Effect of air humidity on human acceptable thermal environments and evaluation

Abstract

Air humidity, coupled with temperature, has been verified with significant effect on human thermal comfort, specifically in warm environments. However, it lacks a holistic understanding on their comprehensive impacts in a wide range of relative humidity, especially in winter. This study aims to evaluate human thermal responses to air humidity, under a wide temperature range, and improve understandings of the role of air humidity on human comfort, with considering the effects of air humidity on human respiratory system. The work is expected to benefit to indoor thermal environment design and building energy saving. To this end, the study conducted a human exposure experiment in a well-controlled climate chamber in summer and winter respectively, with a combination of 20 temperature and relative humidity conditions (temperature: 16/20/24/28°C (winter), 23/26/29/32°C (summer), relative humidity: 15%/30%/50%/70%/85%). Subjective evaluations of 20 subjects in each experimental condition were measured and total 800 samples were collected for analysis. Two-way ANOVA and ANOVA were conducted to test the significance of temperature and relative humidity on thermal perception evaluations of subjects. The results showed that the air humidity had neglectable effects on human humidity sensation in neutral and slightly warm environments; while significant increases of humidity sensation votes were found under warm conditions (summer: 32°C; winter: 28°C). With long term exposure, low relative humidity had remarkable stimulus on human body in cold environments, due to lower moisture content in air enhancing the evaporation on skin surface and mucosa; while the acceptability for air quality of subjects increased with increasing relative humidity under hot environment, which was attributed to long time thermal adaptation and low sensitivity to humidity changes. The heat loss from human respiration was calculated and related to human thermal sensation (TSV) and air freshness sensation (AFV): With increasing respiratory heat loss per 1 W/m2, TSV decreased by 0.66 scale and AFV increased by 0.22 scale. To quantify such effects, the standard effective temperature (SET) was adopted as independent variable and the linear regressions between TSV and air quality sensation (AQV) and SET were built. Accordingly, taking TSV ∈ (–0.5, +0.5) (90% people satisfaction) and (–0.75, +0.75) (80% people satisfaction) as baselines, the upper and lower limits of SET for summer and winter were obtained. Combined with experimental conditions, the corresponding acceptable temperature-relative humidity ranges regarding to equivalent SET were determined, with the same thermal comfort. The obtained temperature ranges were wider than that of ASHRAE 55 standards, which were inferred to differences of clothing insulation and thermal adaptation for different populations. Further, the results indicated that the extreme low and high air humidity affected the heat loss at skin surface and evaporative heat loss from respiratory, reducing the acceptable temperature limits. Overall, the work quantifies the coupling effect of air temperature and humidity on human thermal comfort; the comfort zones enable to obtain the range of relative humidity with given temperatures, and the range of temperature with given relative humidity, which is beneficial for designers and operators for building indoor thermal environments. Further work is discussed regarding to the microenvironment in clothes, especially for the negative effects of clothing absorption, which is worthy of consideration for heating design in cold-humid environments like hot summer and cold winter zone in China, to ensure human comfort and health.