Researchers argue that other climatic characteristics, such as humidity, should be considered to properly explain the impacts of acute heat stress scenarios
Researchers from the Barcelona Institute for Global Health (ISGlobal), a centre supported by the "la Caixa" Foundation, and the London School of Hygiene and Tropical Medicine (LSHTM) warn in a study published in npj Climate and Atmospheric Science that relying solely on expected temperatures in communicating dangerous heatwave conditions can be insufficient to inform people about the true health risks. The team advocates for expanded use of so-called heat stress indices, which, in addition to temperature, consider other meteorological parameters such as humidity, to better explain the effects of extreme heat stress situations.
“It is the wider set of meteorological conditions that can affect one’s response to outdoor heat – starting from the ambient temperature, as well as humidity of the surrounding air, the prevailing wind conditions, exposure to the direct sun versus being in shade, and finally the total duration of exposure to such conditions”, says Malcolm Mistry, from the London School of Hygiene and Tropical Medicine and Ca’ Foscari University of Venice, and a co-author of the study. “Communicating expected maximum temperatures only,” he continues, “may not always reflect the true danger of ongoing heat waves. For example, the same ambient temperature of 35°C can be uncomfortable at low humidity but dangerous to human health at high humidity, even for a short period of time.”
The researchers examined recent record-breaking heatwaves in Europe, North America, and Asia and compared maximum daily temperature maps with maximum daily heat stress indices (humidex and indoor wet bulb globe temperature). The geographical areas where the heat indices suggested the greatest risk of heat stress did not always correspond to the regions with the highest temperatures.
Another heat wave studied by the team was the huge heatwave that rocked India and Pakistan in May 2022, when temperatures reached 50°C. The heat indices demonstrated once again that danger zones were not limited to those with the highest temperatures.
"Episodes of extreme heat that push humans to the edge of survival, like the one recorded in India and Pakistan in May 2022, are likely to become more frequent as climate models highlight, and we need to do everything we can to be as well prepared as possible to deal with them," says Xavier Rodó, head of ISGlobal's Climate and Health programme and one of the authors of the study, “Unfortunately, health data from this heatwave was not available and would certainly be useful to the scientific and medical communities to both better understand the nature and extent of the effects of such mega heat waves, and to educate us on how to prepare for such outcomes”.
The study emphasises that, while each person's threshold of heat resistance varies due to a variety of individual factors, various heat stress indices have been developed to describe the impact of meteorological conditions on the human body, including the point at which the conditions experienced can become a threat to human health. Humidex (Hu), apparent temperature (AT), wet bulb globe temperature (WBGT), heat index (HI), and universal thermal climate index (UTCI) are a few well-known examples. This is especially important in light of the so-called "humid heat waves" that are predicted to become more often as a result of climate change.
Encouragingly, heat indices are being increasingly implemented in weather forecasts by meteorological services around the world. Some examples include humidex in Canada, UTCI in Germany and Heat Index in the USA. “What is still missing is having heat indices communicated to the population in a regular manner, as it is traditionally done using temperatures”, states lead author Ivana Cvijanovic. “This could be helped by the scientific community reaching a consensus on which heat index is best to communicate and which danger levels to use.”
“Lessons learned from the recent major heatwaves suggest that improved protocols for action during heatwaves are needed. Once the meteorological warning has been issued there is need for a clear chain of responsibilities. The authorities need to act promptly and know when to close schools or stop outdoor sports activities, open cooling centers for socially vulnerable populations and ensure a sufficient emergency response.”, says Ivana Cvijanovic.
“Educating the general population on how to behave during heat waves is very important too. Knowing how to recognize the signs of heat related illness (and what to do in case of) and creating a “no one left behind” culture with people checking on their elderly neighbours or anyone seemingly vulnerable can help save lives. We also need to have solutions in place in case of power outages - when we cannot rely on cooling mechanisms such as air-conditioning. Finally, we must not forget animals and pets, as they will need protection during these times too”, concludes Cvijanovic.
Along with the paper, a team from ISGlobal has created a heat index calculator. The easy-to-use tool calculates the heat index and provides guidance on the level of risk along with recommendations on how to stay safe when the conditions are hazardous. To operate the calculator, users just need to introduce temperature and relative humidity values, both of which are provided by many of the domestic thermometer models in the market, and that way they can obtain an estimation of the conditions in any of the rooms of their homes.
Although there are other heat index calculators available online, this is the first one to use the extended and corrected heat index that was recently developed by UC Berkeley researchers. The tool is intended to provide estimates of indoor risk and is openly available at www.isglobal.org/heat-index-calculator.