Istanbul's more than 14 million residents are sweltering in the hottest week of this summer, with felt temperatures hitting around 40 degrees Celsius according to meteorologists.The Turkish State Meteorological Service has issued a heat wave warning across the country, stating that the temperature is between 5 degrees Celsius and 9 degrees Celsius above seasonal averages."The highest temperature this week will be 34 degrees Celsius in many districts of Istanbul," Ahmet Uçar, Weather Forecasts Desk manager of the service told Anadolu Agency (AA)."It will rise as high as 37 degrees Celsius in some districts of the city this week, but because of the high humidity rate, the felt temperature can be above 40 degrees in the city."Istanbul last experienced such heat 15 years ago when temperatures hit 41 degrees.Almost all provinces in the Marmara, southeastern Anatolia and central Anatolia regions are experiencing extreme heat this week.Temperatures in provinces such as Edirne, Kırklareli, Balıkesir, Bursa and Kocaeli will also be above 40 degrees Celsius, Uçar added."The hottest provinces are Şanlıurfa and Batman in the southeast with temperatures hitting around 44 degrees," he said.Both meteorologists and health officials are advising people to remain indoors between 10.00 a.m. and 4.00 p.m. – times when sunshine is at its most intense.It is also very important to increase fluid intake when the temperature is high, Özgür Karcıoğlu, an emergency physician at Istanbul's Acıbadem Bakırköy Hospital told AA."We may need to double the fluid intake in such hot temperatures but this should be pure water, not beverages like tea or coffee."Heat can be dangerous especially for elderly people, children, infants and heart patients. "They are much more responsive to fluid loss and extreme heat. They should be much more careful about these," Karcıoğlu said.The heat wave will continue in the country until the first week of August, according to meteorologists.