Prolonged exposure to high temperatures can lead to heat exhaustion and eventually to heatstroke. Vulnerable people can begin to suffer problems when the temperature rises into the thirties. By the time the thermometer tops 45C, everybody is potentially at risk.
Heat exhaustion refers to overheating of the body due to excessive loss of water or, in rare cases, salt depletion. People who suffer from heat exhaustion have often been taking part in strenuous physical exercise.
Symptoms include thirst, headache, pallor, dizziness and possibly nausea or vomiting.
In severe cases, the heart rate increases and the sufferer may feel disoriented. Heatstroke or sunstroke occurs when the body's thermoregulatory system stops working and body temperature rises to approximately 42C.
Dr Ron Behrens, of the Hospital for Tropical Diseases in London, told BBC News Online that the elderly and the very young were particularly vulnerable to the effect of high temperatures. He said obese people, and those with medical conditions such as heart disease and diabetes were also potentially at risk.
He said: "In a heatwave the body has to spend much of its energy trying to keep the core temperature down and this increases the stress on everything else."
However, he said anybody could be at risk if they did not take sensible steps.
There are a number of sensible things that everybody should do to avoid illness when temperatures soar. Increase your intake of non-alcoholic, non-carbonated, caffeine free beverages such as water and fruit juice,wear clothing that is light in colour and loose fitting ,avoid the outdoors during extreme heat,stay out of the sun,eliminate strenuous activity.
Some experts also believe it is a good idea to cut down on the amount of proteins that you eat during a heatwave, as they can increase metabolic activity and generate heat in the body.