The objective of this study was to investigate a recent trend in the incidence of heat disorders that have been officially registered as occupational diseases by the Ministry of Labor in Japan. Also, the study aimed to explore the meteorological factors relating to the incidence of them. All cases of heat disorders included in the original reports of occupational diseases, which had been submitted to the Ministry of Labor between 1995 and 1998, were made the subjects of the analysis in the study.

There were 80 cases of occupational heat disorders reported in 1995, 49 in 1996, 51 in 1997 and 56 in 1998, respectively. The higher incidence was found in outdoor works such as construction, transportation, patrol, agriculture, forestry, golf course work, etc. The higher incidence was also found in the afternoon (14:00-16:00) between July and August. About 70% of the victims were middle-aged or elder-aged workers. The workers with the shorter years of experience tended to suffer damage. More than 80% of the victims were employees of small enterprise under 50 employees.

Next, we analyzed relationship between the incidence of heat disorders due to outdoor works and the outdoor meteorological condition at the time when they occurred. The outdoor meteorological condition (air temperature, relative humidity, wind velocity and sun radiation) was estimated by the data of a meteorological observatory nearest to the place where they occurred. The air temperature, relative humidity, air velocity and sun radiation at the time when the heat disorders occurred ranged between 19.7 and 39.3 ºC, between 23 and 95%, between 0.6 and 7.7m/sec and between 0.06 and 3.24MJ/m², respectively. The frequency of the heat disorders rapidly increased around air temperature of 30 ºC and the highest frequency (34%) was found between 32 and 34 ºC. But they also occurred between 28 and 30 ºC (12%) and even under moderate temperature conditions less than 28 ºC (8%) concomitant with the higher relative humidity, regardless of wind velocity and sun radiation.

The present study clearly shows that occupational heat disorders are still now one of the most important problems among occupational diseases in physical work environments in Japan although the high risk workplace has changed from mining and steel industries to outdoor works in summer. Furthermore, it suggests that the risk of heat disorders at outdoor work in summer increases around air temperature of 30 ºC, but it also suggests that some risk of heat disorders exits even under moderate temperature conditions with high humidity.