The Importance of Humidity in the Relationship between Heat and Population Mental Health: Evidence from Australia

Despite many studies on the effects of heat on mental health, few studies have examined humidity. In order to investigate the relationship among heat, humidity and mental health, we matched data from the Social, Economic and Environmental Factors (SEEF) project with gridded daily temperature and water vapour pressure data from the Australian Bureau of Meteorology. Logit models were employed to describe the associations among heat (assessed using temperature, degrees C), humidity (assessed using vapour pressure, hPa) and two measures of mental health, (i) high or very high distress (assessed using K10 scores >= 22) and (ii) having been treated for depression or anxiety. We found a one-unit increase in temperature and vapour pressure was associated with an increase in the occurrence of high or very high distress by 0.2% (p < 0.001, 99% CI: 0.1 - 0.3%) and 0.1% (p < 0.001, 99% CI: 0.0 - 0.3%) respectively. However, when humidity rose to the 99th percentile of the sample, the estimated marginal effect of heat was more than doubled (0.5%, p < 0.001, 99% CI: 0.2 - 0.7%). Neither heat nor humidity was related to having been treated for depression or anxiety in the last month. Humidity compounds the negative association between hot weather and mental health and thus should be taken into account when reforming the health care system to respond to the challenge of climate change.