Clothing Reflectance Effects on Mean Radiant Temperature and Outdoor Thermal Comfort under High Solar Radiation

Urban heat stress is intensifying due to climate change and urban heat island (UHI) effects, increasing the importance of personal thermal adaptation in outdoor environments. While meteorological factors are well represented in thermal comfort models, the optical properties of clothing, particularly solar reflectance, remain insufficiently quantified. This study investigates the influence of clothing material reflectance on mean radiant temperature (MRT), predicted mean vote (PMV), and predicted percentage of dissatisfied (PPD) under hot outdoor conditions. The global solar reflectance of 15 clothing materials was measured using spectrometric analysis in accordance with JIS R 3106 and ISO 9050 standards. These measured values were incorporated into a human heat-balance model derived from ISO 7730, in which MRT was adjusted to account for shortwave solar radiation absorbed by clothing and surrounding surfaces. Hourly simulations were conducted using representative summer meteorological conditions for Osaka, Japan. Measured clothing reflectance ranged from 24.4% to 62.4%, resulting in substantial differences in absorbed solar radiation. The highest-reflectance clothing material (Type-2) reduced absorbed solar radiation by approximately 361 W m −2 compared with the lowest-reflectance material (Type-5) under peak summer conditions. Consequently, peak adjusted MRT was reduced by approximately 1.9 °C, while PMV and PPD decreased by 0.17 and 4.6%, respectively. Analysis of heat-balance components indicates that differences in radiative heat gain dominate the observed comfort variations. The results demonstrate that clothing reflectance is a significant and quantifiable parameter in outdoor thermal comfort assessment. Promoting high-reflectance fabrics may provide a practical, low-cost personal adaptation strategy for mitigating heat stress in urban environments.