In present study, the modification effects of humidity in temperature-related health impacts have been discerned in China. This is different from previous studies that merge several meteorological factors into a single indicator. Conversely, we assessed the health risks of temperature and humidity exposures in all combinations. The present study also identified the types and thresholds of compound temperature-humidity exposure with high risks. We further assessed the changes in the intensity and spatial distribution of risks due to compound events exposure during historical period (1981–2010) and future period (2020–2100) under different warming scenarios. Results provide a more robust and realistic projections of future heat-related health risks in a warming world. The analysis introduced in this study extends beyond the previous studies by highlighting the multiple meteorological parameters in defining adverse weather conditions and risk projections, which are distinct from those that using temperature alone to predict health impacts.
Our result suggests that the compound exposure of high-humidity and temperature has higher risks than that of compound low-humidity and temperature events. One reason that high humidity makes hot or cold weather more unbearable is that the higher the relative humidity, the higher or colder the temperature actually feels, and thus reducing human tolerance to temperature. In addition, certain viruses, such as influenza, survive longer in cold and humid environments, further increasing the risk of infection. While during the high temperature with high humidity event, higher amount of water vapor will reduce the evaporation rate of skin moisture. As a result, it reduces the effectiveness of sweating in cooling the body, thus increasing the incidence of heat-related diseases 27,28
Temperature-related impacts are often related to multiple drivers. However, the complexities are currently neglected with common heat-related health assessment, assuming that a single temperature value can adequately predicts health impacts 29–32. Although an array of heat indices has been developed to account for the effects of multiple meteorological factors, such as temperature, humidity and wind. These indices eventually merged into a single and new index, such as WBGT, AT. In that case, the risk of different combination of temperature and humidity cannot be distinguished. To be specific, if the value of WBGT during high temperature and low humidity event is the same as that during low temperature and high humidity, then the risks reported by WBGT would be the same, which would mask the health impacts of different weather exposures. Oversimplification of these complex responses can result in underestimate the risks of temperature-related health impacts as illustrated by examples in this research.
To identify the adverse meteorological conditions posed high risks on human health, this study comprehensively evaluated the health risks under different meteorological conditions, so as to determine the high-risk temperature and humidity thresholds. Conventional investigation of temperature-related health impacts generally set the threshold of adverse temperature empirically 33,34. For example, the intensity of extreme heat or heat wave events was 95th, 98th or 99th percentile of the temperature range. However, the present results reveal that the threshold of compound events is lower than that of extreme heat or heat wave events. The reason behind this might be that the combined effects of temperature and humidity may amplify the effect of temperature on health. Therefore, a combination of non-extreme temperature and humidity exposure will produce high risks.
With the process of climate change, annual total number of ambulance dispatches attributable to compound events showed an inverted U-pattern that increase first and then decrease. This is because ANs caused by cold events will continuous to decline, while ANs caused by heat events will increase in the early and middle of the 21st century, and tend to be stable at the end of the 21st century. Due to the impacts of climate change, different emission scenarios will change the composition of ANs related to compound events. Nearly a third of ANs are caused by cold events under the low emission scenario, while heat and humidity events take the majority under the higher emission scenarios. In addition, higher emission scenarios would exacerbate the spatial heterogeneity of effects of compound events on ambulance dispatches in China. Although concerns about changes in climate extremes over China have been widely raised over the past decades, no research to date have considered health risks of compound temperature-humidity events over China, which may have more severe impacts on human society than single climate extremes and lie a lot of uncertainty to reveal.
This study has several limitations. First, the risks of compound temperature-humidity events exposure were derived from 13 cities in China. Although these cities have a certain national representation, future assessment that include data from more regions of China are needed to verify the reliability of the findings. Second, the thresholds of compound events with high-risks were limited to morbidity, thus generalizing the results to other health outcomes such as mortality should be done with caution. Third, besides temperature and humidity, other factors such as meteorological conditions (sunlight and wind), human activity (metabolic heat production), and clothing can also influence the heat stress imposed on human body. Whilst recognizing these limitations, findings from this study can serve as a basis for understanding the characteristics of compound weather exposure with high-risks, and can be further refined in the future work to improve the robustness and reliability of projections of heat strain and health concerns.
Considering adverse meteorological conditions as a compound event and decomposing them to discern the thresholds that trigger health impacts can improve risk modelling and assessment, thus providing a pathway to better understand the health impacts of climate change. Compound exposure of multiple meteorological factors exacerbates the adverse health impacts, with non-extreme temperature and humidity can pose high risks. With the ongoing climate change and population changes, the frequency and spatial extent of compound events are expected to increase, result in amplifying their impacts on health. Our findings support arguments for embedding more sophisticated human heat stress models into health impacts assessment and climate projections, thereby providing more robust and realistic projections of future heat-related health risks in a warming world.