Seasonal Variation in Mortality from Ischemic Heart Disease and Cerebrovascular Disease in Okinawa and Osaka: the Possible Role of Air Temperature

Okinawa is located in a subtropical area and is well-known for low mortality due to ischemic heart disease (IH-D) and cerebrovascular disease (CVD). However, the factors that contribute to these low mortality rates remain unclear. We examined the seasonal variation in the mortality due to IHD and CVD among Okinawa and Osaka residents, aged 45 to 84 years, between 1992 and 1996. In addition, we studied if there was a relationship between the monthly mortality rate from IHD or CVD and the monthly mean daily air temperature in Naha City and Osaka City. Data on the monthly mean daily air temperature was obtained from the meteorological stations in Naha City and Osaka City. Our results showed that there were inverse correlations between the monthly mean daily temperature in a city and each of the monthly mortality from IHD in Okinawa (r=-0.794, p<0.01), the monthly mortality from CVD in Okinawa (r=-0.837, p<0.001), and the monthly mortality from CVD in Osaka (r=-0.954, p<0.001). In Osaka, the monthly mortality rate from IHD was at or near its minimum value when the mean daily temperature was approximately 25 degrees C (in September), and it increased in a linear fashion as the mean monthly temperature fell (r=-0.975, p<0.001). The difference between the monthly mortality from IHD or CVD among the Okinawa and the Osaka residents increased in the winter season in comparison with that in the other seasons, with the exception for IHD in July and in August. These findings indicate that the lower mortality from IHD and CVD in Okinawa is affected, at least in part, by Okinawa's warm winter.