Minimal heating dose: a novel biological unit to measure infrared irradiation

Abstract

Infrared (IR) rays, which comprise approximately 40% of the solar radiation which reaches the earth's surface, have received relatively scant attention. As no standard method has yet been agreed upon for the biological evaluation of IR irradiation, the objective of this study is to suggest a new unit for IR irradiation. The skin temperature of 38 Korean volunteers was measured after IR irradiation with varying irradiance. Skin temperature after IR irradiation at an irradiance of 2.02 W/cm2 remained unchanged after 652+/-22 s (mean+/-standard error), which corresponds, in this case, to a total radiation dose of IR 1317.3+/-44.84 J/cm2. This quantity was designated as the minimal heating dose (MHD). We also demonstrated that MHD increased with increasing IR irradiance at lower IR irradiance (1.17 and 2.02 W/cm2), whereas it became constant at higher irradiance (2.87 and 3.22 W/cm2). No statistically significant correlations were detected between MHD and volunteers' ages, erythema index, or melanin index. We propose 'MHD' as a biological unit for the measurement of IR irradiation.