What Is Heat Stress that Scientists Call to Include in Heat Alerts?

Heat waves and extreme temperature alerts have become commonplace in many countries. However, these alerts rarely include other factors that dangerously affect human beings and that also hinder their ability to adapt.

Advisories from weather services about heat waves should be based not only on temperatures but also include heat stress indices that take into account factors such as humidity, wind and sun exposure, according to a recently published scientific study. A high level of humidity in the environment and the absence of wind can make, for example, an extreme temperature of 37ºC more harmful to health than the same temperature in a dry environment, making it difficult for the human body to cool down.

This is one of the main points of the study prepared by an international scientific team from the Barcelona Institute for Global Health (ISGlobal) and the London School of Hygiene and Tropical Medicine (Lshtm) published in the journal Npj | Climate and Atmospheric Science de Nature. In the study, they warn that relying solely on temperatures may be insufficient to inform the population about the true health risks of a heat wave, and they request that this data be included in the alerts.

What is heat stress?

“In a simple way, it is about taking into account that temperature is not the same as heat”, explains Xavier Rodó, one of the authors of the study. “The difference is in how the body physiologically perceives the combination of high temperature and high humidity”, he says. “This is what the different heat stress indices take into account in a way, along with other parameters like wind and radiation, but basically the central aspect is humidity”.

The Spanish scientist and head of ISGlobal’s “Climate and Health” program highlights that “more than anything else, what the study does is emphasize that communication by meteorology services and how extremes, waves of heat, is based solely, at least here in our country and in other countries, on maximum temperatures. However, he believes that the footsteps of other countries such as the US, Canada and Germany, which have already included heat stress indices when reporting heat waves, should be followed “as a more adequate measure of hazard, from an alert situation, basically because at the same temperature, at different humidity, the risk is different”.

With humidity above 50% and high temperatures, the body loses the ability to dissipate excess heat by not being able to sweat the same, so it cannot cool down, which can pose harmful health risks. “This is the difference introduced by heat stress indices and this is quite simple, on a scientific level it is not an advance. Yes, it is from the point of view of communication: how does this reach the general population and more so at a time when this type of situation occurs more frequently”, adds Rodó.

There are different indexes used worldwide

Although each person’s threshold for heat resistance varies depending on a number of individual factors, different indices of heat stress have been designed to describe the impact of weather conditions on the body, including the point at which the conditions experienced can become a threat to human health. Just as there is no single heat alert level for everyone, there is also no single heat stress index.

Some of the best known examples are humidex (Hu) -used in Canada-, the heat index (HI) -used in the US- and the universal thermal climate index (UTCI), used in Germany. However, the actual message of heat wave danger through the news and media continues to be linked for the most part to maximum temperatures. Very rarely does it include information on the expected values of these indexes due in part to public ignorance.

At this point, the experts insist that depending on the humidity, 36ºC in a certain place can be, in one case, very uncomfortable and, in another, dangerous. “In these 2 cases, if 2 different alert levels are not established and the substantially different expected health impacts are not clearly communicated, it is easy for the local population to lose perception of the different levels of danger associated with both 36ºC events”, they wrote in the study. This is especially important when considering so-called ‘humid heat waves’, the frequency of which is feared to increase with climate change.

Humid heat waves can make conditions around temperatures previously considered safe in a given location (during dry heat waves) dangerous, they add. Despite this, the researchers behind the study are encouraged that some countries already use these indexes. “What is still missing is that the heat indices are communicated to the population on a regular basis, as is traditionally done with temperatures”, says the study’s lead author, Ivana Cvijanovic. “It could contribute to this if the scientific community reached a consensus on which heat index is better to communicate and which levels of danger to use”.

Beware of the “Danger Zones”

To prepare the study, the scientific team studied recent record-breaking heat waves in Europe, North America and Asia and compared the maps of daily maximum temperatures with those of the maximum heat stress indexes for each day. The geographic areas where the heat stress indices revealed the highest risk did not necessarily coincide with the regions where the highest temperatures were recorded.

For example, during the heat waves in Europe in June and July 2019, records show that central and northeastern Spain experienced the highest temperatures. However, when calculating the heat stress indexes, the research team found that the areas with the most critical conditions were in France, Belgium and the Netherlands, countries that registered an excess mortality of 2,500 deaths.

Another of the cases mentioned was the episode of extreme heat that occurred in parts of western Canada and the northwestern United States in June 2021. Although the maximum temperatures were registered in the states of Washington and Oregon, the stress indexes revealed that Canadian provinces such as Alberta, the Northwest Territories and British Columbia also experienced dangerous conditions, the latter recording 600 heat-related deaths.

“Lessons learned from recent major heat waves suggest that action protocols need to be improved. Once the weather alert has been issued, a clear chain of responsibilities is necessary”, says Ivana Cvijanovic. “Authorities must act promptly and know when to close schools or halt outdoor sports activities, open cooling centers for socially vulnerable populations, and ensure a sufficient emergency response. Educating the general population on how to behave during heat waves is also very important”, she adds.