Skin
Sunburn is caused by excessive exposure to the sun’s ultraviolet (UV) rays. The more the body is subjected to sunburn, the more likely a person is to suffer from skin cancer. Too much UV radiation can damage skin cell DNA. DNA tells the cells how to function so, as the damage worsens with each repeated sunburn, cells can start growing out of control, which leads to skin cancer.
Brain
It can be hard to think clearly when you’re too hot. Studies have shown that extreme heat can affect mental performance. Hot weather has been linked to reduced cognitive function, judgment errors and higher risk of occupational injury. At extremely high temperatures, the blood-brain barrier begins to break down. Proteins and ions accumulate in the brain, causing inflammation.
Many people report feeling irritable on hot days, and evidence suggests that extreme heat negatively affects mental health. A recent study in New York found that, on hot days, there were more emergency hospital visits due to substance abuse, mood and anxiety disorders, schizophrenia and dementia. Another study linked rising temperatures to higher suicide rates.
Sweat
The hypothalamus is the body’s thermostat. It senses temperature changes, inside the body and out, and makes adjustments to keep it within one or two degrees of 37C. When the brain senses that the body is warming, the hypothalamus sends messages to blood vessels near the skin, telling to them to dilate. This increases the amount of blood at the surface of the skin allowing it to lose heat – as long as the air is cooler than the body.
If the air outside the body is hotter than inside, the body’s sweat glands – all 1.6 to 5 million of them – get to work. Sweat is secreted on to the surface of the skin, where its evaporation has a cooling effect – because it takes heat energy from the body to turn the liquid into vapour.
But sweating isn’t a perfect system. An active person can sweat up to 10 litres of water a day, and if this is not replaced, it can lead to dehydration. A body that is dehydrated can no longer cool itself by sweating. Also, if a body gets too hot, blood flow to the skin and sweating can both stop: in this case, body temperature soars and brain cells suffer irreversible damage.
Lungs
Hot weather can impact air quality, making it harder to breathe. High temperatures are typically accompanied by still air which allows pollutants to stagnate. Ground-level ozone is a harmful gas that forms when pollutants emitted by cars, power plants and industrial sources react chemically to sunlight.
One of the major constituents of smog, ground-level ozone can reduce lung function, and is a major factor in asthma morbidity and mortality. One 2008 study found that for every degree celsius the temperature rises, ozone pollution will kill an additional 22,000 people worldwide.
Fatigue
Hyperthermia is the term for the body reaching an abnormally high temperature. This happens when its various heat-regulating mechanisms fail to cope.
If body temperatures rises to around 38C, the brain tells the muscles to slow down, and fatigue sets in. This is called heat exhaustion. Symptoms of heat exhaustion also include dizziness, visual disturbances, intense thirst, nausea, palpitations and numbness.
If body temperature is not reduced, heat exhaustion can worsen to heatstroke. Heatstroke, which kicks in at 40C, should be treated as a medical emergency. Symptoms include dry, hot skin and mental dysfunction. Left untreated, heatstroke can cause seizures, coma and death.
Heart
As the body heats up, blood vessels dilate, and this lowers blood pressure, making a person feel dizzy and sick. At worst, when the body is deprived of normal bloodflow, the gut can leak, blood vessels can be damaged – causing blood to coagulate – and cells can disintegrate as their proteins break down. If blood pressure drops too far, the risk of heart attacks rises.
When the body senses it is too hot, the medulla oblongata – the part of the brain that controls vital processes including heartbeat, breathing and blood pressure – tells the heart to increase the amount of blood pumped for every beat. But with blood pressure lowered, the heart has to work harder to push the blood around the body, so the heart rate increases. At extreme temperatures, the quantity of blood circulated increases dramatically. The heart becomes exhausted and bloodflow can suddenly plummet. Experts say that as global temperatures rise so will mortality rates – and these deaths will be largely due to the stress extreme heat puts on the cardiovascular system.
Katherine Latham is a science and environment journalist
