Int J Biometeorol (1995) 38:194-198 © Springer-Verlag 1995 ORIGINAL ARTICLE I.G. Tselepidaki • D.N. Asimakopoulos • K. Katsouyanni C. Moustris • G. Touloumi • A. Pantazopoulou The use of a complex thermohygrometric index in predicting adverse health effects in Athens Received: 5 January 1994 / Revised: 6 January 1995 / Accepted: 8 January 1995 Abstract Mortality and morbidity indices are known to depend on changes in meteorological conditions. In Ath- ens, severe adverse health effects following extreme heat conditions have been reported. The usefulness has been investigated of the complex thermohygrometric index (THI), a simple index based on maximum daily tempera- ture and relative humidity, in predicting the health effects of specific meteorological conditions. The values of THI were found to correlate well with more complex biocli- matic indices; the THI could successfully replace tem- perature and humidity in predicting the daily number of deaths through multiple linear regression modelling. Thus the introduction of THI levels more than 28.5 ° C and between 26.5 and 28.5 ° C, through dummy vari- ables, in a regression model explained 40% of the vari- ability in the number of deaths during the months of July and August. During days with THI values less than 26.5 ° C the mean number of deaths was 33.5, compared to 41.8 when THI was between 26.5 and 28.5 ° C. The daily number of deaths increased to 108.2 when THI exceeded 28.5 ° C. From this study, the exact level of THI at which public health measures must be taken was not clear and more work is needed to identify it. However, given its simplicity, the use of THI for predicting meteorological conditions which are adverse to health would appear to be promising in preventive medicine and in health ser- vices planning. Key words Discomfort index • Effective temperature • Heat wave • Mortality I.G. Tselepidaki • D.N. Asimakopoulos • C. Moustris Laboratory of Meteorology, University of Athens, 33, Ippokratous street, GR-106 80 Athens, Greece K. Katsouyanni ( ~ ) . G. Touloumi. A. Pantazopoulou Department of Hygiene and Epidemiology, University of Athens, Medical School, 75, Mikras Asias street, GR-115 27 Athens, Greece Introduction The meteorological and climatological pattern in the coastal Hellenic regions comprising the environs of the city of Athens, can be conveniently divided into two main seasons. (a) The cold period, from October to March, when the daily mean temperatures in Athens vary between 10 and 20 ° C and the daily mean maximal temperatures between 14 and 25 ° C, with daily mean relative humidities be- tween 63 and 71%. (b) The warm period, from April to September, when the daily mean and the mean maximal temperatures vary from 17 to 29°C and from 21 to 33°C respectively, while the daily relative humidities are significantly low- er, varying between 43 and 58% (National Observatory of Athens, Annual). Mortality and morbidity have been shown to depend on environmental temperature and humidity, usually fol- lowing a sinusoidal curve over 1 year, with a maximum during the winter months (Kilbourne 1986). However, in places where extreme heat may occur during the sum- mer, short-term, sharp increases in adverse health indices have been observed (Stewart et al. 1989). Furthermore, these increases also appear to depend on other environ- mental and socio-economic factors (Martinez etal. 1989); Athens has been shown to be a high risk area in this respect (Moustris etal. 1990; Katsouyanni etal. 1988, 1993). It follows that public health officials and physicians in such areas should be specially trained to diagnose and treat heat-related illnesses and also to un- dertake the planning of heat-wave prevention strategies to eliminate or, at least, limit the adverse consequences. Whilst investigating the climatological characteristics of each month during the warm period, in Athens, it can easily be seen that the period during which discomfort could occur lies between June and September, when the mean daily maximal temperature exceeds 29 ° C. During this period the prevailing meteorological conditions strongly depend on the local Etesian winds, which are caused by the combination of anticyclones centred over