In this study, we developed a heat and steam generating (HSG) sheet that generates steam for several hours, and analyzed its effects on thermoregulatory responses and thermal sensation. In seven healthy male subjects the lumbar region (lower back) was warmed for four hours in a cool environment of 20°C and 40%RH. For each subject, three experiments were performed: 1) warming the lower back with a HSG sheet (12×20 cm), 2) warming with a heat generating (HG) sheet without steam, and 3) no warming

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... (control). The skin and subcutaneous temperatures in and around the warmed area, as well as rectal temperature were measured. The subjects reported whole body thermal comfort, and local sensations in the warmed area. While the temperature under the sheet was similar between the HSG and HG sheets (approximately 38.5°C), skin temperature at 2 cm from the sheet was about 0.9°C higher (p<0.05) for the HSG sheet than the HG sheet. The subjects reported less whole body coldness during the warming by the HSG sheet than the HG sheet. They also reported stronger and more spreading warm sensation in the warmed region for the HSG sheet than the HG sheet. In a separate experiment using an apparatus with a heat flux sensor, higher heat flux was observed for the HSG sheet than the HG sheet, even though both sheets gave the same surface temperature. These results suggest that warming the skin with steam is more efficient than warming it with dry air.