Background/aim Thailand has no heat health warning system. However, there is an increasing number of heat-related illnesses (HRI) yearly. Thus, the objective of this work was to investigate the statistical associations between climatic variables and heat-related illnesses with an aim to set up a suitable threshold level for Heat Health Warning System in Thailand.

Methods Daily HRI of hospital admissions from the ICD10 database with diagnosis T67 (Effects of heat and light) were collected between January 2010 to December 2014 from the Bureau of Policy and Strategy, Department of Disease Control, Ministry of Public Health, Thailand. Daily temperature and humidity from the same period were obtained from Meteorological Department Ministry of Digital Economy and Society. The heat index was calculated according to the Steadman equation. Time series and Poisson regression analysis were used to find out the relationship between HRI and heat index controlling for day of the week and holiday indicator, for lag times of 1–7 days.

Results There were 6,895 HRI visits. The overall incidence was 2.14 visits per 1 00 000 persons per year. The majority of patients were female. The highest incidence was in the 80–84 years old group with an age-specific incidence rate of 7.21 per 1 00 000 persons per year. The relative risks of HRI visits in the country at 25th and 75th percentile of the mean of heat index at lag 0 were 31.44 and 42.53, respectively. The in-country regional relative risks at the 25th percentile of the Southern, Northern, Central and Northeast regions were 5.01, 15.79, 55.37 and 26.38, respectively. Similarly, the relative risks at the 75th percentile of Southern, Northern, Central and Northeast regions were 5.56, 21.76, 79.59, and 39.75, respectively.

Conclusion The level of heat index has a positive association with heat-related illnesses visits. A suitable warning threshold level of heat index for Thailand will be investigated in the next stage of the study.