[Comparative study of daily maximum exposure to temperatures during hot summer days in 3 Japanese cities]

Health risk assessment and developing measures to deal with global warming (including increased heat waves) have become urgent global issues. In the present study, we measured the personally exposed temperature (Tp) during summer among residents in major Japanese cities to investigate the relation to daily maximum ambient temperature (Tmax), which is generally been used as an index of temperature exposure in epidemiological studies.

Personal exposures to temperature (Tp) were measured for a week with portable monitors (HOBO H8 Loggers, Onset Computer Corporation) for 194 subjects (101 males and 93 females, aged 21-82 years) in 3 cities, i.e., Sapporo, Tokyo and Naha (Okinawa), from July to September, 2003 (73 days).

Even on days with a Tmax of 30-35 degrees C, associated with significantly increased risk of mortality in Tokyo, neither average Tp for 7-19 o'clock nor the value for 13-15 o'clock appeared to rise beyond 30 degrees C in Tokyo and 31 degrees C in Naha. It was, thus, apparent that Tp's are generally controlled to not exceed these values at least during daytime, suggesting that they could be regarded as a threshold for heat stress tolerance. On the other hand, although average Tp's for night time (0-7 o'clock) were also found to be asymptomatic at 29 degrees C in Tokyo and 30 degrees C in Naha, they were generally too high to be free from heat stress including sleep disturbance in both cities as indicated in our questionnaire study. For both cities, in days with the Tmax above 30 degrees C, the average Tmin was 26 degrees C, while average and minimum Tp's during the night time were 28 degrees C and 27 degrees C, respectively. The correlation coefficients with Tmax were generally low; 0.35 for average Tp during daytime, 0.42 for Tmin and 0.27 and 0.19 for average and minimum night time Tp, respectively.