Most people would prefer to stay indoors during very hot weather, but for subdivided flat tenants, temperatures in their homes can sometimes be up to five degrees higher than those outside. A green group took 24-hour temperature measurements at three-minute intervals in five subdivided flats over the period from July 22 to August 10. It found that the temperatures indoors at the hottest time of the day were usually higher than temperatures outside. "The cost of electricity is a big financial burden on these families. Sometimes they do not turn on the air-conditioner even when it's very hot. It's affecting their health," said William Yu Yuen-ping, chief executive officer of World Green Organisation, which conducted the survey. In the five flats in Kwun Tong, Mong Kok, Tai Kok Tsui and Tin Shui Wai, the temperature usually rose in the morning, fell towards evening, but rose again by about one degree Celsius later in the night. Yu said the small spaces and lack of ventilation were to blame as the heat from cooking and electrical appliances stayed in the flat. The highest temperature recorded, 35.7 degrees Celsius, was in a 100 square foot subdivided flat in Kwun Tong on July 30 at around 4pm. The Hong Kong Observatory recorded air temperatures in the region as 30.7 degrees Celsius at that time. The unit was one of 12 subdivisions of a flat and external structures built on the building's podium that housed the Zhang family of two adults and three children, aged one, three and eight. "It is so hot that once you enter the flat you are covered with sweat," said Mrs Zhang, 30. She said she had suffered from heat stroke twice in her home this summer. She let her eldest son do his homework in the public library and took the younger children to the mall around noon. "It's like we have a home that we cannot return to." She tried to use the air-conditioner as little as possible, but the children were unable to sleep without it. Her electricity bill was about HK$600 last month. World Green Organisation called on the government to allocate funding to help these low income families use energy efficiently, and to give them electricity subsidies so their quality of life could be improved. It estimated there were about 210,000 households in "energy poverty", defined by their spending 10 per cent or more of their income on power bills.