Increase in incidences of emergency conveyance related to heat stroke in 2006-2013 in Saitama, Japan – local climate change and aging of society

Before the 1990's, heat stroke was not recognized as a social issue in Japan. It mostly occurred during exercise outdoors. However, the number of incidences of emergency conveyance and death due to heat stroke in the last 10 years was increased to six times that in the previous 30 years. A trend of global warming in Saitama, a part of the Tokyo metropolitan area, has become significant in the past 30 years. Also, with the local inland climate, the daily maximum temperature in Saitama is higher than that in Central Tokyo. Since 1994, seasonal heat waves have occurred increasingly. The first significant disaster related to heat stroke was recorded in 2007. On August 16th, a maximum temperature of 40.9 oC, a new Japanese record at that time, was recorded in Kumagaya City in northern Saitama. Six hundred eighty-one calls for emergency conveyances were received and 20 people died. The second significant event occurred in the summer of 2010. The mean summer temperature was the highest in the past 113 years in Japan, exceeding the past record set in 1994. Three thousand six hundred seventy-nine calls for emergency conveyances were received (a total of 53,843 in Japan) and one hundred twenty-four people died (a total of 1,718 in Japan). Since 2011, more than 3,000 people have required emergency conveyance every summer. Using the 2006-2012 recorded data, a relationship between the maximum daily temperature and incidence rate of heat stroke was shown by a logistic regression curve. However, large variations in the number of incidences of emergency conveyance are seen at around the maximum temperature of 35 oC. To perform a higher precision evaluation of heat stroke risk, the 3-day-accumulated Apparent Temperature defined by Steadman in 1984 was calculated, and the number of calls for emergency conveyance was found to be high when the 3-day-accumulated Apparent Temperature was more than 1,900 oC. During 2006-2012, the number of cases of heat stroke indoors increased, and more than 70 % of those afflicted were elderly persons. This finding reflects the rapid advance of aging as a social background. Factors for heat stroke increase are 1) rapid increase to high temperature in the daytime, and 2) consecutive high-temperature days of more than 35 oC. As another aspect of the social background, the increasing in the number of single-elderly-person households in the past 30 years may promote the aggravation of heat stroke.