What is the thermal stress that scientists asked to include in the heat alerts

Heatwaves and extreme temperature alerts have become commonplace in many countries, but rarely do these alerts include other factors that dangerously affect human beings and that hinder their ability to adapt.

The warnings of the meteorological services about heat waves should be based on just the temperatures, rather include thermal stress indices that have in account factors such as humidity, wind and sun exposureaccording to a recently published scientific study.

A high level of humidity in the environment and the absence of wind can, for example, make an extreme temperature of 37 ºC 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 studio elaborated by an international scientific team of the Institute of Global Health of Barcelona (ISGlobal) and of the School of Hygiene and Tropical Medicine of London (Lshtm, for its acronyms in English) published in the magazine Npj | Climate and Atmospheric Science in Nature.

The studio warns that basing solely on temperatures may be insufficient to inform the population about the real risks to health from a heat wave, and they request that these data be included in the alerts.

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What is the thermal stress

“In a simple way, it is a matter of bearing in mind that temperature is not the same as heat”, explains BBC Mundo Xavier Rodó, one of the authors of the study.

“The difference is in how the body physiologically perceives the combination between a high temperature and a high humidity”, he indicates.

“This is what somehow the different indices of thermal stress take into account, along with other parameters such as wind and radiation, but basically the central aspect is the humidity”.

The Spanish scientist responsible for the “Climate and Health” program of ISGlobal points out that “the studio more than anything he does is putting emphasis on the communication on the part of the meteorological services and how they communicate the extremes, the winds of heat, is based solely, as a minimum here in our country and in other countries, on the maximum temperatures”.

However, it is believed that the steps of other countries such as the USA, Canada and Germany should be followed, which have already included thermal stress indices at the time of communicating the heat waves “as a more adequate measure of the danger, of a situation alert, basically because at the same temperature, at different humidities, the risk is different”.

With humidity above 50% and high temperatures, the body loses the ability to dissipate the excess heat, as well as not being able to perspire the same, which is why it cannot get cold, which can pose harmful risks to health.

“This is the difference that introducen los indices of thermal stress and this is quite simple, at a scientific level it is not an advance. It is from the point of view of communication: how it gets to the population in general and more at a time when this type of situation occurs with greater frequency”, adds Rodó.

Different indices

Since the threshold of resistance to heat for each person varies depending on a series of individual factors, different thermal stress indices have been designed to describe the impact of meteorological conditions on the body, including the point at which the conditions experienced can be become a threat to human health.

Also, there is no single heat alert level for everyone, nor is there a single heat stress index.

Some of the best known examples are humidex (Hu) -used in Canada-, the heat index (HI) -used in the USA- and the universal thermal climate index (UTCI), used in Germany.

However, the real message of danger of heat wave through the news and the means of communication continues being linked in its major part to the maximum temperatures and rarely include information about the predicted values ​​of these indicesalso partly due to public ignorance.

At this point, the experts insist that depending on the humidity, 36 °C in a certain place can be, in one case, very uncomfortable and, in another, dangerous.

“In these cases, if the different alert levels are not established and the substantially different repercussions for health that are expected are not clearly communicated, it is easy for the local population to lose the perception of the different levels of danger associated with the two 36 °C events”, they wrote in the studio.

This is especially important when considering the calls “humid heat balls”whose frequency is feared to increase with climate change.

Wet heat waves can make the conditions around temperatures that were previously considered safe in a certain place (during dry heat waves) become dangerous, aggregate.

However, for the researchers of the study, it is encouraging that some countries are already using these indices.

“What is still missing is that the heat indexes are communicated to the population on a regular basis, as is traditionally done with temperatures”, says the lead author of the study, Ivana Cvijanovic.

“It could help that the scientific community reached a consensus on what heat index is best to communicate and what levels of danger to use”.

“Danger zones”

To develop the study, the scientific team studied recent heat waves that broke records in Europe, North America and Asia and compared the maps of daily maximum temperatures with the maximum thermal stress indexes of each day.

The geographical zones where the thermal stress indices revealed the greatest 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, the records show that the center and northeast of Spain experienced the highest temperatures.

However, when calculating the thermal stress indices, the research team confirmed that the areas with the most critical conditions were found in France, Belgium and the Netherlands, countries that registered an excess of 2,500 deaths.

Another of the cases mentioned was the episode of extreme heat that occurred in areas of western Canada and the northwest of the United States in June 2021.

While maximum temperatures were recorded in the states of Washington and Oregon, thermal stress indices revealed that Canadian provinces such as Alberta, the Northwest Territories and British Columbia also experienced dangerous conditions, with the latter registering 600 heat-related deaths.

“The lessons learned from the recent big heat waves suggest that it is necessary to improve the performance protocols. Once the meteorological alert has been issued, a clear chain of responsibilities is needed”, says Ivana Cvijanovic.

“The authorities must act promptly and know when to close the schools or interrupt outdoor sports activities, open refrigeration centers for socially vulnerable populations and guarantee a sufficient emergency response.

“Educating the general population about how to behave during hot weather is also very important”, he adds. (I)