Global pattern of human thermal adaptation and limit of thermal neutrality: Systematic analysis of outdoor neutral temperature

Abstract

Neutral temperature is the temperature at which people feel neither cool nor warm. Previous regional studies have found that neutral temperature can change with season, which can be attributed to the psychological adaptation effect of people. However, few studies have examined whether the adaptation pattern can be observed across wide geographical and climatic regions. In some cases, outdoor comfort surveys cannot determine neutral temperatures because thermal sensation votes obtained do not reach neutrality. It reflects that thermal adaptation has a temperature limit, and thermal neutrality can hardly be achieved outside the limit. This study aims to establish linear models between neutral temperature and background temperature (survey-period and climate normal) using neutral temperature data reported in international studies. The limit of thermal neutrality will also be determined in the best-fit model. The neutral temperature data in 19 empirical outdoor thermal comfort studies published between 2009 and 2017 were used in the linear regression models. Survey-period mean air temperature was found to be the best predictor of neutral temperature (R2 = 0.539, p = 0.007). The effect of long-term acclimatization may be less important than it was previously thought because survey-period temperature can explain over a half of the variability in neutral temperature. The temperature limit of thermal neutrality was defined by the lowest and the highest neutral temperatures in the best-fit model; the limit was between 12.9 and 32.3 °C in air temperature. The findings indicate that the adaptation effect has a worldwide pattern as neutral temperature always lies closely to background air temperature.