Research links extreme heat to mortality in Rio de Janeiro

The intense heat in Rio de Janeiro is not just an appeal for going to the beach or a reason for discomfort and stress for those with no way of cooling off. The high temperatures for which the marvelous city is known are related to an increased mortality in the capital of Rio de Janeiro. This is what research by João Henrique de Araujo Morais, a doctoral student at the Sergio Arouca National School of Public Health (Ensp/Fiocruz) and researcher at the Epidemiological Intelligence Center of the Municipal Health Secretariat of Rio de Janeiro, shows. The study conducted by the Public Health Epidemiology Program student points out that the risk is greater for the elderly and people with certain diseases. The study analyzed all 466,000 records of natural deaths between 2012 and 2024, and more than 390,000 deaths from 17 selected causes – 12 of which had a considerable increase in mortality for the elderly in extreme heat.

Figures were analyzed separately according to the classification of Heat Levels (NC) of Rio de Janeiro City Hall's Heat Protocol, launched last year. NCs range from 1 to 5 and indicate risks and actions that should be taken in each case. Heat Level 4, when the temperature is above 40 °C for four hours or more, is associated with a 50% increase in mortality from diseases such as hypertension, diabetes, and kidney failure among the elderly. "At Level 5, two hours with a Heat Index equal to or above 44 °C, this same increase is observed and aggravated as the number of hours increases. Therefore, the study confirms that, at these extreme levels defined in the protocol, the health risk is real," said the author.

It has been observed that heat poses a greater risk to the elderly and people with diseases such as diabetes and hypertension, as well as Alzheimer's, kidney failure, and urinary tract infections. Araujo says that this was to be expected. "These results are in line with the existing literature on the subject. Most studies on heat and mortality focus on cardiovascular and respiratory diseases, for which the physiological effects on the human body are well known. However, some studies also report these effects for metabolic diseases, urinary tract diseases, and diseases such as Alzheimer's, which we discuss in the article. Some studies have also shown this connection for neoplasms, which was not found in this study."

The results of the study are a wake-up call. After all, amid worsening climate emergencies, the last two years have been the hottest in Rio's history. And 2024 was the first to surpass the 1.5 °C mark in the Earth's average temperature increase compared to pre-industrial levels, according to Copernicus, the European Union's Earth Observation Programme.

In this sense, João Henrique de Araujo emphasizes the need for plans to adapt cities to the effects of climate change. "Other studies have already shown an increase in the frequency and intensity of heat waves not only for the city of Rio, but for various urban areas in Brazil. Individual measures are important, but public policies are needed to adapt activities and protect the population. Specific populations are known to be at high risk – such as workers directly exposed to the sun, street populations, the most vulnerable groups (children, the elderly, people with chronic illnesses), and populations living in so-called Urban Heat Islands. It is therefore hoped that the actions taken in the Rio de Janeiro City Hall's Heat Protocol, such as providing hydration and cooling points, adapting work activities, constant communication with the population, and suspending risky activities at critical levels, will be disseminated and adopted in other cities as well, to protect the health of the population, especially the most vulnerable."

Innovative metrics

The research has developed a new method for measuring exposure to heat and the related risks with the creation of a metric called Heat Exposure Area (AEC), which further considers the time a person is exposed to heat, something that other measures, such as average temperature or average thermal sensation, do not take into account. According to the study, the length of exposure to intense heat has an important link with mortality, especially among the most vulnerable people. For the elderly, for example, exposure to an AEC of 64 °Ch increases the risk of death from natural causes by 50%, and with an AEC of 91.2 °Ch, the risk doubles.

To explain what this means, the study recalls the heat wave of November 2023. On November 17th, the day that young Ana Clara Benevides died after falling ill at the Nilton Santos stadium, where singer Taylor Swift was performing, the heat index in Rio was above 44 °C for eight hours, generating a record AEC. The following day, November 18th, saw the highest number of deaths of elderly people from specific causes: 151 in total.

Therefore, the research offers a tool that has proven to be more efficient than other metrics in identifying days of extreme heat and thus helping to prevent the possible consequences. The study compares two dates to show how AEC works. On January 12th, 2020, the heat index was 32.69 °C, while on October 7th, 2023, it was 32.51 °C – almost the same. However, on the second day, the heat lasted longer, resulting in an AEC of 55.3 °Ch, which is 20 times higher than the 2.7 °Ch of the first day. This shows the impact of heat exposure time on human health.

"By considering only summary measurements (averages or maximums) we can underestimate abnormally hot days. The metric, in turn, can identify this and be used to define protocols similar to the one developed here in Rio. When we compare the performance of the models, i.e. how well they were able to explain mortality, we see that including the models using the AEC metric had the best performance for 13 of the 17 causes studied," explains the doctoral student.