition from it to heatstroke. In contrast to sunstroke, which requires exposure to sunlight, heatstroke can occur “in pitch darkness” if temperatures are high, according to Dr Bernd Böttiger, head of the department of anaesthesiology and intensive care medicine at Cologne University Hospital and federal coordinator of German Red Cross state doctors. Here’s how heatstroke can occur. Our core body temperature is normally around 37 degrees Celsius (98.6 degrees Fahrenheit). Heatwaves can put the body under severe strain, and a point may be reached when it can’t cool down by sweating, particularly if you don’t drink enough fluids. “The body is no longer able to dissipate heat,” Erbguth says. At this point the body’s core temperature may rise to 40 degrees Celsius or more, which has consequences for various body systems. “The coagulation and circulatory systems are the first to collapse,” Erbguth says. What to do if you sweat too much, and why we sweat at all It’s important to know that blood vessels dilate at high temperatures, increasing the flow of overheated blood to the skin, where heat is dissipated to the environment via the evaporation of sweat. This cools both the skin and blood near its surface, which then flows at lower temperatures to deeper body regions. But blood coagulates if heatstroke occurs, according to Erbguth. “The small clots clog small blood vessels.” The kidneys, along with the liver and heart, can no longer function properly. This can lead to multiple organ failure. Erbguth says that “humid heat is much worse than dry heat”, because increased water vapour in the air reduces evaporation of sweat on the skin and with it the body’s ability to cool down. Symptoms of heatstroke include sudden tiredness, a headache and a vague feeling of dizziness, Erbguth says. “They can progress to nausea, vomiting and then loss of consciousness.” Böttiger says that hot, dry skin and an accelerated heart rate are also signs of heatstroke. If you suspect someone has heatstroke, call the local emergency number. It is 999 in Hong Kong and the United Kingdom. In North America it is 911. Böttiger says that while waiting for help, you should get the affected person into the shade as quickly as possible. Then cool the person’s body with ice cubes or cold packs wrapped in a cloth. Erbguth warns that putting ice directly on the skin can cause frostbite. If the person is conscious, offer them something to drink. Böttiger says water, fruit tea and juice mixed with carbonated mineral water are all suitable. Paramedics can give the person cooling intravenous infusions. If the victim’s coagulation system has already collapsed, anticoagulants will be administered at hospital. How to recognise heatstroke and what to do In the case of sunstroke, “the meninges [thin layers of tissue that cover and protect the brain] and brain warm up. And an increase of just a degree or two is all it takes to cause a kind of meningitis [inflammation of the meninges],” Erbguth says. “The brain isn’t the most robust of organs.” At particular risk are people with no, little or very fine hair on their head, such as the elderly, babies and young children. “The top of infants’ skulls is thin,” Böttiger says. Some sunstroke symptoms are similar to those of heatstroke, such as headaches and dizziness. Other symptoms include neck pain and stiffness, sensitivity to light and a flushed, hot head, Böttiger adds. Symptoms may not appear until several hours after being out in the sun, and in young children may include a high fever. If you suspect someone to have sunstroke, get them to a cool place, cool their body with wet cloths and elevate their upper body. They should also drink a lot of fluids. If the person’s condition worsens and they start to lose consciousness, call the local emergency number. Like what you read? Follow SCMP Lifestyle on Facebook, Twitter and Instagram. You can also sign up for our eNewsletter here.